Tundra to Tropics: Living Tapestry of Our Plant Communities

Tundra to Tropics: 42

Living Tapestry The first Americans came from northeast Asia during the last Ice Age. Entering North America of Our Plant in the area of the Bering Strait, they encountered a continent Communities untouched by human culture. The plant communities they saw were shaped by the environ- By John M. Safley, Jr., and ment—an environment that did Donald T. Pendieton not include humankind. The land they occupied had meadows and steppe-tundra in the uplands. Shrub thickets grew on moist sites, and trees lined rivers and smaller streams. Grazing animals roamed in small herds across the land. Although the millennium of their arrival is still in doubt, it is likely that those peoples arrived over 20,000 years ago. By 10,000 years ago, at the end of the Pleistocene epoch, human cultures were to be found over most of North America. These first American cultures had far-reach- ing impacts upon the native flora and fauna. In contrast to the stone-and- bone technology of the first Americans, Europeans brought iron, steel, and the horse. These enabled those settlers to suc- ceed, but not without significant alteration of the land. Clearing forests for settlements, farms, and wood products profoundly changed the landscape. Later chapters will address uses to which the land and its resources have been put This John M. Safley, Jr., chapter will present a broad pic- and Donald T. Pendie- ture of the native vegetation en- ton are in the countered by European colo- Ecological Sciences Division, Soil Conser- nists. We shall show how these vation Service. plant communities have mutual Our Plant Communities 43 relationships among themselves general, trees require more and to the physical environment moisture so they are associated with more humid areas. Many Habitat Factors grasses and shrubs require less Plant communities consist of water and can grow in drier plants that are adapted to condi- climates. tions of soil, climate, and topography. Some factors within these Evaporation, Change broad classes such as water, Evaporation is also an impor- light, temperature, humidity, and tant physical factor in determin- soil fertility affect plants di- ing the distribution of plants. It rectly. Others, such as soil affects both the amount of water structure and slope aspect, tend directly available for plants and to affect plants indirectly. the amount of water needed by Each habitat has a different plants. The ratio of precipitation combination of environmental to evaporation is quite impor- factors. Plants that tolerate a tant Evaporation potential tends specific combination can survive to increase from northern to in that habitat, depending upon southern latitudes; it also in- their ability to compete for site creases from higher to lower resources. In a community, elevations. many individual plants of many Change occurs in all plant species interact with their envi- communities. Change in general ronment and with each other. is orderly and follows a trend Plants produce material for from simplicity to complexity. their growth and survival by pho- This orderly change is called tosynthesis. This process of succession. converting solar energy into Early stages of succession are chemical energy can also be characterized by low species di- called primary production. Net versity and low biomass (plant primary production is the matter). The ratio of gross pro- amount of plant material stored duction to biomass is high and or remaining as plant tissue production exceeds respiration. after a plant has used what it Energy is used by many individ- needs for maintaining its life uals of a few species and pro- processes. duction per unit is high. An eco- In general, plants require system in the early stages of about 450 grams of water to pro- succession is very sensitive to duce one gram of dry matter. disturbance. Thus, the abundance and distri- In mature communities, there bution of precipitation are major is a greater accumulation of bio- factors controlling the distribu- mass and, although production tion and abundance of plants. may be high, the ratio of gross Some plants are more efficient production to biomass is low, than others in extracting mois- and production equals respira- ture from their environments. In tion. There is an increase in bio- 44 Physical, Biological, and Social Components mass, stratification, complexity, The growing season is short and and diversity of flora. Thus dis- temperatures are low. Sunlight turbance to one component does is continuous and intense not have so serious an effect on throughout the arctic growing the total system. These commu- season and is high in ultravi- nities have a high degree of olent light in alpine habitats. stability. Precipitation comes mostly as snow; permafrost and the depth The Tundra Carpet to which it thaws in summer are Tundra vegetation grows be- important in determining avail- tween the northern limit of trees able soil water. Summer winds and the region of perpetual snow produce high rates of drying and and ice in the northernmost re- place the vegetation under con- gion of Alaska (arctic tundra). siderable stress. It occurs on mountains as far south as Mexico that have a tim- Two T3rpes in Arctic ber line (alpine tundra). Arctic tundra communities are Vegetation is low, dwarfed, and of two types, Low and High Arc- grows in a thin mat In areas of tic. The Low Arctic tundra oc- more moderate climate, there is curs just north of the timberline a high proportion of sedges and and is characterized by low grasses and there are various shrubs, predominantly willows heath species, broad-leaved and birches up to about three herbs, lichens, and mosses. In feet in height The understory areas where soil moisture is not consists of dwarf heath species, limiting, shrub species are domi- cottongrass, many species of nant On wetter sites, there are sedge, mosses and lichens, and sedge-dominated meadows and a wide variety of herbs. Along bogs. rivers and streams, a tall shrub In drier locations, and in the community dominated by alders, northernmost harsher environ- birches, and willows occurs; ments, tundra vegetation is not these species can grow up to 10 continuous. In these areas, feet high. small variations in topography The second type of Arctic tun- and exposure cause marked dif- dra is the High Arctic. Along ferences in vegetation in only a coastal areas and in areas of ad- few feet On the harshest sites, equate soil moisture, this com- lichens are the only plant life. munity resembles Low Arctic Tundra looks like a carpet with tundra. However, High Arctic its lack of tree-form vegetation. tundra is more likely to resem- The number of species is small ble a semidesert In drier, more compared with temperate floras, severe climates, plants may oc- and the number of species de- cupy only 2 percent of the creases markedly northward. ground surface. In these areas, mosses and lichens predominate. Our Plant Communities 45

