Chapter 5

Vegetation of North America North of Mexico

Michael G. Barbour

Norman L. Christensen

COVERING nearly 60° of latitude and 145° of longitude, the North American continent extends through a striking diversity of climates and supports a rich array of vegetation types. Most of the world's major plant formations are represented. Tundra, boreal forest, montane conifer forest, temperate deciduous forest, prairie, steppe, woodlands of deciduous and evergreen trees, semiarid desert scrub, Mediterranean scrub, and tidal marshes occupy significant portions of the continent. Prominently missing are extremely arid deserts and several tropical grassland, savanna, and forest types.

We have been assisted in our review by two recent volumes on North American vegetation: B.F. Chabot and H.A. Mooney (1985) and M.G. Barbour and W.D. Billings (1988). We have also drawn heavily on the plant geography text by R.Daubenmire (1978). Less formal summaries prepared by M.G. Barbour, J.H. Burk, and W.D. Pitts (1987) and J.L. Vankat (1979) were also useful. We have surveyed regional summaries and research papers but have tried to limit references cited to modern summaries, older classics, and a few research articles that support important concepts about a particular vegetation type. Therefore, the review we present is not exhaustive, nor does it include aspects on the history of vegetation mapping in North America. Furthermore, we have concentrated on the current distribution of natural vegetation, rather than on human-modified landscapes or on movements of floras or vegetation through geologic time (but see A.Graham, chap. 3 and P.A. Delcourt and H.R. Delcourt, chap. 4).

The themes we emphasize in this chapter are: (1)relationships between vegetation and regional climate, (2) environmental gradients and episodic stresses that create vegetation gradients over space and time, and (3) the nature of ecotones that exist between all major vegetation types. We begin our survey with tundra vegetation---at the northern limits of plant life---and work clockwise through the continent, to boreal forest, eastern deciduous forest, grassland, desert, woodland, western conifer forest, and coastal vegetation. A map of the major vegetation types of North America north of Mexico is given in figure 5.1. Because of the map's large scale, it does not show many ecotones, mosaics, or local subtypes of vegetation. Every regional specialist who examines our map will likely find some detail to be in error. We alert the reader to the fact that figure 5.1 represents an oversimplification of nature, and we apologize for necessary local distortions.

We dedicate this chapter to John W. Harshberger, who wrote the earliest summary of North America vegetation in 1911.

Arctic Ecosystems

Overview

The Arctic, that region north of the climatic limit of boreal forest, is vegetated by a diverse assemblage of plant communities. L.C. Bliss (1981) distinguished the Low Arctic region, dominated by tundras, from the High Arctic region in which polar deserts are prevalent. We use the term "tundra" to refer to a continuum of treeless plant communities varying in stature from shrub heaths (1--5 m high) to low sedges, grasses, and cryptogams (fig. 5.2). Plant cover is virtually 100% in tundras, but generally less than 50% (often less than 5%) in polar deserts (L.C. Bliss 1988).

Low Arctic landscapes occur as far south as 55° N latitude in eastern Canada, but generally north of 60° N latitude in western North America. This southern limit coincides roughly with the southern extent of continuous soil permafrost (L.C. Bliss 1988). Other factors regulating the tundra-taiga ecotone include length of the growing season (generally fewer than 90 days) and patterns of disturbance (especially fire) (W.C. Oechel and W.T. Lawrence 1985). High Arctic ecosystems are largely confined to the islands of Canada's Northwest Territory and northern Greenland. Suitable sites for plant growth at the northern limit of High Arctic polar desert appear to be restricted by water-saturated soils in the spring and water deficits in the summer (L.C. Bliss 1988). Together, arctic ecosystems cover over 4 million km², or approximately 19% of North America north of Mexico.

Compared to temperate and boreal ecosystems, arctic ecosystems are quite young. During Pliocene times (6--3 million years before the present, M.Y.B.P.), nearly all of the present Arctic was covered by coniferous forest. Tundra may have occurred on the most northern Canadian islands. The majority of arctic species are thought to have evolved from ancestors in montane central Asia, with fewer having been derived from Rocky Mountain species (W.D. Billings 1974; L.C. Bliss 1988). At the onset of the Pleistocene the circumpolar arctic flora probably included over 1500 species, but successive glacial advances and accompanying climate change have reduced this number to fewer than 1100 (D.Löve and A.Löve 1974), of which about 700 occur in North America.

Human utilization and impacts on arctic landscapes have been modest. The native Inuit and Eskimos have made only limited use of tundra plants. More recently, local areas have been affected by grazing of livestock. Exploitation of mineral resources, especially oil, presents a potential for alteration of some arctic landscapes.

As might be expected, arctic plants are well adapted to grow at low temperatures. Optimal temperatures for shoot growth are generally between 10° and 20° C, and many species can maintain net photosynthesis at temperatures near 0° C. G.R. Shaver and W.D. Billings (1977) found that root growth in Eriophorum angustifolium was optimal at 1°--4° C. In general, plant height is correlated with the height above ground at which summer temperatures are warmest (L.C. Bliss 1988). Furthermore, shrub, cushion, and mat growth forms create a significant boundary layer in which air temperatures may be 5°--10° above ambient. Arctic species are also adapted to take advantage of the abbreviated growing season. Shoot growth in most vascular plants begins immediately after snowmelt, utilizing photosynthetic reserves from the previous year (F.S. Chapin III and G.R. Shaver 1985). Buds are "preformed" at the end of each growing season to facilitate rapid deployment of shoots and roots at the beginning of the next growing season.

In polar desert regions, short growing seasons and water deficits limit annual net primary production to only 5--30 g/m² (L.C. Bliss 1988). In Low Arctic wet tundra ecosystems, net primary production is 150--200 g/m²/yr. Low atmospheric nutrient input, soil anoxia, and temperature-limited rates of mineralization result in widespread deficiencies of nitrogen and phosphorus in these habitats (F.S. Chapin III and G.R. Shaver 1985). In well-drained Low Arctic shrub tundras, primary production is 250--400 g/m²/yr.

Regional Variation

Total plant cover and stature, as well as relative abundance and diversity of vascular plants, generally decrease with increasing latitude. Diversity and relative abundance of lichens and bryophytes, however, are highest in High Arctic ecosystems. Within a particular region, vegetation composition and structure may vary considerably along microclimatic and hydrologic gradients, and in relation to patterns of snowmelt and disturbance.

Tall shrub tundra occurs on well-drained, nutrient-rich sites in the Low Arctic. These areas are most common on river flood plains or other areas with a deep active layer (L.C. Bliss 1988). Shrubs in genera such as Alnus, Betula, and Salix grow to a stature of 1--5 m on such sites.

Low shrub tundra (shrub stature 40--60 cm) ecosystems are common on rolling upland sites across the southern Low Arctic. Diminutive individuals of the same Alnus, Betula, and Salix spp. found in high shrub tundra occur with Arctostaphylos alpina, A. rubra, Empetrum nigrum, Ledum palustre, Rubus chamaemorus,Vaccinium uliginosum, and V. vitis-idaea, in varying combinations. In some locations Carex spp. and a diverse array of lichens and mosses are especially abundant. The relative importance of these taxa often varies in relation to seasonal patterns of snow accumulation and melt (L.C. Bliss 1988). It is believed that changes in climate and fire frequency during the past two centuries have caused replacement of forest tundra by low shrub tundra in many areas near the arctic/boreal ecotone (I.G.W. Corns 1974; J.C. Ritchie 1977, 1984).

Dwarf shrub heath tundras occur as small patches (usually less than 1 ha) on well-drained soils on floodplain and upland areas. Low stature (10--20 cm) shrubs belonging to the Ericaceae, Empetraceae, and Diapensiaceae dominate. The patchy distribution of these ecosystems is related to local heavy accumulation (greater than 20--30 cm) of winter snow (L.C. Bliss 1988). Thus, the relative importance of this vegetation type in various arctic landscapes varies with topographic patterns that regulate snow distribution. Cotton grass--dwarf shrub heath tundras are most common in upland areas between the mountains and wet coastal plain in Alaska and the Yukon. The presence of prominent tussocks of Eriophorum vaginatum (tussock grass) in a heath matrix distinguishes this community from other heath tundras. Considerably less diverse dwarf shrub heath tundras may occur in local mesic sites associated with late snowmelt in the High Arctic (L.C. Bliss 1988; L.C. Bliss and J.Svoboda 1984).

Graminoid-moss tundras dominate poorly drained areas across the Low Arctic. Sedges, such as Carex and Eriophorum spp., and grasses, including Alopecurus,Arctagrostis, Arctophila, and Dupontia, dominate these wetlands along with a diverse assemblage of bryophytes. Local variation in relative abundance of graminoid-moss tundra species often occurs as a consequence of small-scale variations in topography and depth of the active layer that influence local soil hydrology. Graminoid-moss tundras are common in High Arctic landscapes where drainage is impeded (L.C. Bliss and J.Svoboda 1984).

L.C. Bliss (1988) distinguished three arctic semidesert communities: (1) Cushion plant-cryptogam communities are dominated by cushionlike or mat-forming herbs, such as Draba corymbosa, Dryas integrifolia, and Saxifraga spp., and lichens and mosses. Scattered graminoids are virtually always present. This vegetation is common on warm, dry sites with a relatively long growing season. (2) Cryptogam-herb vegetation, in which lichens and mosses account for 50--80% cover, occurs on moister sites. (3) Graminoid steppe, dominated by Alopecurus alpinus and Luzula confusa, has a distribution that appears to be related to slate-derived soils on some Canadian islands (L.C. Bliss and J.Svoboda 1984).

Polar desert ecosystems include herb barrens and snowflush communities and occur on upland plateaus above 200m. Plant cover in herb barrens varies with the availability of soil; in rocky areas cover may be less than 5%. Common vascular plants include Draba corymbosa, D. subcapitata, Minuartia rubella, Papaver radicatum, Puccinellia angustata, and Saxifraga oppositifolia. Many of these areas may have been vegetated only since the Little Ice Age (400--130 yr B.P.). Snowflush communities occur where snow meltwater is present all summer. These sites vary considerably in species composition and are characterized by greater plant cover and greater species diversity than are adjacent barrens.

Boreal Forests

Overview

Boreal forest or taiga is typified by relatively dense stands of evergreen coniferous trees of modest stature and comparatively low species diversity (fig. 5.3). The boreal forest formation is actually composed of a complex of plant communities that include bogs and meadows in addition to forest stands. This formation extends as a continuous band across North America, coinciding with H.Walter's (1979) Zonobiome VIII. Boreal forest reaches its southernmost extent in southeastern Canada and the Great Lakes states, and extends northwest into central Alaska. It commonly spans over 10° latitude and dominates 28% of the North America landmass north of Mexico, making it the most extensive formation in North America.

The northern transition from boreal forest to tundra is probably related to short growing seasons (fewer than 90 days) and permafrost conditions unfavorable to tree growth (D.L. Elliott and S.K. Short 1979; R.A. Black and L.C. Bliss 1978, 1980; D.L. Elliott-Fisk 1983). In the western Canadian Rocky Mountains, boreal forest intergrades and shares many species with montane coniferous ecosystems. In southern Manitoba and western Ontario, taiga borders on prairie. Across the Great Lake states and southern Canada, the boreal forest borders on mixed deciduous forest.

Conifers (Abies, Larix, Picea, and Pinus) comprise the bulk of taiga biomass. Most ecosystems, however, also include a variety of deciduous tree, shrub (especially Ericaceae), and herb species. In general, species diversity within individual communities increases with the length of the growing season, favorable drainage, and increasing soil fertility (D.L. Elliott-Fisk 1988).

The modern boreal flora is generally thought to be derived largely from the Arcto-Tertiary geoflora (J.A. Larsen 1980). Nearly all of the extant tree genera were present in late Tertiary times. Most of the region now occupied by taiga was glaciated repeatedly during Quaternary times, when boreal taxa were found as far south as the Carolinas and the lower Mississippi Embayment (P.A. Delcourt et al. 1980; W.A. Watts 1980). Boreal forest is currently distributed across terrain that was covered by continental glaciers just 14,000 years ago. These landscapes have supported boreal forests for only 3000--6000 years (J.C. Ritchie and G.A. Yarranton 1978). D.L. Elliott-Fisk (1988) attributed the relative impoverishment of the boreal flora in part to Quaternary disturbance and the youth of the landscape.

Human impacts on boreal forest landscapes include timber and mining activities, alteration of fire events, and hydrologic modification. In more northern regions, destruction of permafrost accompanying human land use has resulted in landscape degradation. Nevertheless, extensive vigorous tracts of boreal forest remain and provide a major renewable timber resource. Because of the limited economic value of boreal wetlands, many of these landscapes have remained virtually unaffected by human activities (D.L. Elliott-Fisk 1988).

Boreal plants must be able to tolerate extremes of environment. Summers are cool and winters are extremely cold. Annual precipitation varies between 300 and 900 mm, with most falling during the summer. On average, the precipitation/evapotranspiration ratio (P/E) is greater than 1.0, but high summer evapotranspiration, coupled with porous soils, can result in local water deficits during the growing season. Nevertheless, water stress generally is not a limiting factor in boreal forests (W.C. Oechel and W.T. Lawrence 1985). H.Walter et al. (1975) found that the distribution of boreal forest was most strongly correlated with growing season length (number of days with a mean temperature greater than 10° C). At the taiga-tundra ecotone, the growing season is less than 90 days, whereas the growing season is greater than 120 days at the transition to temperate ecosystems.

Local distribution of boreal communities is heavily influenced by topography and soils. Glacial features such as kettle holes, eskers, moraines, and outwash plains create considerable topographic and soil variation at nearly all spatial scales. Spodosols (podzols), i.e., soils with an infertile, leached A horizon and a well-defined B horizon high in iron and aluminum, dominate upland areas. On glacial tills and outwash plains, clay layers may impede drainage and greatly influence forest composition (M.R. Roberts and N.L. Christensen 1988). Histosols (organic soils) are prevalent in wetlands.

In northern areas, permanently frozen ground or permafrost significantly influences vegetation distribution. The depth of thaw determines the volume of soil available for root growth and resource exploitation. Vegetation changes that affect gain and loss of soil heat can result in considerable changes in permafrost distribution (W.C. Oechel and W.T. Lawrence 1985).

Many boreal forest species are able to carry out net photosynthesis over a range of temperatures from less than 0°C to greater than 40° C (T.Vowinckel et al. 1975). Such wide temperature tolerance is due in part to acclimation during the growing season (W.C. Oechel and W.T. Lawrence 1985). Rates of photosynthesis are generally greater for deciduous taxa than for evergreen ones. Evergreen species, however, are generally able to photosynthesize at lower temperatures and thus experience a longer growing season.

Net primary production (NPP) on upland sites varies from 480 g/m²/yr on sterile sandy soils to 1280 g/m²/yr on mesic fertile sites (J.S. Olson 1971). In black spruce muskegs, NPP may be less than 130 g/m²/hr (K.Van Cleve et al. 1983). Nutrients, particularly nitrogen and phosphorus, are limited in most boreal ecosystems. This may be in part responsible for the prevalence of evergreen taxa (F.S. Chapin III 1980). Nearly all boreal forest vascular plants possess mycorrhizae that aid in nutrient uptake (D.Malloch and B.Malloch 1981).

Regional Variation

J.A. Larsen (1980) divided the North American taiga into seven regional zones based on physiography and variations in the relative importance of dominant species and associations. The Alaskan region (zone 1) is dominated by Picea glauca on mesic sites and P. mariana on poorly drained sites. Abundant fire and floodplain disturbance result in a complex mosaic of successional types with abundant Alnus crispa, Populus balsamifera, Betula papyrifera, and Salix spp. The Cordilleran region (zone 2) is influenced by the diverse topography of the northern Rocky Mountains. Picea mariana--dominated forests are found here primarily on north-facing slopes. The interior region (zone 3) extends from the Cordilleran foothills to the edge of the Canadian Shield. In addition to Picea spp., Pinus contorta var. latifolia and Pinus banksiana are important. On the Canadian Shield (zone 4), Picea mariana forests characterize the north and grade into a mixture of forest types to the south. Abies balsamea becomes increasingly important here. J.S. Rowe (1977) identified 18 different boreal associations in J.A. Larsen's Gaspé-Maritime region (zone5). Abies balsamea is often dominant, and pure stands of Picea mariana are rare. The Labrador-Ungava region (zone6) is characterized by vegetation continua associated with moisture and geologic gradients. Species of Abies balsamea, Betula papyrifera, Larix laricina, Picea spp., Pinus banksiana, Populus balsamifera, P. tremuloides, and Thuja occidentalis are important. Along the northern boreal boundary (zone 7), Picea mariana is dominant, with P. glauca occurring on well-drained sites and Larix laricina in wet areas.

D.L. Elliott-Fisk (1988) classified upland ecosystems into closed forest, lichen woodland, and tundra-forest ecotone. Spruce-feathermoss forests are the most widespread closed forest type. Either Picea glauca or P. mariana may dominate the canopy of such forests, with a nearly continuous bryophyte stratum of Hylocomium splendens and Pleurozium schreberi. The flora of understory vascular plants is most diverse beneath Picea glauca--Abies balsamea--dominated canopies. Lichen woodlands are characterized by scattered individuals of Picea glauca or P. mariana and an understory covered by the lichens Stereocaulon paschaleor Cladonia stellaris (K.A. Kershaw 1977). These woodlands are considered by some to be successional following fire (J.C. Ritchie 1962; K.A. Kershaw 1977) and by others to be stable climaxes (J.A. Larsen 1980; D.L. Elliott-Fisk 1983).

The tundra-forest ecotone is composed of scattered Picea mariana, sometimes occurring in clumps, within a surrounding matrix of tundra. This ecotone is relatively narrow in the east and west, but it is 200--300 km wide in central Canada (D.L. Elliott-Fisk 1988). Although the general location of this ecotone is certainly related to climate (D.Löve 1970), tree distribution in this ecotone is also influenced by fire (R.A. Black and L.C. Bliss 1978; D.L. Elliott-Fisk 1983) and hydrology (D.L. Elliott-Fisk 1988).

Shrublands dominated by Alnus crispa, Betula glandulosa, and Salix spp. are common throughout the taiga. Near the northern treeline, such communities may be stable and more properly classified as tundra, but in other areas they are often successional to forest (D.L. Elliott-Fisk 1988).

Wetlands constitute a major part of the boreal landscape. Ombrotrophic bogs---peatlands that receive nutrient inputs only from the atmosphere--- are dominated by Sphagnum and a diverse assemblage of Ericaceous shrubs. Herbs in the Asteraceae, Cyperaceae, Orchidaceae, and Poaceae are also characteristic of such wetlands (J.A. Larsen 1982). Picea mariana and Larix laricina are the most common tree species in such bogs. Fen-type wetlands are considerably less acidic and more nutrient enriched. They share many tree species in common with bogs, but little overlap occurs in their herbaceous flora (J.A. Larsen 1982).

Eastern Deciduous Forests

Overview

The eastern deciduous forest province contains a diverse array of forests dominated by winter-deciduous trees (figs.5.4, 5.5). The closed canopies of these forests reach to a height of over 25 m, with a complex understory. The broad northern transition of deciduous forest to taiga begins at about 45° N latitude. The western grassland--deciduous forest ecotone is often abrupt, but it meanders between 90° W and 100° W longitude. The eastern deciduous forest occupies approximately 11% of the continent and straddles H. Walter's (1979) Zonobiomes V (warm temperate maritime) and VI (temperate with a cold winter---nemoral). In the southeastern coastal plain, coniferous and broad-leaved evergreen trees become more common. This region is discussed separately.

The northern transition from deciduous to boreal forest is thought to be a consequence of decreasing growing season length. Growing seasons must be sufficiently long to allow deciduous species to produce enough photosynthate to amortize the cost of new leaves each year (D.J. Hicks and B.F. Chabot 1985). The infertile soils typical of boreal climes probably also favor evergreen conifers relative to deciduous trees (F.S. Chapin III 1980). In general, deciduous forest gives way to grassland as water deficits increase to the west. In Minnesota, where cooler temperatures result in lower evapotranspiration, about 60 cm of rain is sufficient to support deciduous forest. In Texas and Oklahoma, however, deciduous forests require 90--100 cm of rain because of higher temperatures, greater evapotranspiration, and longer growing seasons. Local variations in hydrology and soil characteristics also affect water availability and the location of this ecotone (E.L. Braun 1950). Finally, animal grazing and fire have clearly influenced the distribution of deciduous forest relative to grassland (A.M. Greller 1988).

During the Tertiary, the progenitors of modern deciduous forest taxa were distributed across what are now boreal regions of North America (J.A. Wolfe 1978). The eastern deciduous forest is thought to have become restricted to its current range during the Quaternary. Repeated Pleistocene glaciations resulted in extensive readjustment of deciduous forest ecotones. For example, during full-glacial periods, deciduous forest species were confined to favorable sites along the Gulf region (P.A. Delcourt and H.R. Delcourt 1977, 1980). Considerable variation existed among deciduous forest species with respect to rates and routes of migration during the last glacial retreat, beginning 14,000 yr B.P. (M.B. Davis 1981, 1983). Indeed, it appears that migrations of some species are still continuing (M.B. Davis 1987).

Very little of the eastern deciduous forest has escaped human impact (J.Bakeless 1961; M. Williams 1989). Native Americans may have influenced forest structure in many areas by burning the understory vegetation to improve browse for game (S.J. Pyne 1982); the extent of such burning is a matter of debate (E.W.B. Russell 1983). With European colonization, the frequency and intensity of fires increased in many locations. Most arable land was cleared for agriculture, and wooded areas were selectively cut and used for livestock grazing (W.Cronon 1983; A.E. Cowdrey 1983).

Widespread land abandonment in many regions over the past century has created a landscape mosaic of successional fields (for a historical account see N.L. Christensen 1989). Abandoned fields are initially invaded by a predictable sequence of herbs, among which the Asteraceae and Poaceae are prominent. Patterns of tree invasion vary by region. In the northeast, successional forests are dominated by a mixture of hardwoods. In the west and middle Atlantic region, Juniperus virginiana is prominent, and in the southeast, Pinus echinata, P. taeda, and P. virginiana are widespread in old fields. These successional forests have become important economic resources in many areas.

Introduction of exotic species has had a very significant impact on the North American deciduous forest. For example, Ailanthus altissima, a native of southern China, is a widespread invader of disturbed areas. Honeysuckle (Lonicera japonica) and kudzu (Pueraria lobata) are other examples of introduced pernicious weeds (R.L. Stuckey and T.M. Barkley, chap. 8). Introduced plant diseases such as the chestnut blight and Dutch elm disease have greatly altered forest structure and composition throughout this region (E.L. Braun 1950).

Within the deciduous forest region are strong latitudinal and longitudinal climatic gradients. In the south, mean annual temperatures approach 19° C, rainfall may exceed 1400 mm, and the frost-free growing season often exceeds 280 days. In the mixed mesophytic forest region near the southern Appalachians, annual rainfall averages more than 2000 mm. Net primary production is highest in this region, exceeding 2500 g/m²/yr on fertile soils (D.L. DeAngelis et al. 1981). In general, rainfall declines to the north and west across this province. For example, at the deciduous forest--grassland boundary in Minnesota, mean annual temperature is 8°--9° C, annual precipitation is 600--700 mm, and the frost-free growing season is generally less than 150 days. On dry, infertile sites, net primary production is only 1200--1500 g/m²/yr (D.L. DeAngelis et al. 1981).

Soils vary considerably with respect to climatic gradients and the character of parent rocks. In general, the soils (Inceptisols, Spodosols, and Alfisols) derived from glacial tills and outwash in the northern portion of this region are considerably less fertile than the ancient, residual soils (mostly Ultisols) to the south (D.Steila 1976; see also D. Steila, chap. 2).

Regional Variation

The eastern deciduous forest biome can be divided into eight regions or associations as indicated in figure 5.1, based on variations in species composition and physiography (E.L. Braun 1950; A.M. Greller 1988). The mixed mesophytic forest represents the center of diversity for deciduous forest vegetation (fig.5.4). The forest canopy includes a diverse array of taxa including Fagus grandifolia, Liriodendron tulipifera, Acer saccharum, and various species of Tilia, Aesculus, Quercus, Fraxinus, and Carya. Castanea dentata was a prominent member of this community prior to the chestnut blight epidemic. These forests also possess very diverse herb and shrub floras.

S.A. Cain (1944) and E.L. Braun (1950) proposed that parts of this region served as refugia for this association during Pleistocene glaciations and that the relative lack of disturbance accounted for their high diversity. We now know, however, that most deciduous forest taxa were displaced hundreds of kilometers farther south during the last ice age (M.B. Davis 1983), and there is no evidence that they did survive in the areas where they are so richly represented now. Reliable rainfall and fertile soils may better explain the complexity of these communities (P.A. Delcourt and H.R. Delcourt 1987). Quercus and Carya spp. increase in dominance, and overall species richness declines in the western mesophytic region, probably in response to decreasing rainfall.

The oak-hickory association extends westward to the grassland ecotone (fig.5.5). In northern areas these forests may be savannalike, dominated by Quercus macrocarpa. Farther south, Quercus stellata, Q. macrocarpa, and Carya spp. dominate. J.L. Vankat (1979) combined E.L. Braun's oak-pine-hickory association with the oak-hickory. He argued that the prevalence of pines (Pinus echinata, P. taeda, and P. virginiana) in this region, which includes the southeastern Piedmont, is largely related to patterns of human-caused disturbance and old-field succession. Quercus alba, Q. rubra, Q. velutina, and Carya spp. dominate relatively undisturbed sites in this area (H.J. Oosting 1942).

Because of the chestnut blight, the composition of the oak-chestnut association has changed considerably (E.W.B. Russell 1987). Presettlement forests were dominated by Castanea dentata, Quercus prinus, Q. rubra, and Carya spp. In the understory Rhododendron spp. were often abundant. Loss of chestnut from the canopy has resulted in increased dominance of Carya and Quercus spp. (C.Keever 1953; J.F. McCormick and R.B. Platt 1980; S.L. Stephenson 1986).

The beech-maple association predominates on glaciated terrain in Ohio, western Indiana, and southern Michigan. Fagus grandifolia and Acer saccharum are canopy dominants, often growing with lesser numbers of Fraxinus and Ulmus spp. Fagus grandifolia is replaced by Tilia americana west of Lake Michigan in the maple-basswood association. Throughout this region Quercus and Carya spp. are important on drier sites, while Fraxinus, Ulmus, and Acer rubrum are important on the wettest soils (A.M. Greller 1988).

The northern hardwoods association is transitional to boreal forest. This association consists of a mosaic of community types largely distributed in relation to soil characteristics and patterns of disturbance. On the most fertile soils in the absence of fire or other disturbance, mixed conifer--hardwood forest, dominated by Acer saccharum, Fagus grandifolia, Tilia americana, and Tsuga canadensis, occur. On sandy, less fertile soils Pinus strobus and P. resinosa are canopy dominants, and on the driest, least fertile soils Pinus banksiana forms dense, even-aged stands. This latter community type is maintained by frequent, often intense wildfires (H.L. Hansen et al. 1974). See R.S. Rogers (1980, 1981) for a description of the understory vegetation of these forests. Toward the northern portion of this transition, boreal species such as Picea glauca (in the west) and Picea rubens (in the east) become more important. Picea mariana and Larix laricina dominate boggy areas, which may be abundant on this landscape. Widespread cutting and burning in many areas have resulted in extensive stands of successional taxa such as Betula, Populus tremuloides, P. grandidentata, and Prunus pensylvanica.

High Elevation Appalachian Ecosystems

Overview

Higher elevations of the Appalachian Mountains support a unique assemblage of subalpine and alpine vegetation types distributed in an ecological "island archipelago" from New England to southwestern North Carolina. The upper elevational limit for deciduous forest is approximately 760m in the Green Mountains of Vermont and White Mountains of New Hampshire. The transition to subalpine vegetation rises to 1280 m in the Catskill Mountains of New York. To the south the highest elevations of the Appalachian complex are below the subalpine transition until one reaches the Allegheny Mountains of West Virginia. Farther south in Tennessee and North Carolina, the Blue Ridge and Unaka mountains (including the Great Smoky Mountains) rise well above the 1400--1500 m elevational limit of deciduous forest.

Spruce-fir forests

As one nears the elevational limit of deciduous forest, red spruce (Picea rubens) increases in importance and forms dense, relatively even-aged stands above this transition. At higher elevations Abies increases in importance and may form pure stands if elevations are sufficient. Other commonly associated species include Betula allegheniensis, B. lenta, B. papyrifera (north), Sorbus americana, Vaccinium spp., and Viburnum spp. Within each region a variety of subtypes have been identified based on subordinate vegetation (D.L. Crandall 1958; E.L. Core 1966). H.J. Oosting and W.D. Billings (1951) argued that, despite the north-to-south elevational shift in the spruce-fir zone, the climatic conditions were generally similar, i.e., generally harsh winters and short frost-free growing seasons (less than 120 days). They noted, however, that temperature extremes and snowfall were less, and humidity and growing season precipitation were greater, in the southern Appalachians than in the mountains of New England. Soils in all areas are usually relatively infertile spodosols, but organic horizons are generally thicker at northern sites (H.J. Oosting and W.D. Billings 1951).

Clear floristic differences exist between spruce-fir forests in the southern Appalachians and those from West Virginia northward. In the south the fir species is the endemic Abies fraseri, whereas Abies balsamea dominates to the north. H.J. Oosting and W.D. Billings (1951) identified a number of bryophyte, herb, and shrub taxa that were also unique to each of these areas. Palynological evidence suggests that many species now unique to northern areas were present in the southern Appalachians during the last ice age. Thus these floristic variations probably reflect climatic conditions unique to each of these regions rather than evolutionary change due solely to geographic isolation.