Alpine tundra occurs on Coniferous Forest mountains that are high enough The boreal forest, a coniferous to have a timberUne. In the east- forest, forms the southern ern United States, tundra occurs boundary of the Arctic Tundra. only on a few peaks in New Its northern limit—the treeline— England. is irregular because of the ef- Alpine tundra in western fects of topography on mountains lies far south of the microclimate. Arctic at high elevations. In Can- Climate is generally less se- ada, alpine tundra occurs as low vere than that of the tundra; the as 600 feet Southward its lowest growing season is cool and short elevation increases about 360 (June through August). Annual feet per degree of latitude into precipitation averages between lower California. In the Rocky IV2 and 2 inches; because sum- Mountains, alpine tundra com- mer temperatures are cool (usu- munities can be found between ally less than 86° F), évapotran- 10,000 and 14,000 feet spiration is low and there are Alpine tundra consists of many lakes and bogs. sedges, grasses, and herbs. On Soils are thin where the region wet sites these plants form a was glaciated. Along the boreal dense mat; on alpine boulder forest's southern boundary, soils fields, plants occupy spaces be- are deep, having resulted from tween the rocks. moraine deposits and glacial Productivity of all tundra com- outwash. In its northernmost munities is very low because of areas, soils may be permanently the cold temperatures, short frozen. growing season, and low soil fer- Along its southern border, bo- tility. Net primary production real forest blends subtly into the ranges from 900 to 3,600 pounds Hemlock-White Pine-Northern of dry weight per acre yearly. Hardwoods forests in the East Production of some High Arctic In south central Canada, the for- communities is as low as 80 est is dominated by aspen and pounds an acre. coniferous stands which form Because its productivity is low, the transition into grassland. In tundra was not much used by the West, it blends into the humans in premodern times. In Rocky Mountain and northwest recent years, grazing by sheep coastal coniferous forests. and other livestock, oil explora- Spruce and fir are the domi- tion and extraction, agriculture, nant species of this forest type. and recreational pursuits have White spruce, black spruce, and contributed to a deterioration in balsam fir are common. Black tundra. Time required for re- spruce and larch are common in growth and restoration of dam- bogs in the southern part of the aged tundra is measured in dec- forest; in northern reaches these ades, not years. trees occupy drier, more exposed sites. 46 Physical, Biological, and Social Components