Natural disturbance is important in Appalachian spruce-fir landscapes. Fungal pathogens and insects such as the spruce budworm cause high tree mortality in some places and account for the relative abundance of successional species such as Betula spp., Prunus pensylvanica, and Sorbus americana. At higher elevations wind and ice damage may create "waves" of tree mortality, producing a mosaic of different age trees (D.G. Sprugel 1976).

Human impacts have greatly altered the structure of Appalachian spruce-fir forests. Logging and human-set fires resulted in the complete loss of many southern forests early in this century (C.F. Korstian 1937). More recently, introduction of an exotic insect, the balsam woolly aedelgid, has resulted in rapid decline of Abies fraseri throughout its range. It is generally agreed that throughout the Appalachians, spruce-fir ecosystems have been hard hit by acid rain and other atmospheric pollutants (National Research Council 1989).

Alpine Tundra

True timberlines, i.e., elevations above which trees cannot establish and grow, occur only on the summits of the White Mountains of New Hampshire. Although the elevation is only 1900 m, the environment here is among the harshest of any alpine habitat (W.D. Billings 1988). These ecosystems have a high (70%) floristic affinity to arctic ecosystems even though they are isolated from the Arctic by over 1000km (L.C. Bliss 1963; W.D. Billings 1988).

In the most exposed snow-free areas Diapensia lapponica dominates with Juncus trifidis. Sedge meadows dominated by Carex bigelowii are common in snowy and foggy locations. Other important species include Arenaria groenlandica, Ledum groenlandicum, Vaccinium vitis-idaea, and V. uliginosum.

Grass and Heath Balds

Grass and heath balds represent the only treeless upland vegetation types in the southern Appalachians. Unique soil and historical factors appear to be much more important than elevation in the distribution of these ecosystems.

Grass balds are most common on mountaintops and ridges between 1700 and 1800m (D.M. Brown 1941; A.F. Mark 1958). Lower elevation grass-dominated areas were likely created and maintained by human disturbance in the past two centuries (P.J. Gersmehl 1971). Dominant plants in high elevation balds include Danthonia compressa, Potentilla canadensis, and Rumex acetosella. Mosses such as Polytrichum commune and P. juniperinum are especially common in some locations (A.F. Mark 1959).

A.F. Mark (1958) argued that high elevation balds were created simultaneously as spruce-fir forests were displaced from mountaintops by climatic warming 8000--5000 years ago. Subsequent cooling resulted in retreat of deciduous forest species. Poor seed dispersal and competition from the already established herbs, however, have prevented reinvasion by spruce and fir. Shallow soils, fire, and animal grazing may also play roles in exclusion of spruce and fir.

Heath balds are closed-canopy shrub communities dominated by Kalmia latifolia, Leiophyllum lyonii, Rhododendron catawbiense, and R. maximum (R.H. Whittaker 1979). They are characteristic of ridges with shallow soils between 1400 and 2000 m elevation. S.A. Cain (1930) suggested that such shrublands were produced and maintained by disturbances such as fire or landslide. Whittaker argued that they were "topographic climaxes" dominated by plants adapted to shallow infertile soils on steep slopes.

Southeastern Coastal Plain Ecosystems

Overview

The southeastern coastal plain is composed of alluvial and marine sediments deposited since Mesozoic times and considerably reworked during the Pleistocene by fluvial and coastal geomorphic processes (G.E. Murray 1961). It covers 3% of the North American landscape. This province is often classified as the southern mixed hardwood region of the deciduous forest, based on the assumption that successional change in the absence of disturbance such as fire would result in a climax forest dominated by a mixture of deciduous and evergreen angiosperms and evergreen gymnosperms (E.L. Braun 1950; E.Quarterman and C.Keever 1962). N.L. Christensen (1988) argued that the unique soils, topography, climate, and disturbance regime of the coastal plain make succession to such vegetation unlikely in many locations. Instead, upland sites are normally vegetated by pine-dominated forests, and low sites support a diverse array of paludal and alluvial wetlands (figs.5.6, 5.7).

Vegetation composition of the coastal plain varied considerably during the Pleistocene. During full-glacial periods the drop in sea level exposed vast areas of the Continental Shelf for colonization by terrestrial plants. Peninsular Florida, for example, was 600 km wide at the peak of the last ice age. During several of the past interglacial periods, sea level may have been 85 m higher than at present (G.E. Murray 1961; T.M. Cronin et al. 1981).

The advent of Native American people on the coastal plain appears to coincide with the close of the last ice age (C.Hudson 1976). Intensive land use by Indians was greatest on river flood plains, near the coast, and around swamp complexes. Indian-set fires probably had a major impact on the entire landscape (A.E. Cowdrey 1983; N.L. Christensen 1988).

European colonization of continental North America began in the coastal plain. Flood plains and many upland areas were cleared for agriculture. The abundant pine forests spawned a timber and naval stores industry that continues to the present (A.E. Cowdrey 1983). Like their Native American predecessors, European colonists were quick to understand the need for frequent fires to maintain many coastal plain communities (S.J. Pyne 1982).

Upland Pine Forests

We divide the upland pine forests into three general types: northern pine barrens, xeric sand communities, and mesic pine communities. Northern pine barrens are confined to the coastal plain north of Delaware Bay. Pine-oak forests dominated by Pinus rigida, Quercus stellata, and Q. marilandica dominate the most favorable sites. The relative importance of oak species increases as fire frequency decreases (R.T.T. Forman and R.E. Boerner 1981). On coarse-textured soils and with increasing fire frequency, tall-stature (greater than 10 m) Pinus rigida trees form a closed canopy with an understory of Gaylussacia baccata, Quercus spp., and Vaccinium spp. Frequent fires maintain the dwarf pine plains, dominated by shrubby ecotypes of Pinus rigida, Quercus ilicifolia, and Q. marilandica. These communities have floristic affinities with the southern Appalachians (e.g., Comptonia peregrina, Kalmia latifolia, and Quercus ilicifolia) as well as the southeastern coastal plain (e.g., Clethra alnifolia, Gaylussacia dumosa, G. frondosa, and Ilex glabra).

Well-drained coarse sands provide xeric habitats on much of the southern coastal plain despite abundant rainfall. The xeric sand communities that occupy such sites include sandhill pine forest and sand pine scrub. Sandhill pine forest occurs throughout this region and is dominated by a broken overstory of Pinus palustris. Understory trees include Diospyros virginiana, Nyssa sylvatica var. sylvatica, and Quercus laevis. Aristida stricta often dominates the herb layer, although on the most sterile sands, lichens and mosses provide the greatest ground cover. Frequent surface fires (every 3--6 years) maintain the open structure of these forests and are essential to the reproduction of Pinus palustris (N.L. Christensen 1988). In peninsular Florida where fires occur at less frequent intervals (every 30--50 years), sand pine scrub is the predominant vegetation. Here, Pinus clausa is the dominant canopy species, and a diverse array of shrubs and low palms dominate the understory. In the absence of fire, these forests slowly succeed to xeric oak woods, or "hammocks," dominated by Quercus virginiana (R.L. Myers 1985).

Mesic pine communities include so-called "flatwoods" and "savannas." Flatwoods are generally dominated by an even-aged closed canopy of Pinus palustris, P. taeda, P. serotina, and/or P. elliottii (fig.5.6). The relative frequency of these pine species is dependent on variations in site fertility, drainage, and fire history (C.D. Monk 1968). Understory vegetation in these forests often includes abundant shrub species such as Ilex glabra, Myrica spp., and Serenoa repens. Pine savannas occur most frequently at transitions between xeric pine communities and wetlands. Depending on hydrologic conditions and fire frequency, these savannas are characterized by a very open canopy of Pinus palustris, P. taeda, or P. serotina. The understory herb layer includes a very diverse assemblage of herbs and forbs, all of which are adapted to frequent surface fires. Abundant insectivorous plants (species of Sarracenia, Pinguicula, Drosera, and Dionaea muscipula) testify to the low availability of nutrients in savanna soils (N.L. Christensen 1988).

Upland Hardwood Forests

In the absence of fire, broadleaf forests do indeed dominate some coastal plain sites. On sandy soils in Florida and across the Gulf, oak hammocks with an overstory of Quercus virginiana occur. These communities often have a shrubby understory that includes Serenoa repens, Ilex, Quercus, and Lyonia. On more mesic sites, particularly on fertile soils derived from limestone or fine-textured sediments, forests dominated by a mixture of evergreen and deciduous trees are prevalent. In Florida and along the Gulf, Magnolia grandiflora and Fagus grandifolia are prominent overstory trees, along with Quercus spp., Liquidambar styraciflua, and Pinus taeda. In the Carolinas such forests are dominated by Carya spp., Fagus grandifolia, Quercus alba, and Q. laurifolia (E.Quarterman and C.Keever 1962).

Alluvial Wetlands

Subdued topography and complex drainage patterns result in an abundance of wetlands on coastal plains landscapes. Wetlands occupy over 10⁷ ha, or 15% of the total land surface of the southeastern coastal plain, and roughly half of these ecosystems are alluvial or associated with streams and rivers. Vegetation varies in relation to frequency of flooding, which affects such ecologically important factors as soil aeration and water and nutrient availability. R.T. Huffman and S.W. Forsythe (1981) proposed a zonal classification of alluvial wetland vegetation based on inundation frequency.

Zone I plant communities occupy permanent water courses and impounded areas. Where water flow is rapid, submerged aquatic plants with streamlined leaves, such as Alternanthera philoxeroides, are common. Where flow is sluggish, emergent broad-leaved taxa such as Pontederia and Sagittaria spp. and floating aquatics such as Azolla, Lemna, and Spirodela are common.

Zone II, river swamp forests, occupy river flood plains that are inundated for much of the year but may be exposed during the growing season. Taxodium distichumvar. distichum is the most typical tree in this zone. Taxodium distichum var. imbricarium (= T. ascendens) is more common on sandy substrates and in impounded areas such as oxbow lakes (fig.5.7). Chamaecyparis thyoides and various species of Nyssa codominate in many locations. In less frequently inundated portions of this zone, Acer rubrum, Liquidambar styraciflua, Quercus laurifolia, and Ulmus americana are important.

Zone III, or lower hardwood swamp forest, soils are saturated 40--50% of the year. Plants in this zone must tolerate inundation in the early part of the growing season and water deficits in the late summer. Nutrient subsidization from alluvial sediments contributes to the high fertility and diversity of these forests. Dominant trees include Quercus lyrata and Carya aquatica. Cornus foemina, Gleditsia aquatica, Ilex verticillata, and Itea virginica are common understory associates.

Zone IV forests occur in backwater and flat areas that are inundated only 20--30% of the year, and very infrequently during the growing season. Quercus laurifolia, Q. phellos, Fraxinus pennsylvanica, and Ulmus americana typify canopy trees in this zone; Carpinus caroliniana, Ilex decidua, Crataegus spp., and Sabal minor are common understory associates.

Zone V, transition to upland, comprises the highest locations of the active flood plain and includes levees, higher terraces and flats, and sand ridges and dunes. Soils here are generally saturated less than 15% of the year. Dominant trees include Quercus michauxii, Q. falcata, and Q. nigra on fine-textured soils, and Quercus virginiana on sandy sites. Understory trees and shrubs include Ilex opaca, Asimina triloba, Lindera benzoin, Serenoa repens, and Sabal palmetto.

Paludal Wetlands

Because input of water to paludal (nonalluvial) wetlands is primarily from rainfall or groundwater flow, plant production is often nutrient limited. Graminoid-dominated wetlands or wet "prairies" are maintained by frequent disturbance such as fire. Dominant genera in these habitats include Carex, Cladium, Juncus, Muhlenbergia, Panicum, Rhynchospora, and Scirpus.

Poor drainage in flat interstream areas results in peat accumulation and bog development over thousands of hectares. Pocosins, or shrub bog vegetation, are characteristic of the centers of such bogs (N.L. Christensen et al. 1981; R.R. Sharitz and J.W. Gibbons 1982). These peatlands typically have a dense impenetrable cover of deciduous and evergreen shrubs with scattered emergent Pinus serotina. Shrub diversity may be quite high (20 species/ha) with Cyrilla racemiflora, Ilex glabra, Lyonia lucida, and Zenobia pulverulenta being most abundant. In the most nutrient-limited areas, shrub height may be less than 1 m. Toward the bog margins on thinner peats, nutrient availability and tree and shrub height increase, with little change in species composition. Fires occur in these ecosystems at intervals of 30--50 years.

Bay forests are common at the margins of such bog complexes and on shallow peats. They also occur in shallow depressions such as lime sinks. These forests are dominated by broad-leaved evergreen trees such as Gordonia lasianthus, Magnolia virginiana, and Persea borbonia. Understory species include many of the shrubs found in pocosins. Atlantic white cedar swamp forests, dominated by Chamaecyparis thyoides, occur in similar habitats but are maintained by fires at 80--100 year intervals.

Cypress heads (also called "domes" or wet hammocks) occur in poorly drained depressions throughout the coastal plain. Taxodium distichum var. imbricarium (= T.ascendens) often forms a monotypic, relatively closed canopy. These areas are generally inundated more frequently than depressions that support bay forest. Compared to Zone I swamp forest, these areas are profoundly nutrient limited. S.Brown (1981) found that annual inputs of phosphorus to cypress heads were only one ten-thousandth that of floodplain cypress swamps.

Tropical Hardwood Hammocks

Within the contiguous United States, only at the very southern tip of Florida does the vegetation have a tropical aspect. The climate in this area corresponds to H.Walter's (1979) Zonobiome II, tropical with summer rains (A.M. Greller 1980). The vast majority of this area is vegetated by wetlands of various kinds, or upland pine forest. Tropical hardwood hammocks, however, occur where soils are well drained and fire is infrequent (R.M. Harper 1911; F.C. Craighead Sr. 1971; D.Wade et al. 1980).

Tropical hardwood hammocks vary in size from less than 0.2 ha to over 50 ha. Although abundant at one time, only a few hundred of these "islands" have survived human land development.

Broad-leaved evergreen taxa with definite tropical affinities include Bursera simaruba, Ficus spp., Metopium toxiferum, and Swietenia mahogani. Temperate species that are important in other coastal plain communities, such as Quercus virginiana, often occur as scattered individuals in these forests. Lianas and vines, as well as epiphytes in the Bromeliaceae and Orchidaceae, give these forests a particularly tropical appearance (F.C. Craighead Sr. 1971).

Grasslands

Overview

Grassland vegetation is dominated by herbaceous plants that form 1--2 canopy layers (fig. 5.8). Woody plants are restricted to local areas of distinctive topography, soil, or protection from fire. "Prairie" denotes relatively humid grasslands of nearly continuous cover, dominated by tallgrass species. "Plains," "high plains," or "steppe" denote more arid, open vegetation dominated by shortgrasses. The grasses are largely perennial, and they may be either rhizomatous sod-formers or nonrhizomatous bunchgrasses.

Grasslands occur in H.Walter's (1979) Zonobiome VII: regions of temperate, arid macroclimates too xeric for closed forest and too mesic and cold for desert scrub. Specifically excluded from this section are meadows within forested areas, tundra, freshwater marsh, tidal saltwater marsh, and tropical savannas.

Poaceae (along with Cyperaceae) dominate the biomass, but forbs (in particular members of the Asteraceae and Fabaceae) account for most of the species. P.G. Risser (1985) estimated that North American grasslands contain 7500 species. Ecotype differentiation among wide-ranging taxa is a common theme (C.McMillan 1959; J.A. Quinn and R.T. Ward 1969). Species richness increases with topographic diversity, precipitation, and distance from human disturbance in the past century (R.T. Coupland 1979). Few taxa are endemic to grasslands, however, and D.I. Axelrod (1985) used this fact to support his contention that grasslands are geologically recent, dating back only 7--5 million years. In addition, large areas of grassland were displaced during Pleistocene glacial advances. J.A. Young et al. (1976) have suggested that the intermountain grassland is especially fragile and easily modified by human disturbance because of its evolutionary youth.

In pristine times, grassland may have covered as much as 25% of the landmass north of Mexico (P.L. Sims 1988), but our own estimate is 21%. It is the second-largest formation in North America. Human activities, however, have reduced the extent of natural grasslands and modified their composition. Modifications resulted from removal of native grazing animals, introduction of domesticated livestock, clearing of land for agriculture, modification of the natural fire regime, and the purposeful or accidental introduction of aggressive weeds from other continents. All six major grasslands of North America were originally dominated by perennial grasses and forbs, but human activities have led to an increase in cover by such other growth forms as annuals, subshrubs, shrubs, and trees.

Although each regional grassland is floristically rich, a given local patch of grassland is dominated by only 4--5 grass species (P.L. Sims 1988). Important growth forms include: (1) perennial sod-forming (rhizomatous or stoloniferous) species; (2) nonrhizomatous or short-rhizomatous perennial bunchgrass species; (3) cool-season C₃ species active in fall, winter, and spring; (4) warm-season C₄ species active in summer; and (5) annuals (J.R. Carpenter 1940; P.G. Risser et al. 1981; P.L. Sims et al. 1978). These growth forms are not mutually exclusive: annuals and perennials can be either cool-season or warm-season types, and certain species can be rhizomatous in some habitats but nonrhizomatous in others.

Bunchgrasses tend to dominate the desert, intermountain, and California regions. Cool-season grasses tend to dominate north of 45° N, in regions with a July minimum cooler than 8° C (P.G. Risser 1985; J.A. Teeri and L.G. Stowe 1976), and also historically in California. The several growth forms appear to exhibit metabolic, phenologic, and morphologic behaviors that adapt them to such stresses as drought, high temperatures, nutrient limitations, grazing, and fire (P.G. Risser 1985). Bunchgrasses exhibit less resistance to intense grazing than do sod-forming grasses, and R.N. Mack and J.N. Thompson (1982) hypothesized that they evolved with lower grazing pressure than did sod-formers.

C.O. Saure (1958), among others, argued that grasslands are so diverse and so often owe their existence to edaphic or fire-related circumstances that there is no "grassland climate." In North America, grasslands occur where annual precipitation is 250--625 mm, a marked dry season of several weeks to several months in length occurs, the P/E ratio is 0.2--1.0, the annual moisture deficit is greater than 300 mm (N.L. Stephenson 1990), the growing season is accompanied by high temperatures and wind, and great variability in weather exists from year to year (P.G. Risser 1985; P.L. Sims et al. 1978). Soils tend to be heavy and deep. Natural fires from dry lightning strikes can be expected with a periodicity of 5--10+ years. Beyond this, climates do vary widely, from continental, to Mediterranean, to dry subtropical (P.L. Sims 1988). Annual net productivity, as a consequence, varies from a high of 500--1000 g/m² in the tallgrass prairie to 50 g/m² in the most arid regions.

Regional Grasslands

The central grasslands consist of three major grassland types. The tallgrass prairie, also known as the true prairie (J.E. Weaver and T.J. Fitzpatrick 1934), extends from southern Manitoba to Texas (D.D. Diamond and F.E. Smeins 1985). It is dominated by Andropogon gerardii, Schizachyrium scoparium, and Sorghastrum nutans. Common associates, often more locally restricted, include Panicum virgatum, Sporobolus asper, S. heterolepis, and Stipa spartea. These grasses are a mixture of sod-formers and bunchgrasses, with foliage 40--100+ cm tall and inflorescences up to 200+ cm tall.

Most of the tallgrass prairie is now in cultivation. Along its eastern edge, at approximately 95° W longitude, the true prairie forms a broad ecotone with two major forest types. In the north, from Saskatchewan and Manitoba to Minnesota, the ecotone with the boreal forest is an aspen parkland 75--175 km wide. Populus tremuloides and P. balsamifera are favored over grasses, apparently because of the decreasing amount of precipitation in the spring months of April through June, which has a negative effect on the reproduction of dominant grasses (J.Looman 1983).

Farther south the ecotone with the deciduous forest is an oak savanna 50--100 km wide. This ecotone also projects for a total of some 250,000 km² east across Illinois, to Indiana, Ohio, and Kentucky, in what has been called the prairie peninsula (E.N. Transeau 1935). Common trees in the savanna include Acer negundo, Carya texana, Quercus macrocarpa, Q. marilandica, Q. stellata, Q. velutina, and Ulmus americana (J.E. Weaver and T.J. Fitzpatrick 1934; R.C. Anderson and L.E. Brown 1986). The factors that historically have been thought to preclude tree invasion along this ecotone include high fire frequency, absence of ectomycorrhizal fungi in prairie soil (R.W. Goss 1960; D.White 1941), and drought episodes.

Consecutively farther west of the tallgrass prairie are the mixedgrass and shortgrass steppes or high plains. The mixedgrass prairie (fig. 5.8) is floristically the most complex of the central grasslands, because it is an ecotone/tension zone between the tallgrass and shortgrass prairies, thus containing taxa characteristic of both (J.S. Singh et al. 1983). As annual climate becomes relatively humid or arid, the species of tallgrass or shortgrass prairies are alternately favored (A.W. Küchler 1972). Dominants of the shortgrass prairie include Agropyron smithii, Bouteloua gracilis, Buchloë dactyloides, Hilaria jamesii, Koeleria cristata, Muhlenbergia torreyi, Sporobolus cryptandus, and Stipa comata. Warm-season grasses predominate.

The shortgrass prairie extends southward like a finger into Mexico, mainly at 1100--2500 m elevation along the flank of the Sierra Madre, from the state of Chihuahua at the United States/Mexico border to 22° N latitude in the state of Jalisco (J.Rzedowski 1978). The grasses are 20--70 cm tall. Generally, less than 50% of the ground is covered by vegetation. Dominants include Bouteloua curtipendula, B. gracilis, and B. hirsuta. Overall, the flora has a pronounced Mexican cast that includes endemic species in Andropogon, Aristida, Erioneuron, Heteropogon, Hilaria, and Lycurus. Human disturbance has led to invasion of Acacia schaffneri,Juniperus monosperma, Opuntia spp., Prosopis velutina, Quercus chihuahuensis, Q. cordifolia, Q. emoryi, and Yucca decipiens in parts of this central Mexican zacatal (J.Rzedowski 1978; see also S.Archer et al. [1988] for a model of Prosopis glandulosa invasion into southern Texas grassland).

The warm desert grassland covers plateaus greater than 1000 m elevation at the edge of the Sonoran and Chihuahuan deserts in western Texas, southern New Mexico, southeastern Arizona, and northeastern Sonora. An outlier is a shrub steppe in southeastern Utah (N.E. West 1988). In pristine times the desert grassland was dominated by short bunchgrasses such as Bouteloua eriopoda, Hilaria belangeri, H. jamesii, H. mutica, Muhlenbergia porteri, and Oryzopsis hymenoides. Within the past century, desert shrubs (mainly Larrea divaricata subsp. tridentata, Flourensia cernua, Prosopis glandulosa, and P. velutina) have expanded in cover, sometimes twentyfold, and have become dominants (L.C. Buffington and C.H. Herbel 1965; R.R. Humphrey and L.A. Mehrhoff 1958; J.Rzedowski 1978).

The intermountain grassland (often called a steppe or shrub steppe) extends from western Wyoming through northwestern Utah, southern Idaho, northern Nevada, and northeastern California, and into the Columbia Basin of eastern Oregon and the Palouse area of southeastern Washington. An outlier is the prairie of the Willamette Valley of Oregon and of coastal northern California, which was dominated by Danthonia californica in pristine times (J.F. Franklin and C.T. Dyrness 1973; H.F. Heady et al. 1988). Artemisia shrubs (A. arbuscula, A. rigida, A. tridentata) are typically associated with the cool-season bunchgrasses Aristida longiseta, Elymus lanceolatus (= Agropyron dasystachyum), E. cinereus, Festuca idahoensis, Koeleria cristata, Pascopyrum smithii (= Agropyron smithii), Poa fendleri, P. nevadensis, P. sandbergii, Pseudoroegneria spicata (= Agropyron spicatum), Sitanion hystrix, Sporobolus airoides, and Stipa comata (A.Cronquist et al. 1972+, vol.1; J.F. Franklin and C.T. Dyrness 1973; P.L. Sims 1988; E.W. Tisdale 1986; N.E. West 1988). This grassland has been severely affected by overgrazing, burning, and the introduction of aggressive annual forbs and grasses, especially Bromus tectorum.

Finally the central California grassland uniquely evolved in a Mediterranean-type climate, and consequently many of its pristine dominants were endemic. This grassland has been completely modified in the past 150 years, and few relict areas remain. From limited information, W.J. Barry (1972) and H.F. Heady (1988) estimated that the original dominants at elevations below 100 m were the cool-season bunchgrasses Stipa pulchra and S. cernua. On gentle slopes just above valley floors, Stipa was joined by Elymus glaucus, Festuca californica, Melica californica, M. torreyana, Muhlenbergia rigens, and Sitanion hystrix. Bunchgrasses contributed 50% cover, with native annual grasses and annual and perennial forbs seasonally filling in the spaces between bunches. The grassland covered 13--14% of California's area, occupying much of the Sacramento Valley, the northern half of the San Joaquin Valley, and coastal valleys from Monterey to San Diego. Today, most of the land originally dominated by bunchgrass vegetation has been converted either to agriculture and urban use, or to an annual grassland dominated by Eurasian species of Avena, Bromus, Hordeum, and Lolium (R.M. Love 1975).

Desert Scrub

Overview

Deserts occupy 5% of the North American landmass north of Mexico. They occur in the southwestern part of North America at elevations below 1700 m, where annual precipitation is less than 400 mm and highly variable from year to year. On a global scale, "true deserts" are areas receiving less precipitation than 120 mm/yr (A.Shmida 1985). North American deserts receive more precipitation and qualify only as "semiarid." They are dominated by desert scrub vegetation consisting of evergreen or drought-deciduous shrubs and subshrubs that provide an interrupted cover (typically 5--25%) and have low annual net productivity (less than 100 g/m² [S.R. Szarek 1979]; figs.5.9, 5.10). Associated growth forms are diverse and include phreatophytic trees (roots penetrate to ground water), stem succulents, bunchgrasses, ephemerals, and the ecophysiological C₃/C₄/CAM photosynthetic syndromes (F.S. and C.D. Crosswhite 1984; P.R. Kemp 1983; F.Shreve 1942).

Desert scrub in the intermountain region (fig.5.11) has developed under a regime of the majority of annual precipitation falling as snow, which is unique in North American deserts. This intermountain (cold) desert falls within H.Walter's (1979) Zonobiome VII, with the temperate grasslands, whereas the warm deserts fall within his Zonobiome III, subtropical arid. Annual water budget deficits are 600--900 mm in the cold desert, but 2000--3000mm in the warm deserts (N.L. Stephenson 1990). In the arid American West, seasonality of precipitation changes from a summer peak in the east to a winter peak in the west. Winter and summer rainfall events differ in intensity and associated temperatures. Consequently, it is not surprising that annual and perennial flora and vegetation vary with longitude (J.A. MacMahon 1979, 1988; J.A. MacMahon and F.H. Wagner 1985; F.Shreve 1942).

The easternmost and largest desert is the Chihuahuan. It has a total area of 450,000 km², is situated at an elevation of 1100--1500 m, and receives a mean annual precipitation of 150--400 mm, 60--80% of which falls in summer. Its soils are often rich in limestone or gypsum and often have endemic plants (J.A. MacMahon 1988). This desert includes western Texas, southern New Mexico, and portions of Chihuahua, Coahuila, Durango, Zacatecas, and San Luis Potosí, south to 22° N latitude (J.A. MacMahon and F.H. Wagner 1985; P.S. Nobel 1985).

West of the Chihuahuan Desert is the Sonoran Desert (fig. 5.9). Its low elevation (below 600 m) and southern position make it the warmest of the North American deserts. Fewer than 1% of annual hours experience temperatures lower than 0° C (M.G. Barbour, J. H. Burk, and W. D. Pitts 1987). Annual precipitation is 50--300 mm, more or less evenly divided between winter and summer. This desert includes southwestern Arizona, southeastern California, most of Baja California, and northwestern Sonora, for a total area of 300,000 km² (J.A. MacMahon and F.H. Wagner 1985; P.S. Nobel 1985). It has a rich and bizarre array of growth forms. The Sonoran and Chihuahuan deserts nearly meet in eastern Arizona, being separated by only 250 km of desert grassland. The two deserts exhibit high floristic similarity; more than 50% of their genera are shared (J.A. MacMahon and F.H. Wagner 1985). Overgrazing, climatic change, and fire suppression have allowed the vegetation of both deserts to expand into adjacent grassland.

North of the Sonoran Desert is the Mojave, smallest of the four deserts (140,000 km² [J.A. MacMahon 1988]; fig.5.10). It includes portions of southern California, southern Nevada, southwestern Utah, and northwestern Arizona. Its position and varied topography (from below sea level to 1200 m elevation) make it a "bridge" between the warm deserts and the cold intermountain desert. Annual precipitation is 40--275 mm, most falling in winter, some as snow. It is the driest of North American deserts and has vegetation with the least cover. Frost occurs 2--5% of yearly hours (M.G. Barbour, J.H. Burk, and W.D. Pitts 1987). Despite the Mojave's ecotonal position, 25% of its total flora is endemic. Even more striking, 80% of its annual flora is endemic (J.A. MacMahon 1988).