Reindeer Lichen Subdominant species include There are relatively few sec- birches, eastern hemlock, and ondary, or subdominant, species various heaths, azaleas, and in the boreal forest Paper birch rhododendrons. is one common subdominant Most shrubs are either heaths or Range in Rockies willows. In the Rocky Mountains, mon- Reindeer lichen is the predom- tane forests occur from northern inant lichen, as it is in the tun- Alberta to New Mexico and from dra. Sphagnum species are im- western South DEikota to central portant in bog formation and Utah. They are not continuous succession. over this range, and where they As with tundra, net primary do occur can be divided into up- productivity for the boreal forest per and lower zones; each has is low. Cool temperatures, short an elevational range of about growing season, and low soil fer- 2,100 feet tility limit production to only The forests grade into subal- about 4,500 pounds of dry mat- pine communities upslope and ter per acre yearly. During the deciduous forests downslope. growing season, however, daily They are dominated by Douglas- net primary production rates are fir in the upper zone and ponde- comparable to those of some rosa pine in the lower. Blue temperate communities. spruce and white fir grow in the Montane coniferous forests are southern Rockies on wetter along the higher ridges and sites. Ponderosa pine forests are mountain crests of the Appala- open and park-like with a grassy chian Mountains, and in similar ground cover of fescues, blue- situations in the Rocky Moun- grasses, and wheatgrasses. tains and Cascade Mountains of In the Cascades and Sierra the West They are quite similar Nevadas, montane forests are to the boreal forest in physical narrowly distributed from British environment and in species and Columbia southward to Califor- life forms as well. nia. Douglas-fir and white fir In the Appalachians, the mon- dominate the upper montane in tane forest extends from New the Cascades; Douglas-fir does England to North Carolina and not grow in upper montane for- Tennessee. In its northern por- ests in the Sierras. tion, it occurs at elevations from Upslope this forest intergrades 2,600 to 4,000 feet Southward into a subalpine forest domi- from New England it occurs at nated by mountain hemlock and ever higher elevations; at its lodgepole pine. Downslope the southernmost limit it is found upper montane grades into a above 5,300 feet ponderosa pine lower montane Along its range from north to forest very similar to forests in south, red spruce replaces bal- the Rocky Mountains. sam fir as the dominant species. Our Plant Communities 47

Montane forests distributed from of the Cascades British and Sierra Columbia to Nevadas are California. 48 Physical, Biological, and Social Components

Coastal Forest Rockies and the grasslands and The northwest coastal conifer deserts of lower elevations. forest parallels the Pacific Coast Pinyon-juniper stands gener- from Alaska to central Califor- ally grow in association with big nia. It occurs from sea level to sagebrush, bitterbrush, oak spe- 5,000 feet and for distances up cies, and other shrubs. Where to 100 miles from the ocean. It the stands are more open, also parallels the montane forest grasses such as bluegrama, in- of the Sierra Nevadas and dianricegrass, needlegrasses, Cascades. wolftail, and muhlys often form a Its climate is temperate be- herbaceous ground cover. cause of the moderating effect of the Pacific. Annual precipitation Deciduous Forest varies from about 28 inches to The deciduous forest of North 100 inches. Humidity is always America occurs in the eastern high, and heavy fogs provide United States from the Great much of the soil moisture during Lakes south to the Florida pen- dry summer months. insula and westward to the In general, final dominants of Ozark Mountains. It extends this forest are grand fir, western about halfway down the Florida hemlock, and western redcedar. peninsula, where it merges with Douglas-fir is the most wide- tropical flora. In the north it spread species; however, be- gradually blends into the boreal cause of its intolerance to shade forest from Minnesota to Maine. and its role in succession fol- At its western limits, the deci- lowing fire, it is considered dom- duous forest grades into grass- inant on disturbed sites. North land as the climate becomes of Puget Sound, Sitka spruce is drier. The western boundary an important species. closely follows a line marking The southern part of this for- rainfall equal to 80 percent of est is dominated by redwood. evaporation potential. Fire has These giants occur in pure also played a role in maintaining stands in areas of heavy coastal this western limit fog, especially coastal valleys Although the deciduous forest and river bottoms in Californiia. is extensive, it is far from uni- From the semiarid regions of form. The terrain varies, as do the southwestern United States the principal species. northwest to the Great Basin Where glaciers covered the (between the Rockies and the northern section, the land is rel- Sierras) and southward into atively flat; there are few sharp Mexico, an association of pinyon boundaries between forest asso- pine and juniper species forms a ciations here. The mountains in widespread vegetation type of the east trend northeast to dwarf trees and large shrubs. It southwest; in this section, dis- occupies the landscape between tinct boundaries generally exist the ponderosa pine forests of the between forest communities. Our Plant Communities 49

Productivity Varies Mountains; to the west and east, For all Ulis diversity, there are it is in valleys. Toward Ohio, it is some common and consistent a mixed hardwood forest with environmental factors: Distinct fewer species. seasons, the occurrence of frosts and/or freezes in winter, Beech, Maple, Basswood precipitation distributed In the northward arm of the throughout the year, and moder- mixed mesophytic forest, both ate- to long-growing seasons American beech and sugar maple (from 140 to 365 frost-free days). increase in dominance. This Net primary productivity of de- beech-maple association occurs ciduous forests varies greatly. north and west of the Alleghen- Because of their latitude, these ies from New York to Ohio into forests tend to have a longer Wisconsin. Common species are growing season than do most red maple, elm, and black boreal forest associations. Soil cherry. Forests of this associa- moisture, exposure, and topo- tion grow best in well-drained graphic position all influence soils. productivity in forests and ac- The maple-basswood associa- count for most of the variation tion is the smallest of deciduous in net production per acre. forests in extent Its range in- Net primary productivity for cludes southwestern Wisconsin, these forest associations ranges southeastern Minnesota, and from 3,600 pounds per acre to northeastern Iowa, Beech is re- 22,500 pounds annually, or placed by basswood; otherwise roughly twice the rate for the bo- this forest differs very little from real forest the beech-maple association. The mixed mesophytic forest The hemlock-hardwoods asso- association occurs at the center ciation, a transitional forest, lies of the forest throughout the between the boreal coniferous Cumberland and Appalachian forests and deciduous forest It plateaus. It is the most diverse extends from Nova Scotia deciduous forest through the Lake States to Min- There are 25 to 30 species that nesota. Across this range, hem- may have climax status, depend- lock stands are interspersed ing upon topography, microcli- among hardwood stands. mate, and soil moisture. The Hardwood stands are domi- most widespread dominants in- nated by sugar maple, American clude sweet buckeye, American beech, birches, and aspen. Spe- beech, several basswoods, and cies associated with hemlocks tuliptree. include white pine and red pine. Away from the central area, Bogs in the region support larch, this forest type is increasingly white cedar, and black spruce. restricted to specific habitats. To the south, it is in moist coves in the Appalachian and Unaka 50 Physical, Biological, and Social Components

Disturbance of the forest by and overcup 0£ik are on moister fire or clearing initiates second- sites. Subdominant species in- ary succession, which typically clude flowering dogwood and ends in a white pine or aspen sweetgum. stand. Old field succession typically passes through a pine stage be- Oak, Chestnut, Hickory fore the hardwood climax forest The oak-chestnut association is achieved. extends from New England to Georgia along the mountains. It Bottom Land, Fire-Swamp retains the name chestnut even Flood plain or bottom land for- though chestnut blight has ests occur throughout the deci- nearly eliminated American duous forests. Those in the chestnut from the forest Before southern part of the deciduous the blight this forest was wide- forests are dominated by oak, spread on slopes and uplands baldcypress, and gums. Those of east and southeast of the mixed the northern deciduous forests mesophytic forest are dominated by elms. Because blight gradually killed the chestnut, associated species maintained their dominance. The oak-hickory association is the most widespread of the deciduous forests because its species are drought-tolerant It extends across much of the Piedmont Plateau and the Atlan- tic and Gulf States Coastal Plains to eastern Texas. From this area it is more or less continuous to Minnesota. In its northwestern limit it becomes parklike. In Texas and Oklahoma, post oak and blackjack oak form open stands, known as the cross timbers, which mark the transition to grassland. White oak, red oak, black oak, post oak, blackjack oak, mock- ernut hickory, bittemut hickory, and shagbark hickory are among the dominant species throughout the oak-hickory association. Beech, sugar maple, willow oak. Our Plant Communities 51 cottonwoods, and willows. The tected from flre are replaced by northern flood plain forests ex- oak-hickory forest tend into the grasslands, form- Undrained depressions form ing what have been called "gal- upland bogs, pocosins (swamps) lery forests." and "Carolina bays" in which The coastal plain of the east- evergreen shrubs predominate. ern U.S. extends from Texas to Permanent standing water in the New Jersey. In many places the lower coastal plain results in soils are sandy and drainage is extensive marshes dominated by poor. Fire in dry seasons and the rushes and grass. Areas that are height of the water table in wet periodically flooded develop seasons are critical to plant swamp forests dominated by community development in this gum and baldcypress. region. Areas in the South influenced Forests of loblolly pine in the by salt spray and intermittent South and of pitch pine in the flooding have an evergreen var- Mid-Atlantic States were and are iant of the oak-hickory associa- maintained by fire; stands pro- tion dominated by live oak.