The intermountain, or Great Basin, desert extends eastward from central Nevada through central Utah and southwestern Colorado to northern New Mexico and Arizona (fig.5.11). Elevations are 1200--1600m. Precipitation (100--300 mm/yr) has a winter peak, and most falls as snow (M.M. Caldwell 1985). Frost occurs in 5--20% of yearly hours (M.G. Barbour, J.H. Burk, and W.D. Pitts 1987). Succulent species are uncommon. The few Opuntia spp. that occur in the Great Basin are obviously frost-tolerant; P.S. Nobel (1985) has documented their degree of frost-tolerance. The portion of this arid area that extends north of 42° N latitude is more properly defined as a shrub steppe (N.E. West 1988); consequently we have described it in the grassland section as "intermountain grassland."

Two major landforms in all four deserts are bajadas (gentle alluvial slopes with coarse soil) and playas (basins with no external drainage). Bajadas support the regional, zonal community type, the biomass of which is typically dominated by C₃ and CAM (Crassulacean Acid Metabolism) syndromes (J.P. Syvertson et al. 1976). Playas support azonal halophytic plant communities, rich in C₄ taxa (J.P. Syvertson et al. 1976). Bajada and playa communities often exhibit sharp boundaries not completely explained by gradients of soil chemistry and structure (D.H. Gates et al. 1956; J.E. Mitchell et al. 1966). Other desert habitats include arroyos (washes or small canyons created by intermittent streams), which support riparian vegetation, sand dunes (accumulated from the dry beds of Pleistocene lakes), which support a highly endemic flora (J.E. Bowers 1984), and oases, which in the Sonoran Desert support striking groves of Washingtonia filifera (R.J. Vogl and L.T. McHargue 1966).

Some desert taxa are geologically old, and it appears that desert environments have existed since the Paleozoic (A.Shmida 1985). The prevailing opinion, however, is that modern North America deserts---and their floras---are geologically recent (D.I. Axelrod 1950, 1979).

Regional Deserts

Chihuahuan Desert vegetation can be classified into half a dozen major communities. The most common one is dominated by Larrea divaricata subsp. tridentata, associated with Acacia neovernicosa, Agave lecheguilla, Dalea formosa, Ephedra spp., Flourensia cernua, Fouquieria splendens, Jatropha dioica, Koeberlinia spinosa, Krameria spp., Opuntia phaeacantha, and a rich collection of Yucca spp. (J.A. MacMahon 1988). Density of individual perennial plants may often exceed 4000/ha, and their cover may often exceed 20%. Leaf succulents (Agave, Dasylirion, Yucca) dominate the visual aspect of Chihuahuan vegetation. The southern extension of the Chihuahuan Desert in Mexico deserves more attention by researchers (J.Rzedowski 1978).

Sonoran Desert vegetation is characterized by an arboreal element (fig.5.9). The diversity of growth forms reaches its peak in Arizona uplands and central Baja California. Overstory elements include columnar cacti (e.g., Carnegiea gigantea and Pachycereus pringlei), or trees such as Cercidium floridum, C. microphyllum, C.praecox, Olneya tesota, Pachycormus discolor, and Yucca valida, the inflorescences of Agave deserti and A. shawii, and the bizarre boojum, Fouquieria columnaris(R.R. Humphrey 1974). Tall shrubs include Acacia greggii, Fouquieria splendens, Krameria grayi, K. parvifolia, Larrea divaricata subsp. tridentata, Lycium andersonii, and Simmondsia chinensis. Subshrubs and small cacti include Encelia farinosa and several species of Ambrosia, Jatropha, and Opuntia. The composition of the herbaceous element depends on the season. Winter annuals flower in April--May, summer annuals flower in August--September. Few species are capable of doing both. Summer and winter annuals differ in morphology, photosynthetic pathway, phenology, and length of life (J.H. Burk 1988; J.R. Ehleringer 1985; T.W. Mulroy and P.W. Rundel 1977). Total plant cover for all four canopy layers can reach 25--50%.

The Sonoran Desert is rich in Cactaceae. Ecological literature is extensive for saguaro, Carnegiea gigantea, which visually characterizes the Sonoran landscape. It exhibits the classic CAM syndrome of traits---shallow roots, slow growth rate, high water-use efficiency, and capability of tolerating relatively high tissue temperatures (greater than 60° C in some cases [P.S. Nobel 1985]). In addition, the saguaro has a life span greater than 200 years. Its young plants are dependent on "nurse plants" for microenvironmental protection. Present population structure suggests senescence, perhaps because of grazing pressure by rodents or larger vertebrates (W.A. Niering et al. 1963, but see J.Vandermeer 1980).

In contrast to the Chihuahuan and Sonoran deserts, the Mojave has a low, 1--2 canopy layer and a rather monotonous aspect (fig. 5.10). Perennial plant density is typically less than 2000 individuals/ha, and cover is less than 10%. The bajadas are thoroughly dominated by Larrea divaricata subsp. tridentata, associated with Ambrosia dumosa, Grayia spinosa, Lycium spp., Krameria parvifolia, and Yucca schidigera (N.H. Holmgren 1972; F.C. Vasek and M.G. Barbour 1988). The Joshua-tree, Y. brevifolia, is more characteristic of the upper elevation limits of the desert than it is of the regional bajadas. Columnar succulents are absent, but smaller conspicuous cacti include Echinocactus polycephalus, Ferocactus cylindraceus (= F. acanthodes), and Opuntia spp. In southern Nevada, the warm desert and cold desert plant communities meet and are differentiated sharply along subtle environmental gradients (J.C. Beatley 1975; W.H. Rickard and J.C. Beatley 1965).

Mojave Desert vegetation is remarkably sensitive to soil compaction (R.C. Stebbins 1974). Several studies have documented the process and timing of secondary succession following disturbance by off-road vehicles, military maneuvers, pipeline burial, powerline construction, and even agricultural clearing (D.E. Carpenter et al. 1986). Succession leads through a seral stage of short-lived subshrubs (species of Acamptopappus, Gutierrezia, Haplopappus, Hymenoclea, Leucelene, Salazaria, Sphaeralcea, Thamnosma) and ultimately back to the original dominants within 65--130(--500+) years.

T.J. Mabry et al. (1977) have summarized the biology of Larrea divaricata subsp. tridentata. This species dominates all three warm deserts. Larrea is not genetically homogeneous over this large range, but it exists as a distinct chromosome race and ecotype in each desert (M.G. Barbour 1969; T.W. Yang 1970). It can spread vegetatively, forming clones estimated to be several thousand years old (F.C. Vasek 1980).

Intermountain desert scrub has low species and growth-form richness (fig.5.11). In general, Artemisia spp. constitute more than 70% of the cover and more than 90% of the biomass (N.E. West 1988). Species and subspecies of Artemisia arbuscula, A. longiloba, A. nova, and A. tridentata are segregated on gradients of moisture and elevation. Associated woody perennials include Chrysothamnus nauseosus, C. viscidiflorus, Ephedra viridis, Grayia spinosa, Purshia tridentata, and Tetradymia glabrata. Clearing, overgrazing, burning, and introduction of alien plants have led to significant modifications in the vegetation. Noxious or poisonous annuals, such as Bromus tectorum, Ranunculus testiculatus, Elymus caput-medusae, Halogeton glomeratus, Onopordum acanthium, Salsola ibirica, S. kali, and Sisymbrium altissimum, have become common (A.Cronquist et al. 1972+, vol. 1; N.E. West 1988; J.A. Young et al. 1988).

Alkaline areas, warmer and drier than sagebrush-dominated areas, support the C₄ shrub Atriplex confertifolia, associated with Artemisia spinescens, several otherAtriplex spp., and a variety of halophytic or glycophytic taxa (W.D. Billings 1949; A.Cronquist et al. 1972+, vol. 1).

Playas are dominated at their least saline edge by Sarcobatus vermiculatus. More saline zones support Allenrolfea occidentalis and Distichlis spicata var. stricta. Other common halophytes include Ceratoides (= Eurotia) lanata, Kochia americana, Salicornia utahensis, Suaeda fruticosa, and S. torreyana. Many of these taxa occur on saline soils in the warm deserts and grasslands as well (A.Cronquist et al. 1972+, vol. 1).

Mediterranean and Madrean Scrublands and Woodlands

Overview

In North America, Mediterranean and Madrean areas replace each other along an axis from central California to central Mexico. The northwest portion of the axis is Mediterranean; the southeast portion is Madrean. They share a common flora and vegetation but experience different climates.

Mediterranean ecosystems are found in five locations throughout the world (M.L. Cody and H.A. Mooney 1978): the Mediterranean region, the Cape region of South Africa, south and southwest Australia, central Chile, and southern Oregon to northern Baja California. The regions all lie between 40° and 32° N or S latitude, occupy west or southwest edges of continents, receive 275--900 mm annual precipitation (more than two-thirds of which falls in winter), experience less than 3% of annual hours with frost, and have hot, dry summers. The vegetations in all five Mediterranean regions share many traits at a superficial level, but M.G. Barbour and R.A. Minnich (1990) summarized many important differences in both abiotic environment and vegetation. In North America, Mediterranean vegetation includes---from the most mesic to the most xeric---mixed evergreen forest, oak woodland and savanna, grassland, and several types of scrub, e.g., northern coastal scrub, chaparral, southern coastal scrub (figs.5.12, 5.13).

Closely related Madrean vegetation extends from central Arizona to southern New Mexico, southwestern Texas, and south along the Sierra Madre Occidental to southern Durango, at about 22° N latitude (D.E. Brown 1982, 1982b; H.A. Mooney and P.C. Miller 1985; C.P. Pase and D.E. Brown 1982, 1982b; J.Rzedowski 1978). Mexico is a center of oak speciation (ca. 150 species), and oak woodlands account for 5--6% of Mexico's land area (R.Daubenmire 1978; J.Rzedowski 1978). In North America north of Mexico, however, Mediterranean and Madrean vegetations dominate only 1% of the land area.

In western North America, Mediterranean vegetation evolved from the Madro-Tertiary geoflora, which had differentiated by late Miocene (A.Graham, chap. 3). As aridity expanded in the southwestern United States, the Madrean geoflora moved north and west from central Mexico, supplanting a retreating Arcto-Tertiary geoflora. By 15--14 million years ago, a Mediterranean climate existed throughout much of low elevation California (D.I. Axelrod 1958, 1975). Evergreen elements dominate Madrean vegetation. In contrast, predominantly deciduous scrub and woodland vegetation ("petran") occurs in more northerly regions (north of 33° N latitude) in the colder areas along the east and west slopes of the Rocky Mountains. This vegetation is discussed later, at the end of the section on Intermountain Region Montane Forests.

Mixed Evergreen Forest

Mixed evergreen forest has a rather dense, species-rich overstory of hardwood and needle-leaf evergreen trees. The shrub and herb strata are depauparate. W.S. Cooper (1922) included it in his "broad sclerophyll forest formation." It is an ecotone, generally at 600--1500 m elevation, between the lower oak woodland and the higher montane conifer forest. The average annual precipitation of 800--900 mm is at the wet extreme of the Mediterranean type climate (M.G. Barbour 1988; J.Rzedowski 1978; up to 1700 mm in Oregon, according to J.F. Franklin and C.T. Dyrness 1973). This forest occurs in interior valleys of Oregon south of 43° N (J.F. Franklin and C.T. Dyrness 1973), is well represented in the Siskiyou Mountains (R.H. Whittaker 1960), extends through the Coast, Transverse, and Peninsular ranges of California, and parallels oak woodland distribution in Mexico along the middle elevation slopes of the Sierra Madre Occidental in Mexico (J.Rzedowski 1978).

In Oregon and California, Quercus chrysolepis is a unifying taxon for this forest. Common associates include Acer macrophyllum, Arbutus menziesii, Calocedrus decurrens, Chrysolepis chrysophylla, Lithocarpus densiflorus, Pinus coulteri, P. ponderosa, Pseudotsuga menziesii, P. macrocarpa, Quercus kelloggii, and Umbellularia californica (M.G. Barbour 1988; J.O. Sawyer et al. 1988). Many of these genera, but only a few of the species, continue into the mixed evergreen forest of central Mexico (J.Rzedowski 1978). Pines are an important element there, in particular Pinus cooperi, P. durangensis, P. engelmannii, P. leiophylla, P. lumholtzii, and Pinus ponderosa var. arizonica (D.E. Brown 1982).

Oak Woodland

Oak woodland generally lies below mixed evergreen forest, but floristic elements of the two may interdigitate, cooccur, or occupy different slope aspects. Oak woodland experiences a significantly drier and warmer climate than that of mixed evergreen forest; mean annual precipitation is about 600 mm (M.G. Barbour 1988; D.E. Brown 1982b).

Oak woodland in Oregon and California is characteristically two-layered, with an overstory canopy 5--15m tall and 30--80% closed, which is composed of evergreen and deciduous trees (fig.5.12). The most characteristic taxa are Quercus agrifolia, Q. douglasii, Q. engelmannii, Q. garryana, Q. kelloggii, Q. lobata, and Q. wislizenii, associated with Aesculus californica, Juglans californica, and Pinus sabiniana (M.G. Barbour 1988; R.Daubenmire 1978; J.F. Franklin and C.T. Dyrness 1973; J.R. Griffin 1988). The understory is dominated by species characteristic of adjacent grassland, although there may be scattered shrubs (Arctostaphylos spp., Heteromeles arbutifolia, Rhus diversiloba, Symphoricarpos albus). Oak woodland typically exists within a complex mosaic of grassland, savanna, woodland, and chaparral, which all share the same macroclimate but differ in fire frequency, soil texture, soil depth, and slope aspect. Regeneration of the deciduous oaks Q. douglasii, Q. engelmannii, and Q. lobata has been poor for the past century, apparently because of high populations of seed predators, compaction of soil by cattle, and competition with introduced annual grasses (for example, see M.T. Borchert et al. 1989).

Madrean oak woodland in Arizona, New Mexico, Texas, and Mexico (encinal, bosque de encino) is more variable in height, has a more continuous shrub layer less than 2 m tall, and exhibits a richer diversity of vines and epiphytes than does California oak woodland. More than one-third of annual precipitation falls in summer---three times as much as in California (H.A. Mooney and P.C. Miller 1985). The vegetation occupies slopes at 900--2100 m elevation. The most characteristic oaks in Arizona and New Mexico are Quercus arizonica, Q. emoryi, Q. gambelii, Q. grisea, Q. hypoleucoides, Q. oblongifolia, and Q. rugosa, and they are associated withPinus cembroides, P. edulis, P. leiophylla, P. ponderosa, and P. strobiformis (R.K. Peet 1988).

In Mexico, oak of varying degrees of deciduousness often cooccur; among them are white oaks Quercus arizonica, the closely related forms Q. santaclarensis, Q. grisea, Q. chihuahuensis, and Q. chuchiuchupensis (= Q. toumeyi), and black oaks Q. albocincta and Q. durifolia (D.E. Brown 1982b; A.S. Leopold 1950). D.E. Goldberg (1982) has decribed how evergreen and deciduous Madrean oak elements segregate along microenvironmental gradients of soil nutrient level, soil pH, and abundance of seed predators. Mexican oak woodlands have been degraded by cutting, overgrazing, and erosion, and they require serious attention by conservationists (J.Rzedowski 1978).