Periodically flooded areas of the eastern coastal plain are dominated by gum and cypress. 52 Physical, Biological, and Social Components

Truth About Deserts The Mojave, the smallest des- Desert formations extend ert, extends from the southern throughout much of western tip of Nevada into adjacent Cali- North America. The common fornia. Though vegetationally conception of them is a dry, hot, similar to the Great Basin, it is a and often dusty landscape in transitional area to the Sonoran which there is little life. Desert farther south. While it is true that precipita- Precipitation, usually less than tion is low, temperatures can 10 inches annually, falls mainly fluctuate widely and in the in winter. Summers are very hot northern formations frosts can and dry; temperatures over 100° occur throughout the year. F are common. Joshua tree is Strong winds and low relative important in areas of lesser ele- humidity are characteristic too. vation and rainfall. Dominant plants can survive The Sonoran Desert occurs in long periods without rainfall southeastern California eastward through adaptations such as into Arizona and southward into shedding leaves or storing water Baja California. This lowland in their tissues. area (less than 3,000 feet in ele- There are two desert forma- vation) has level plains with tions: The Cold Desert, which some small mountains. Precipi- corresponds to the Great Basin tation varies, ranging from about area, and the Hot Desert, which 2 inches in the west to about 28 occurs in the southwestern inches in the mountains. United States and extends into The Sonoran Desert is the Mexico. Net primary production most diverse of all North Ameri- is low, ranging from 60 to 2,250 can deserts. Low plains there are pounds per acre yearly in the dominated by creosote bush and Hot Desert The wide variability bur sage, with palo verde and sa- is a function of rainfall. guaro cactus becoming prevalent The Cold Desert has two major as elevation increases. plant communities. One, domi- The Chihuahuañ Desert oc- nated by big sagebrush, is in the curs at elevations from 4,000 to northern part or at higher eleva- 6,100 feet from west Texas and tions throughout The other is southern New Mexico into Mex- dominated by shadscale (or salt ico. Rainfall comes mostly in desert shrub). Rainfall in this summer and varies from 3 to 12 desert ranges from 4 to 12 inches yearly; winter frosts do inches per year. occur. Many shrub species grow, as Parts of the Hot Desert do a variety of succulents. Creo- The Hot Desert formation has sote bush, mesquite, ocotillo, three subunits: The Mojave, Chi- and tarbush are found across huahuañ, and Sonoran deserts. this desert Many species of yucca and agave are also mem- bers of this flora. Our Plant Communities 53

Grasslands Grasses are found in almost all plant communities. Plant communities dominated by grasses are subject to wide variations in season-to-season and year-to-year temperature and precipitation. Because grasses can tolerate these fluctuations better than can trees, there is a broad grass- dominated region from southern Saskatchewan and Alberta to eastern Texas and from Indiana westward to the woodland zone in the Rockies. Grasses are a codominant type of vegetation east of the tallgrass prairie; they occur as an understory of savanna ecosystems. Net primary productivity of U.S. grasslands ranges from 700 to 4,000 pounds per acre annually. Many wet meadows and subirrigated sites exceed 5 tons per acre yearly. Nationwide, however, 40 percent of U.S. grasslands yield less than 1,800 pounds per acre annually and 75 percent less than 2,700 pounds.