Chaparral

Chaparral is a dense, one-layered scrub, 1--3 m tall, composed of rigidly branched C₃ shrubs with small, sclerophyllous, leaves and extensive root systems (although the root:shoot ratio is less than one). Herbs are infrequent. Fires cycle every 25--75 years, and most woody species are capable of stump sprouting. Ground cover is close to 100% and the leaf area index = 2. Net annual productivity is 400--800 g/m² (T.L. Hanes 1988; J.E. and S.C. Keeley 1988; H.A. Mooney and P.C. Miller 1985). The scrub vegetation called "chaparral" may be subdivided into three different types: Californian chaparral, Madrean (or interior) chaparral, and southern coastal scrub (fig.5.13).

Chaparral often occurs on steep slopes with coarse soil at 400--800 m elevation on coast-facing slopes, at 900--1800m elevation for interior locations, and at 1700--2400m elevation in Mexico (C.P. Pase and D.E. Brown 1982). Californian chaparral occurs from Baja California to southern Oregon; Madrean chaparral extends from northern Mexico to northwestern Arizona, north central New Mexico, and western Texas. Madrean and Californian chaparral are both considered to be climax types, despite the frequency of fire (C.P. Pase and D.E. Brown 1982).

Many species range throughout Californian and Madrean chaparral, but each area has its separate dominants. Californian chaparral is characterized by Adenostoma fasciculatum, Arctostaphylos spp., Ceanothus spp., Heteromeles arbutifolia, Garrya spp., Quercus dumosa, Rhamnus californica, R. crocea, and Rhus spp. (T.L. Hanes 1988). Chaparral in southern California and northern Baja California is more diverse in growth forms and woody species than the chaparral in northern California (H.A. Mooney and D.J. Parsons 1973). Madrean chaparral north of the international border is dominated by Quercus turbinella, associated with Arctostaphylos pringlei, Ceanothus greggii, Cercocarpus betuloides, Garrya spp., Rhamnus spp., Rhus spp., and several other scrub oaks.

In Mexico, Quercus intricata often dominates this chaparral, associated with Q. pringlei and Q. pungens. This Madrean chaparral is floristically and ecologically closely related to Californian chaparral, despite the fact that the climate is not Mediterranean. Madrean areas experience significant amounts of summer as well as winter rain. Consequently, J.L. Vankat (1989) suggested that "if chaparral is climatically determined, it is associated with a warm temperate climate that has seasonal drought, the timing and duration of which may not be critical."

A variation of chaparral, called southern coastal scrub, dominates slopes below chaparral in southern California and in Baja California (fig. 5.13). The adjective "coastal" is a misnomer, but it is too well accepted in the literature to replace. This scrub does occur on ocean-facing slopes, but it also covers desert-facing slopes some distance inland. The vegetation has a lower, more open canopy than chaparral, with a lower leaf area index of 1.3. The foliage is soft, pubescent, gray, and drought-deciduous, rather than hard, glabrous, and evergreen. Net annual productivity is half that of chaparral. Dominant taxa include Artemisia californica, Encelia californica, Lotus scoparius, Eriogonum fasciculatum, Malosma laurina, Salvia spp., Rhamnus ilicifolia, Rhus spp., Toxicodendron spp., and Yucca whipplei. Succulent species increase to the south, in Baja California. Coastal scrub is sometimes seral to chaparral, sometimes climax; as a consequence, typically narrow ecotones between the two may correspond either to persistent microenvironmental differences or to disturbance boundaries. The ecology, dynamics, and distribution of coastal sage scrub have most recently been described by J.T. Gray and W.H. Schlesinger (1981), H.A. Mooney (1988), and W.E. Westman (1981, 1983).

Pacific Coast Coniferous Forest

Overview

This forest has been called the most luxuriant, most productive vegetation in the world (R.Daubenmire 1978; J.F. Franklin and C.T. Dyrness 1973). It is dominated by a rich diversity of massive, long-lived tree species, underlain by equally rich canopies of shrubs, herbs, and cryptogams. It occupies a coastal, low elevation strip (0--1000 m) that extends from 36° N latitude in Monterey County, California, to 61° N latitude at Cook Inlet, Alaska, an area of about 3% of the North American landmass.

Climate is maritime, with narrow diurnal 6°--10° C (daytime maximum minus nighttime minimum) and seasonal 7°--17° C (mean of warmest month minus mean of coldest) thermoperiods. Hard frosts and persistent snow are uncommon. Studies in growth chambers with seedling conifer elements of this forest, such as Sequoia sempervirens, show that optimum growth occurs at zero thermoperiod (H.Hellmers 1966). Annual precipitation is 800--3000 mm, 80% or more of which falls 1 October--1 April (that is, in the cool season). Summers are relatively dry, but they are moderated by cloud cover and fog drip.

Net annual productivity is 1500--2500 g/m², and standing aboveground biomass is 80,000--90,000(--230,000) g/m². Both ranges exceed the values for any other forest (J.F. Franklin and C.T. Dyrness 1973). Furthermore, the productivity is high despite an average age of overstory dominants at 400--1200 years. Hardwoods are minor elements in this forest, generally restricted to specialized or seral habitats, and they have been declining in diversity and abundance for at least the past 1.5 million years. Conifers account for more volume over hardwoods by 1000:1 for several reasons: moderate temperatures permit net photosynthesis outside the normal growing season (as little as 30% of annual net photosynthesis may occur during the growing season); conifer needles are tolerant of the moderately high water stresses that develop during summer (south of 51° N); conifers have a high nutrient-use efficiency, and time periods between disturbance---on the order of several centuries---suit the life cycles of populations with massive, long-lived individuals (J.F. Franklin and C.T. Dyrness 1973; J.P. Lassoie et al. 1985; N.L. Stephenson 1990; R.H. Waring and J.F. Franklin 1979).

Regional Vegetation

R.Daubenmire (1978) has broken this coastal strip, which he calls the Tsuga heterophylla province, into three areas and forests. The northernmost forest, from Cook Inlet to the southern tip of Alaska, has a relatively simple overstory. Tsuga heterophylla is the dominant species, extending from low elevations to the subalpine zone. Minor species include Chamaecyparis nootkatensis, Picea sitchensis, and Tsuga mertensiana. Picea is seral. Scattered shrubs of Cornus canadensis, Rubus pedatus, and Vaccinium spp. overlay a continuous carpet of mosses. Sphagnum moss is capable of invading this climax forest, raising the water table, and setting in motion a retrogressive succession back to bog and wet meadow (W.H. Drury Jr. 1956).

A central section, much richer in tree species, extends from the southern Alaska--British Columbia border to the Oregon-California border. Tsuga heterophylla is dominant, and important associates include Picea sitchensis (along the most coastward strip), Pseudotsuga menziesii, Abies amabilis, and Thuja plicata (fig. 5.14). Taxus brevifolia is a common subcanopy tree, and in the ground layer are many understory shrubs, ferns, herbs, bryophytes, and lichens (J.F. Franklin and C.T. Dyrness 1973; V.J. Krajina 1965). Similar vegetation extends inland, on west-facing slopes, to the central Rocky Mountains as a peninsula created by storm tracks that extend as far southeast as Yellowstone National Park and as far northeast as Jasper National Park (44--53° N latitude [R.K. Peet 1988]). On the most mesic sites---sandwiched between lower elevation Pseudotsuga forest and higher elevation Picea subalpine forest---Tsuga heterophylla dominates luxuriant forests, with Thuja plicata and Abies grandis as characteristic associates. Basal areas are 100--200 m²/ha, no different from those of western Washington and Oregon; ericaceous shrubs, however, are less common while herb diversity is higher (R.Daubenmire 1978).

The successional nature of these forests has been difficult to discern because of the long-lived nature of most species, including seral species. For example, Pseudotsuga menziesii is a common seral tree in the region that can maintain populations or individuals for 400--1000 years while slowly being replaced by Tsuga heterophylla (J.F. Franklin and C.T. Dyrness 1973). Tsuga heterophylla is understood to be the climatic climax species, because of all major western forest species it is the most moisture-demanding, the least heat-tolerant, the least drought-tolerant, and among the most shade-tolerant (W.K. Smith 1985; D.Minore 1979).

A variant of the Tsuga heterophylla forest occupies the immediate coast, generally within a few kilometers of the ocean except where river valleys and level terrain (such as the west side of the Olympic Peninsula) offer more area below 600 m elevation. Acer macrophyllum and Picea sitchensis are more abundant, the nurse log phenomenon of tree sapling establishment is very evident, the overstory canopy is less completely closed (especially where Thuja plicata and Picea sitchensis are common), individual trees are more massive, and the understory is either more dense (especially Gaultheria shallon and Vaccinium spp.) or more open due to elk grazing. Vegetation in Olympic National Park epitomizes this variant, often referred to as a "temperate rainforest."

The southern part of this Pacific Coast forest, from the California-Oregon border to the Big Sur coast at 36° N, is dominated by Sequoia sempervirens. Abies grandis, Lithocarpus densiflorus, Pseudotsuga menziesii, and Tsuga heterophylla may be associated, but they are seral (R.Daubenmire 1978; P.J. Zinke 1988). Coast redwood generally occurs within 35 km of the coast and below 1000 m elevation. It dominates a more or less continuous forest zone in northernmost California, but much of its range is characterized by discontinuous groves. Since approximately 1850, when commercial logging of redwood began, 90% of old-growth forest acreage has been removed by clear-cutting (P.Hyde and F.Leydet 1969).

Sequoia occupies both riparian floodplains and slopes. It is uniquely tolerant of fire and silt deposition following flood because of its ability to form adventitious roots and shoots. The time interval between fire or flood disturbances appears to be 30--60 years. Associated species are not as tolerant of disturbance. E.C. Stone and R.B. Vasey (1968) hypothesized that redwood is seral and will not maintain itself without such disturbance, but S.D. Viers Jr. (1982) accumulated data for some stands suggesting that disturbance is not always essential.

Western Montane Coniferous Forests

Overview

Coniferous forests clothe the slopes of the Rocky Mountains and their extensions into Mexico (Sierra Madre Oriental and Sierra Madre Occidental), the Sierra Nevada--Cascade axis, the Coast Range of British Columbia, Washington, Oregon, and northern California, the Transverse and Peninsular ranges of southern California and Baja California, and scattered ranges and northern plateaus of the intermountain region (figs. 5.14, 5.15). They extend from 65° N to 19° N latitude and from 100° W to 140° W longitude. North of Mexico, these forests dominate 7% of the landmass.

Floristically, this enormous expanse is not homogeneous. For example, montane areas of north and south extremes of the Cascade--Sierra Nevada axis exhibit a Sorensen community similarity coefficient of only 30 (0 = total dissimilarity, 100 = identity [D.W. Taylor 1977]). The region does have a long, common paleobotanical history through the Tertiary (D.I. Axelrod and P.H. Raven 1985; D.I. Axelrod 1976), and some species have wide distribution limits through much of the area (though they may have climax status in one region and seral status in another, or they may be differentiated into regional subspecific taxa). Among the trees, only Populus tremuloides is found throughout the area; Pinus ponderosa and Pseudotsuga menziesii occur through most of the area (H.A. Fowells 1965).

Zonation of forest types along elevation gradients is a common theme recognized since C.H. Merriam (1898), and the forests characteristic of each elevational zone share many physiognomic similarities, from one mountain chain to another mountain chain. Low elevation forests tend to be rather open savannas or woodlands, intermingled with species from grasslands, broad-leaved woodlands, deserts, or chaparral. Frequent fires seem essential to maintain the open, two-layered structure, and these forests have been modified by overgrazing and fire suppression policies of the past century. Midmontane forests are typically rich in overstory and understory woody species. Upper montane and subalpine forests are denser, simpler, and experience deep, long-lasting snowpacks.

Local zonation is affected by aspect, exposure, soil depth, distance from the ocean, and latitude, as well as by elevation; consequently zonation is not consistent from place to place, even within a floristically homogeneous region. In addition, western montane forests are characterized by frequent disturbance---fire, wind, insects, disease, browsing, avalanche, landslide, weather extremes, volcanism, and logging---and as a result a mosaic of communities exists within a given elevational zone (R.K. Peet 1988). J.R. Habeck (1988) has also pointed out that the vegetation is still recovering from glacial retreat and the Hypsithermal interval, and he concluded that any modern description only succeeds in capturing "a moment in an ever-changing phenomenon."

Climate throughout most of the region, except the Madrean, has a pronounced winter peak in precipitation. Annual precipitation typically ranges from 600 mm at low elevations (Pinus ponderosa) to more than 2000 mm at high elevations (spruce-fir), and annual productivity ranges from 600 to 1100 g/m² (R.K. Peet 1988). The pinyon-juniper lower fringe, where present, receives as little as 250 mm of precipitation and exhibits significantly lower productivity (D.E. Brown 1982; A.Cronquist et al. 1972+, vol. 1; J.F. Franklin and C.T. Dyrness 1973; N.E. West 1988). Fire cycles may be as long as 200--400 years in subalpine spruce-fir and as short as 5--12 years at low elevations (R.K. Peet 1988).

Rocky Mountain Forests

Detailed reviews of these forests have been written by R.Daubenmire (1943) and R.K. Peet (1988). Each divides the Rocky Mountains into somewhat different regions and zones; for consistency, we shall use Peet's more recent treatment. He divides the range into four areas: far north (approximately 53°--65° N latitude), north (45°--53°), south (35°--45°), and Madrean (19°--35°), the last being largely a simplification for convenience, given the high degree of variation within it (J.Rzedowski 1978).

Far northern Rocky Mountain forests are relatively simple, and they broadly intergrade with surrounding lowland boreal forest. The common montane tree species, Abies bifolia, Picea engelmannii, and Pinus contorta, are all capable of hybridizing with their boreal counterparts, A. balsamea, Picea glauca, and Pinus banksiana, respectively. Many understory herbs---e.g., species of Clintonia, Cornus, Linnaea, Pyrola, Sorbus---are shared between montane and boreal forests (R.Daubenmire 1978). Picea glauca dominates lower and midmontane slopes, and A. bifolia dominates the subalpine. Timberline is at 1400 m elevation. Toward the northern end of the Rocky Mountains, the range of P. glauca becomes constricted to a narrowing band at low elevations, and the range of Picea engelmannii increases in the subalpine (P.Achuff 1989). This brief summary does not do justice to the complexities of vegetation within British Columbia. A fine vegetation map, at a scale of 1:2 million, identifies a dozen montane biogeoclimatic zones; we have mentioned but a few (British Columbia Ministry of Forests 1988).

Vegetation of the northern Rocky Mountains, as mentioned earlier, has a Cascadian aspect, due to penetration by westerly storm tracks. J.R. Habeck (1988) recently reviewed this region in detail. Low elevations, 800--1500 m, support a Juniperus scopulorum--Pinus ponderosa woodland intermixed with Cercocarpus ledifolius and Purshia tridentata shrubs. A low elevation ecotype of Pinus flexilis is also present. The montane zone, up to 1800 m, is a productive forest rich in tree species such as Abies grandis, Pseudotsuga menziesii, Thuja plicata, and Tsuga heterophylla. Above, in the subalpine, Abies bifolia and Picea engelmannii dominate mesic sites, while Pinus albicaulis and Larix lyallii occur on more extreme sites. The deciduous Larix is unique in the northern Rocky Mountains. It is able to establish stands above evergreen conifers, in what would otherwise be tundra, and it creates a suitable microenvironment for montane herbs to follow upslope with it (S.F. Arno and J.R. Habeck 1972). Timberline in the northern Rocky Mountains lies at 2700 m. Timberline rises to the south at a rate of +100 m per degree latitude (R.K. Peet 1988).

Zonation in the southern Rocky Mountains proceeds from a pinyon-juniper woodland through a Pinus ponderosa parkland, a Pseudotsuga menziesii forest (with Abies concolor and Picea pungens), and a Pinus contorta forest to a dense Abies bifolia--Picea engelmannii subalpine forest (with an open Pinus aristata woodland on exposed sites). Although Pinus contorta is largely a seral species throughout western forests, at its southern limit it is capable of climax status. In the Rocky Mountains it is typically serotinous (cones remain closed at maturity), but the degree of serotiny is known to vary with stand history (P.S. Muir and J.E. Lotan 1985).

The Madrean portions of the Rocky Mountains are complex in their flora and topography, being less continuous. Zonation typically proceeds from a pinyon-juniper woodland (north) or an oak-pine mixed evergreen forest (south) through a Pinus ponderosa parkland, and a dense Pseudotsuga menziesii forest (associated with Abies concolor and Pinus flexilis, among several other taxa with more Madrean affinities) to a subalpine forest of Picea and Abies. Ponderosa pine parkland has 50--75% tree cover, 8--40m tall, overtopping a well-developed grass understory (C.P. Pase and D.E. Brown 1982). Pinus ponderosa changes from var. scopulorum in the north to var. arizonica in the south. In Arizona and New Mexico, Picea engelmanii and Abies bifolia "var. arizonica" dominate mesic sites in the subalpine, while Pinus aristata and P. flexilis dominate dry sites (C.P. Pase and D.E. Brown 1982b).

Pacific Northwest Montane Forests

In the Coast Range, and along the west face of the northern Cascade Range, montane vegetation above the temperate rainforest proceeds through an Abies amabilis zone to a Tsuga mertensiana zone and an Abies lasiocarpa subalpine zone (J.F. Franklin and C.T. Dyrness 1973). The Abies amabilis forest receives a modest 1--3 m deep snowpack and annual precipitation greater than 2000 mm. From Mt. Rainier south, its overstory, including Abies procera, Pinus monticola, and Pseudotsuga menziesii, overlies rich ericaceous shrub, herb, and moss canopies. To the north, Tsuga heterophylla is a common associate. The higher Tsuga mertensiana forest receives heavy snowpacks up to 7.5 m deep, and its lower limit appears to coincide with freezing elevation in winter (E.B. Peterson 1969). Associates include Abies amabilis and Chamaecyparis nootkatensis.

Eastern slopes of the Cascade Range, and mountains to the east, exhibit a complex zonation that proceeds from a Juniperus occidentalis-- Artemisia savanna through a Pinus ponderosa forest (sometimes with Pseudotsuga menziesii) to a mesic Abies lasiocarpa or Tsuga mertensiana forest above 1500 m elevation.

Extensions of the Coast and Cascade ranges south of 43° N latitude and into northern California experience a Mediterranean precipitation regime. As a consequence, they have a Sierran zonation of species and are included in the next section.

Montane Forests of Alta and Baja California

These forests have recently been summarized by M.G. Barbour (1988), P.W. Rundel et al. (1988), J.O. Sawyer and D.A. Thornburg (1988), and R.F. Thorne (1988). The region may be subdivided into a northern portion (southern Cascade, Siskiyou Mountains, Klamath Mountains, north Coast Range of California, Sierra Nevada) and a southern portion (Transverse and Peninsular ranges, extending south to the San Pedro Mártir of Baja California).

In the northern portion, typical zonation proceeds upward from a mixed evergreen forest through a relatively open mixed conifer forest, to a denser Abies magnificaforest, and to an open mixed subalpine woodland.

The mixed conifer forest has received considerable research and sylvicultural attention because of its commercial value. The overstory exhibits shared or shifting dominance by five conifer species: Abies lowiana (= A. concolor var. lowiana), Calocedrus decurrens, Pinus lambertiana, P. ponderosa, and Pseudotsuga menziesii. At the lower edge Pinus ponderosa predominates; at the upper edge Abies lowiana predominates. (This white fir is the western counterpart of the Rocky Mountain A. concolor. It is capable of hybridizing with A. grandis of the Pacific Northwest [J.F. Franklin and C.T. Dyrness 1973].) Within this zone are 75 groves of Sequoiadendron giganteum, the most massive trees in the world (fig. 5.15). Their habitat restrictions, community dynamics, geologic record, and problematic future have been described by D.I. Axelrod (1986c) and P.W. Rundel (1971, 1972).

All forests in this lower montane zone are tolerant of, and dependent on, frequent ground fires to maintain their openness and mixed dominance (B.M. Kilgore 1973; J.L. Vankat and J.Major 1978). Fire suppression and postlogging management have seriously modified these forests; more than half the pristine old-growth acreage has been cut (R.J. Laacke and J.F. Fiske 1983). Another threat is air pollution damage, once restricted to southern California, but now documented for the southern Sierra Nevada as well (W.T. Williams et al. 1977).

The upper montane zone is dominated thoroughly by Abies magnifica on mesic sites and by Pinus contorta or Populus tremuloides on wet sites. Abies magnificaforests receive the highest snowpacks of any Californian vegetation, 2.5--4m deep and lasting 200 days. The lower limit of the forest appears to correspond with average elevation of freezing temperature in winter (M.G. Barbour et al. 1991). They are floristically simple forests---often monospecific in the overstory and with few understory shrubs and herbs---but the trees are impressive in size, with mature ones 30--45 m tall and 1.5 m dbh (diameter at breast height) (M.G. Barbour and R.A. Woodward 1985). Abies magnifica forms hybrid swarms with A. procera of the Pacific Northwest. Pinus contorta here is the nonserotinous var. murrayana(H.A. Fowells 1965).

The mixed subalpine woodland, in contrast to the Abies forest, has only 5--40% canopy cover provided by trees 10--15 m tall. Unlike the subalpine zone of the Rocky Mountains and the Pacific Northwest, Picea and Abies are absent. Dominants include Pinus albicaulis, P. balfouriana, P. contorta var. murrayana, P. flexilis, P. monticola, and Tsuga mertensiana. We have adopted the "mixed subalpine" name applied by the Society of American Foresters (F.H. Eyre 1980) to emphasize the pattern of shared or shifting dominance by these six taxa. Timberline increases in elevation from 2900 m in the southern Cascades to 3400 m at the southern limit of the Sierra Nevada.

In southern California and Baja California, the mixed conifer forest becomes simplified to a yellow pine forest, with mixtures of Pinus jeffreyi and P. ponderosa that depend on elevation and aridity. In Baja California mountains, fire suppression has never been practiced. There, P. ponderosa is absent, and the lower montane forest is dominated by P. jeffreyi and P. quadrifolia (R.A. Minnich 1987; M.-F. Passini et al. 1989). Above this is an open upper montane forest with Abies concolor, Calocedrus decurrens, Pinus lambertiana, and P. jeffreyi. Juniperus occidentalis var. australis occurs on rock outcrops throughout this zone, as it does to the north. The subalpine zone, 2500--3000+ m in elevation, supports either relatively dense Pinus contorta forest or open P. flexilis woodland (R. F. Thorne 1988). Southern mountains are not tall enough to exhibit a climatic timberline.

The eastern, desert-facing slopes of Californian mountain ranges fall steeply, have shallower soils, and support less continuous forests with less well-defined zones. The flora has a pronounced desert aspect.

Intermountain Region Montane Forests

Between the Cascade-Sierra axis and the Rocky Mountains, and between 45° N and 37° N latitude, are mountain chains isolated like islands amidst a sea of shrub steppe and desert scrub. Because of their relative aridity, and because of the accidental vagaries of plant migrations during the Pleistocene, zonation and species richness on these peaks are not as complex as on the Rockies or the Sierra Nevada. These montane forests have been well described by W.D. Billings (1951), A.Cronquist et al. (1972+, vol. 1), and F.C. Vasek and R.F. Thorne (1988).

Elevations between 1500 and 2500 m have a pinyon-juniper woodland, one of the most extensive vegetation types in southwestern North America, covering 170,000 km² (D.E. Brown 1982; N.E. West 1988). The woodland is composed of trees 3--7 m tall with rounded crowns that generally do not touch each other. They overlie an understory of cold desert shrubs, C₃ bunchgrasses, and forbs with varying cover. Modern woodlands are denser than those of 150 years ago, perhaps because grass cover in pristine woodlands was able to carry fire more frequently than the overgrazed modern cover. The most widely ranging components are Juniperus osteosperma, generally at lower elevations, and Pinus monophylla, generally at higher elevations, but A.Cronquist et al. (1972+, vol. 1) and N.E. West (1988) reported several other species of pine and juniper that may be more important locally. Not discussed here are other Pinus-Juniperus woodlands along the eastern flank of the Rocky Mountains and into the Madrean region of Mexico (R.K. Peet 1988; J.Rzedowski 1978).

Above pinyon-juniper is an Abies concolor montane forest, followed by a subalpine woodland (approximately 3000m elevation) dominated by Pinus flexilis and/or P.longaeva. The latter species (the Great Basin bristlecone pine) contains individuals that are approaching 5000 years, older than those of any other taxon. On some very arid intermountain ranges, the Abies zone is absent and pinyon pines run up the entire slope, stopping only when reaching the shade of bristlecone pines. On some relatively mesic ranges, the montane zone includes Pinus contorta, P. ponderosa, and Pseudotsuga menziesii, and the subalpine zone includes Abies lasiocarpa and Picea engelmannii.

A deciduous scrub may, in places, either replace the pinyon-juniper woodland or lie just above it as a dry margin to montane forest. This scrub has been called petran chaparral or Great Basin montane scrubland. It differs from typical Californian or interior chaparral by species composition and predominance of deciduous elements (D.E. Brown 1982b). Quercus gambelii (in the broad sense) is dominant, associated with Amelanchier alnifolia, A. utahensis, Arctostaphylos patula, Ceanothus fendleri, C. velutinus, Cercocarpus montanus, C. ledifolius, Cowania mexicana, Symphoricarpos spp., and Prunus virginiana, among others. The scrub is 1--6 m tall and ranges from dense to open; its physiognomy and herbaceous composition have been degraded by overgrazing throughout its distribution.

Tidal Wetlands

Overview

Salt marshes are coastal meadowlands subject to periodic flooding by the sea (fig. 5.16). They are typically restricted to shorelines with low-energy waves, such as estuaries or areas behind barrier islands or spits, but otherwise they can occur on stable, subsiding, or rising coasts (V.J. Chapman 1976). The vegetation is usually a single, nearly closed layer of perennial herbs. The flora is rather simple. For example, tidal marshes along the Atlantic and Gulf coasts of North America combined support only 347 taxa of vascular plants, those along the Pacific Coast from the tip of Baja California to Point Barrow have 78 species, and the marshes of Hudson Bay contain 28 species (B.L. Haines and E.L. Dunn 1985).

Perhaps as a consequence of the simplicity of the flora, several species have enormous ranges that take them through nearly all of North America (Atriplex patula, Distichlis spicata, Salicornia virginica, Spartina alterniflora, Triglochin maritima). Poaceae, Juncaceae, and Chenopodiaceae dominate the temperate zone salt marshes, but subtropical and arctic marshes are more diverse at the family level (L.C. Bliss 1988; W.A. Glooschenko 1980; R.F. Thorne 1954). Subtropical coastlines in North America, generally south of 29° N latitude, tend to be dominated by a group of woody taxa collectively called mangroves. Mangrove swamps are not salt marshes in the physiognomic sense defined above, but for completeness they are included in this section.

Environmental stresses on salt marsh vegetation include flooding---with attendant mechanical disturbance and anaerobic conditions---and salinity. As the land slopes upward from the sea, these two stresses decline in intensity. As a consequence, vegetation is divided into two or more zones. The most typical zones are low marsh and high marsh. Low marsh extends approximately from mean sea level to mean high water and is flooded once or twice a day, whereas high marsh is irregularly flooded once a month or less, only at highest tides or during storms. The ecotone between the two is characteristically abrupt, and its location has been ascribed variously to physical factors and competition (M.D. Bertness and A.M. Ellison 1987; B.E. Mahall and R.B. Park 1976).

The low marsh tends to be a narrow fringe, only 1--30 m wide, whereas the high marsh may be hundreds or even thousands of meters wide (J.P. Stout 1984). With farther distance inland or inward along river mouths, salinity declines and complex, diverse, brackish marshes become dominant. These are not discussed in this section. Areal estimates for tidal salt marshes along the coast of the United States have been compiled, but not for Canada. Our estimate, largely based on a map in B.L. Haines and E.L. Dunn (1985) and our own calculations, is that these marshes occupy 1% of the North America landmass.

Net annual productivity varies with dominant species and latitude, but overall for North America it is 300--2000 g/m²/yr (M.Josselyn 1983; D.M. Seliskar and J.L. Gallagher 1983; J.P. Stout 1984), the highest values being for the Gulf Coast, which has a year-long growing season.

Regional Salt Marsh Vegetation

Arctic salt marshes are scattered, and their short plants (3--5 cm tall) exhibit only 15--25% cover. No estimates of their area or productivity seem to exist in the literature. Their extent is limited by annual ice scour, lack of fine sediment, limited tidal amplitude, low salinity of coastal waters, and a short growing season (L.C. Bliss 1988). The low marsh is dominated by Puccinellia phryganodes, and the high marsh is a mixture of Atriplex patula, Calamagrostis deschampsioides, Carex bipartita, C. maritima, C. neglecta, C. ramenskii, C. salina, C. subspathacea, C. ursina, Chrysanthemum arcticum, Cochlearia officinalis, Dupontia fisheri, Festuca rubra, Glaux maritima, Plantago maritima, Potentilla egedii, Ranunculus cymbalaria, Salicornia europaea, Scirpus maritima, Senecio congestus, Stellaria crassifolia, S. humifusa, Suaeda maritima, and Triglochin maritima (L.C. Bliss 1988; V.J. Chapman 1976; W.A. Glooschenko 1980, 1980b; W.A. Glooschenko et al. 1988; K.A. Kershaw 1976; K.B. Macdonald and M.G. Barbour 1974).