Tallgrass: The tallgrass prairie, often referred to as the true prairie, once dominated a landscape of over 40 million acres along the eastern edge of the Great Plains and eastward into Illinois, Indiana, and Missouri. At its southern extremity in Texas and Oklahoma, average an- The Sonaran is the most Desert extends diverse of all nual precipitation ranges from from southeast- North American 30 to 40 inches and evaporation ern California deserts. Sagu- is correspondingly high. At the eastward into aro cactus northern end, precipitation Arizona and becom.es preva- ranges from 20 to 26 inches. south into Baja lent as eleva- California. It tion increases. 54 Physical, Biological, and Social Components

Deep-rooted, tall-growing of the Rocky Mountains. It en- grasses—including bluestem, compasses some 280 million indiangrass, little bluestem, and acres; the topography is gently switchgrass—were the dominant sloping to extremely flat Average plants. Woody vegetation was annual precipitation ranges from found only along water courses. one-third to one-half of potential evaporation, or from 10 inches Soils and the Com Belt in the north to more than 25 in The soils, which formed under the south. these tall grasses and which in Dominant vegetation of this turn supported these productive grassland consists of buffalo- plant communities, are the deep, grass, wheatgrass, and blue dark, fertile soils that now con- grama. Net primary production stitute the prime farmlands of ranges from 450 to 2,700 pounds the Corn Belt per acre. It was not uncommon for the Sandwiched between the tall- best sites in the tallgrass prairie grass prairie and the shortgrass to produce from 3,000 to over plains is a narrow belt transi- 5,000 pounds of dry matter per tional between the two and dom- acre annually. inated by grasses common to both. This area is often referred Shortgrtiss: The shortgrass to as the mixed prairie or mixed prairie stretches from the Cana- grass prairie. dian border to west central Texas. East to west, this grass- Sagebrush and Grass land extends from the middle of West of the plains grasslands, the Great Plains to the foothills grasses are often codominant

U"^.». ■.'.■m- Our Plant Communities 55 with drought-resistant shrubs. rain from February through Oc- The big sagebrush ecosystems of tober. The mean temperature for the Great Basin, Columbia and this period is 64° F. There is a Colorado Plateaus, and the Wyo- moderate dry season from Octo- ming Basin are examples. ber to February. Several wheatgrass, bluegrass, Forests on the Florida coast fescues, and bromes are codomi- and on the Keys are made up of nant herbs. Associated shrubs trees with hard, evergreen include bitterbrush, rabbitbrush, leaves; twisted trunks; and and other sagebrush species. dense wood. The trees make a These plant communities oc- deep shade that allows few her- cur in areas 600 to 9,900 feet in baceous species. Lignumvitae, elevation where average annual gumbo limbo, Florida strangler precipitation ranges from 5 to 16 fig, and palms are among the inches. Average annual herbage more common species. production ranges from 180 to 1,800 pounds per acre depending Hammocks, Mangroves on the soil, climate, topography, In the glades south of Lake and vegetation on the site. Okeechobee are habitats called hammocks, which are raised Tropical communities: above their wetter surroundings. Within the United States, tropi- Their dominant species are cal vegetation occurs only on the mixed with some tropical spe- southern tip of Florida and on cies and a few northern deci- the Florida Keys. This flora oc- duous species. Sabal palmetto is curs at or below 25 feet sea level a common hammock species, as and receives over 50 inches of are live oak and pignut hickory. Coastal areas in this region tend to have a mangrove climax. Red mangrove grows offshore and merges into black mangrove which, in turn, intergrades into button mangrove on the shore. The tangle of mangrove roots and stems can help to build the shoreline by trapping sediment. Productivity of these tropical communities is high due to a long growing season and the intense sunlight S In areas west Jescues and of the plains bromes are co- grasslands, dominant with wheatgrass, drought-resist- bluegrass. ant shrubs.