The Atlantic Coast low marsh, from the Gulf of St. Lawrence to southern Florida, is dominated by cordgrass, Spartina alterniflora (fig. 5.16). It is a dense, usually monospecific community. Away from the tide line, tall cordgrass (1--2 m) gives way to sterile, short cordgrass (less than 1 m). Transplant and genetic studies have demonstrated that these are ecophenes (not genetically fixed) rather than ecotypes (C.M. Anderson and M.Treshow 1980; I.Valiela et al. 1978). The upper marsh is dominated by Spartina patens, associated most commonly with Distichlis spicata, Juncus gerardii, Limonium carolinianum, Salicornia virginica, Suaeda linearis, and Triglochin maritima (M.D. Bertness and A.M. Ellison 1987; V.J. Chapman 1976; W.A. Glooschenko 1980; B.L. Haines and E.L. Dunn 1985; R.J. Reimold 1977). This corresponds to the "northern cordgrass prairie" of A.W. Küchler (1964).

Along the United States portion of the Gulf of Mexico, cordgrass continues to dominate the low marsh. The high marsh is thoroughly dominated by Juncus roemerianus, but associates include Batis maritima, Borrichia frutescens, Carex spp., Distichlis spicata, Fimbristylis castanea, Limonium carolinianum, Salicornia virginica, Scirpus spp., Sesuvium portulacastrum, Spartina patens, and Suaeda linearis (R.H. Chabreck 1972; V.J. Chapman 1976; L.N. Eleuterius 1980; J.P. Stout 1984; R.F. Thorne 1954). This is the "southern cordgrass prairie" of A.W. Küchler (1964). Juncus roemerianus is the most abundant salt marsh species on the Gulf Coast; it also extends east and north to Virginia (where it replaces J. gerardii) and inland to brackish marshes. It exists in tall (1--2 m) and sterile short (less than 1 m) forms, which occupy different zones, just as cordgrass does.

South of 29° N latitude, on both sides of the Florida peninsula, mangroves assume increasing dominance (A.F. Johnson and M.G. Barbour 1990). Rhizophora mangleis the most seaward species, followed by a zone of Avicennia germinans (= A. nitida), Laguncularia racemosa, and Conocarpus erecta. The Avicennia-Lagunculariazone includes the salt marsh species Batis maritima, Salicornia virginica, Sesuvium portulacastrum, and Suaeda linearis. Sand, marl, or limestone substrates here also support Monanthochloë littoralis and Sporobolus virginicus (R.F. Thorne 1954).

The Pacific Coast salt marsh progresses northward through subtropical, temperate, and cool temperate vegetation regions (K.B. Macdonald and M.G. Barbour 1974). A number of subtropical high marsh plants also occur as elements of strand vegetation here and along the Gulf of Mexico (M.G. Barbour et al. 1987; A.F. Johnson 1977, 1982; A.F. Johnson and M.G. Barbour 1990; P.Moreno-Casasola 1988).

From central Baja California to northern California, the temperate low marsh is dominated by Spartina foliosa and the high marsh is dominated by Salicornia virginica (associated with its parasite Cuscuta salina), Distichlis spicata, Frankenia grandifolia, Grindelia stricta, Jaumea carnosa, Limonium californicum, Suaeda californica, and Triglochin maritima (M.Josselyn 1983; K.B. Macdonald and M.G. Barbour 1974; B.E. Mahall and R.B. Park 1976; L.Neuenschwander et al. 1979; J.B. Zedler 1982). From Oregon through western Alaska, Puccinellia spp. replace cordgrass in an open low marsh, and the high marsh is characterized by Carex lyngbyei, which also extends into brackish areas. Associated taxa include Deschampsia caespitosa, Distichlis spicata, Glaux maritima, Grindelia stricta, Jaumea carnosa, Juncus spp., Plantago maritima, Potentilla egedii, Salicornia virginica and its parasite Cuscuta salina, Stellaria humifusa, and Triglochin maritima (J.F. Franklin and C.T. Dyrness 1973; W.A. Glooschenko 1980; K.B. Macdonald and M.G. Barbour 1974; D.M. Seliskar and J.L. Gallagher 1983).

Beach and Frontal Dune Vegetation

Overview

"Beach" is that strip of sandy substrate adjacent to open coast that extends from mean tide line to the top of the frontal dune---or in the absence of a frontal dune, to the farthest inland reach of storm waves (fig. 5.17). It is characterized by a maritime climate, high exposure to salt spray (oceanic coasts) and sand blast, and a shifting, sandy substrate with low water-holding capacity and low organic matter content, and it is subject to episodic overwash. Reviews of entire coastlines (G.J. Breckon and M.G. Barbour 1974; M.G. Barbour et al. 1985; M.G. Barbour, M.Rejmanek, et al. 1987; P.Moreno-Casasola 1988) indicate that many beach taxa have wide latitudinal and longitudinal distributions, the limits of which relate more to substrate texture and chemistry, wave energy, and frequency of disturbance than to zonal climate.

The dominant growth forms are (1) rhizomatous perennial grass or (2) prostrate, succulent, perennial forb (M.G. Barbour 1992). Woodiness, sclerophylly, and annuality are uncommon features (M.G. Barbour 1992). Plant cover tends to be low (5--20%), and any given beach is characterized by only a dozen or fewer taxa. As in the salt marsh, species tend to be zoned with distance back from tide line: some taxa characteristically dominate the leading edge of vegetation and others dominate more inland strips. Species richness, growth-form diversity, and plant cover all increase with distance inland. The frontal dune may be densely covered by grasses or forbs---indeed, its existence is due to the sand-stilling nature of vegetation. To our knowledge, productivity estimates have not been made for beach vegetation in North America; judging from biomass estimates of Pacific and Gulf Coast strands made by M.G. Barbour and R.H. Robichaux (1976) and F.W. Judd et al. (1977), they may approximate those of steppe and desert. Strand vegetation occupies much less than 1% of the North American landmass, and the land it occupies is mobile. Beaches can build seaward during calm seasons and be eroded during storm seasons.

Barrier beaches on islands may also migrate over time when erosion and building occur simultaneously, but on different parts of the island. Unfortunately, the aesthetic qualities of beachfront land have attracted road and home construction, leading to a degradation of the beach habitat along most coastlines (e.g., R.Dolan et al. 1973; Corps of Engineers 1973). Revegetation, groin construction, and beach "nourishment" (additions of dredged sand) are some techniques that have been used for strand reclamation, with a record of only limited success.

To the lee of the frontal dune, or beyond the reach of storm waves, a variety of habitats and a mixture of communities exist: low swales with marsh plants, where deflation has brought down the soil surface close to ground water; stable dunes with woody vegetation, where plant cover is more nearly continuous and soils are colored by significant amounts of organic matter; and transition zones with an admixture of typically inland taxa. An elegant study by J.S. Clark (1986) demonstrated that because the disturbance frequency is so high, these communities are not in equilibrium with their present physical environments. The vegetation of these more inland habitats are too complex for us to summarize in this brief continental review, so we limit our discussion to beach and frontal dune ("strand").

Regional Vegetation

Beaches facing the Arctic Ocean are few and narrow, and they have a rather abrupt transition to tundra. Their dominant taxa have a circumarctic flavor, and they extend southward on both Atlantic and Pacific coasts for various distances: Angelica lucida, Cochlearia officinalis subsp. groenlandica, Elymus mollis (= E. arenariussubsp. mollis and Lymus mollis), Festuca rubra, Honkenya peploides, Lathyrus japonicus, Ligusticum scoticum, Matricaria ambigua, Mertensia maritima, and Senecio pseudo-arnica (L.C. Bliss 1988; G.J. Breckon and M.G. Barbour 1974; K.B. Macdonald and M.G. Barbour 1974). The grasses Elymus and Festuca are dominants.

Pacific Coast beaches may be divided into 5--7 ecofloristic zones, and some of these correspond to climatic zones in the Köppen system (G.J. Breckon and M.G. Barbour 1974; M.G. Barbour et al. 1975). Flora with arctic and subarctic ranges, such as Elymus mollis, characterize beaches to 54° N latitude, but a temperate ecofloristic zone occupies the region 54°--34° N. Common taxa here include Abronia latifolia, Ambrosia chamissonis, Atriplex leucophylla, Calystegia soldanella, Lathyrus littoralis, and Poa douglasii. Introduced plants, Ammophila arenaria (which has largely displaced Elymus mollis), Cakile maritima, C. edentula, and Mesembryanthemum chilense, also exist along this portion of the coast. South of 37° N latitude, grasses no longer dominate the foredune, and they are replaced with prostrate forbs such as Abronia maritima (M.G. Barbour and A.F. Johnson 1988). Beaches in the region 34°--24° N latitude are arid, and their flora has a desert aspect. South of 24° N latitude, such tropical species as Ipomoea stolonifera, Scaevola plumieri, Sesuvium verrucosum, and Sporobolus virginicus are dominants (A.F. Johnson 1977, 1982, 1985).

Along the United States shore of the Gulf of Mexico, 73 common taxa form three vegetation regions (M.G. Barbour, M.Rejmanek, et al. 1987). Uniola paniculatadominates everywhere except Louisiana, where it is replaced by a dune ecotype of Spartina patens. Spartina patens ranges extensively on the eastern seaboard, from 50° N latitude in Canada to 16° N latitude in Central America. In the northern part of its range, it occurs exclusively in salt marshes; in the south, exclusively on dunes; but in the middle of its range, distinct ecotypes occur in dune, marsh, and swale habitats (J.A. Silander 1979; J.A. Silander and J.Antonovics 1979).

Other important local or regionwide Gulf strand plants include Cenchrus incertus, Croton punctatus, Ipomoea stolonifera, Iva imbricata, Oenothera humifusa, Panicum amarum, Schizachyrium maritimum, and Sesuvium portulacastrum. Many of these taxa continue south along the Mexican portion of the Gulf (I.Espejel 1986; P.Moreno-Casasola 1988; J.D. Sauer 1967). C₄ taxa constitute more than 40% of all plant cover in the subtropical Gulf, in contrast to less than 25% along temperate North American shores (M.G. Barbour et al. 1985). A special calcareous shell substrate---which occurs in southern Florida, the Florida Keys, and parts of Mexico---creates narrow beaches that support a unique community that deserves more study (M.G. Barbour, M.Rejmanek, et al. 1987; A.F. Johnson and M.G. Barbour 1990; P.Moreno-Casasola 1988).

We have not found a thorough regional review of Atlantic Coast strand vegetation. There appear to be three ecofloristic zones along that coast. Several Gulf taxa continue around the Florida peninsula and up the Atlantic Coast to approximately the Virginia--North Carolina border (approximately 37° N latitude), and their presence defines a southern zone (fig. 5.17). Barrier islands are characteristic of this region, just as they are of the Gulf Coast (R.Dolan et al. 1980; J.G. Ehrenfeld 1990; S.P. Leatherman 1982; G.F. Levy 1990; C.A. McCaffrey and R.D. Dueser 1990). Uniola paniculata is the beach dominant, with Andropogon virginicus, Cenchrus tribuloides, Croton punctatus, Diodia teres, Erigeron canadensis, Euphorbia (= Chamaesyce) polygonifolia, Heterotheca subaxillaris, Hydrocotyle bonariensis, Iva imbricata, Oenothera laciniata, Panicum amarum, Physalis maritima, and Triplasis purpurea among the important associates (J.M. Barry 1980; N.L. Christensen 1988; P.J. Godfrey and M.M. Godfrey 1976; R.Stalter 1974). Ecotypic differences between Gulf and Atlantic populations of Uniola have been described by E.D. Seneca (1972).

The mid-Atlantic zone is dominated by Ammophila breviligulata, most commonly associated with Andropogon virginicus, Arenaria peploides, Artemisia stelleriana,Cakile edentula, Euphorbia polygonifolia, Lathyrus japonicus, Polygonum glaucum, and Solidago sempervirens (A.F. Johnson 1985b; W.E. Martin 1959; B.Robichaud and M.F. Buell 1973).

Moving west along the St. Lawrence River, one leaves a saline coast and enters the lacustrine beach system of the Great Lakes. The vegetation of sandy coasts here has been described in detail for only a few places (e.g., R.G. Morrison and G.A. Yarranton 1973, 1974; J.S. Olson 1958). Its character has been regionally summarized by M.A. Maun (1992). Beach dominants include the grasses Ammophila breviligulata and Calamovilfa longifolia, and the forbs Cakile edentula, Corispermum hyssopifolium, Euphorbia polygonifolia, and Salsola kali. All of these, save Calamovilfa, extend to the Atlantic Coast, and some evidence indicates that ecotypic differentiation between maritime and lacustrine habitats has occurred (R.S. Boyd and M.G. Barbour 1986; P.R. Baye 1990). Behind the foredune ridge is a grassland-heath scrub with such tallgrass prairie species as Andropogon gerardii, A. scoparius, Sorghastrum nutans, and Stipa spartea. The grasses are associated with Arctostaphylos uva-ursi, Juniperus communis, and Prunus pumila shrubs.

Returning to the maritime Atlantic Coast, one sees that arctic and subarctic elements are gradually added with increasing latitude, with dominance shifting from Ammophila to Elymus along the Newfoundland Coast (approximately 50°--52° N latitude). The Atlantic Coast to the north is characterized by a circumarctic element described at the beginning of this section. Beach vegetation of the Canadian maritimes has been described in detail by D.Thannheiser (1984) and G.Lamoureux and M.M. Grandtner (1977, 1978). A littoral fringe, dominated by Cakile edentula, is replaced farther inland by Elymus mollis, Honkenya peploides, and Mertensia maritima (the latter is especially common on shingle or coarse sand beaches). More inland yet, dunes are dominated by Ammophila breviligulata, Festuca rubra, and Lathyrus japonicus.

Conclusion

Our tour of North American vegetation began with the Arctic, and it has ended there. We can conclude for virtually every vegetation type that the depth of our understanding does not match the scale of its distributional range. In some areas, alpha-level descriptions of modern and past vegetation still remain to be accumulated and published. In other regions, such important ecosystem attributes as productivity and nutrient cycling are essentially unknown. And in most cases, we have very rudimentary knowledge about the autecology of more than one or two dominant species: details about their demography, ecophysiology, response to human-mediated disturbance, and biotic relationships with associated plants and animals await discovery. Compared to some parts of the world, such as Europe, our mastery of vegetation is very superficial.

The passage of time will not guarantee better understanding, because the extent of natural vegetation annually grows smaller. Vast areas of some types of western and northern vegetation still remain as wilderness, but many of our deciduous forests, grasslands, deserts, woodlands, montane coniferous forests, beaches, and marshes are endangered. Historical accounts that document what we had 100--200 years ago (J.Bakeless 1961; M.Williams 1989) are, in turns, exhilarating and depressing as a sense of discovery turns into a sense of loss.