Final Paper Outline
CHAPTER 23
Smith, T. M., & Smith, R. L. (2015). Elements of Ecology (9th ed.). Boston, MA: Pearson.
23.1 Terrestrial Ecosystems Reflect Adaptations of the Dominant Plant Life-Forms
Given that the broad classification of terrestrial biomes presented in Figures 23.1 and 23.2 (forest, woodland/savanna, shrubland, and grassland) reflects the relative contribution of three general plant life-forms (trees, shrubs, and grasses), the question of what controls the distribution of biomes relative to climate becomes: Why are there consistent patterns in the distribution and abundance of these three dominant plant life-forms that relate to climate and the physical environment? The answer to this question lies in the adaptations that these three very different plant life-forms possess, as well as the advantages and constraints arising from these adaptations under different environmental conditions.
Although the broad categories of grasses, shrubs, and trees each represent a diverse range of species and characteristics, they have fundamentally different patterns of carbon allocation and morphology (see Chapter 6). Grasses allocate less carbon to the production of supportive tissues (stems) than do woody plants (shrubs and trees), enabling grasses to maintain a higher proportion of their biomass in photosynthetic tissues (leaves). For woody plants, shrubs allocate a lower percentage of their resources to stems than do trees. The production of woody tissue gives the advantage of height and access to light, but it also has the associated cost of maintenance and respiration. If this cost cannot be offset by carbon gain through photosynthesis, the plant is unable to maintain a positive carbon balance and dies (see Chapter 6). As a result, as environmental conditions become adverse for photosynthesis (dry, low nutrient concentrations, or short growing season and cold temperatures), trees decline in both stature and density until they can no longer persist as part of the plant community.
Within the broad classes of forest and woodland ecosystems in which trees are dominant or codominant, leaf form is another plant characteristic that ecologists use to classify ecosystems. Leaves can be classified into two broad categories based on their longevity. Leaves that live for only a single year or growing season are classified as deciduous, whereas those that live beyond a year are called evergreen. The deciduous leaf is characteristic of environments with a distinct growing season. Leaves are typically shed at the end of the growing season and then regrown at the beginning of the next. Deciduous leaf type is further divided into two categories based on dormancy period. Winter-deciduous leaves are characteristic of temperate regions, where the period of dormancy corresponds to low (below freezing) temperatures (Figures 23.3a and 23.3b; also see discussion of plant adaptations in Chapter 6 ). Drought-deciduous leaves are characteristic of environments with seasonal rainfall, especially in the subtropical and tropical regions, where leaves are shed during the dry period (Figures 23.3c and 23.3d). The advantage of the deciduous habit is that the plant does not incur the additional cost of maintenance and respiration during the period of the year when environmental conditions restrict photosynthesis.
Evergreen leaves can likewise be classified into two broad categories. The broadleaf evergreen leaf type (Figure 23.4a) is characteristic of environments with no distinct growing season where photosynthesis and growth continue year-round, such as tropical rain forests. The needle-leaf evergreen form (Figure 23.4b) is characteristic of environments where the growing season is very short (northern latitudes) or nutrient availability severely constrains photosynthesis and plant growth.
A simple economic model has been proposed to explain the adaptation of this leaf form (see discussion of leaf longevity in Chapter 6, Section 6.11). The production of a leaf has a “cost” to the plant that can be defined in terms of the carbon and other nutrients required to construct the leaf. The time required to “pay back” the cost of production (carbon) will be a function of the rate of net photosynthesis (carbon gain). If environmental conditions result in low rates of net photosynthesis, the period of time required to pay back the cost of production will be longer. If the rate of photosynthesis is low enough, it may not be possible to pay back the cost over the period of a single growing season. A plant adapted to such environmental conditions cannot “afford” a deciduous leaf form, which requires producing new leaves every year. The leaves of evergreens, however, may survive for several years. So under this model, we can view the needle-leaf evergreen as a plant adapted for survival in an environment with a distinct growing season, in which conditions limit the plant’s ability to produce enough carbon through photosynthesis during the growing season to pay for the cost of producing the leaves.
On combining the simple classification of plant life-forms and leaf type with the large-scale patterns of climate presented previously, we can begin to understand the distribution of biome types relative to the axes of temperature and precipitation shown in Figure 23.2. Ecosystems characteristic of warm, wet climates with no distinct seasonality are dominated by broadleaf evergreen trees and are called tropical (and subtropical) rain forest. As conditions become drier, with a distinct dry season, the broadleaf evergreen habit gives way to drought-deciduous trees that characterize the seasonal tropical forests. As precipitation declines further, the stature and density of these trees declines, giving rise to the woodlands and savannas that are characterized by the coexistence of trees (shrubs) and grasses. As precipitation further declines, trees can no longer be supported, giving rise to the arid shrublands (thorn scrub) and desert.
The temperature axis represents the latitudinal gradient from the equator to the poles (see geographical labels on x-axis of Figure 23.2). Moving from the broadleaf evergreen forests of the wet tropics into the cooler, seasonal environments of the temperate regions, the dominant trees are winter-deciduous. These are the regions of temperate deciduous forest. In areas of the temperate region where precipitation is insufficient to support trees, grasses dominate and give rise to the prairies of North America, the steppes of Eurasia, and the pampas of Argentina. Moving poleward, the temperate-deciduous forests give way to the needle-leaf–dominated forests of the boreal region (conifer forest or taiga). As temperatures become more extreme and the growing season shorter, trees can no longer be supported, and the short-stature shrubs and sedges (grasslike plants of the family Cyperaceae) characteristic of the tundra dominate the landscape ecosystems of the arctic region.
In the following sections, we will examine the eight major categories of terrestrial biomes outlined in Figure 23.2. We begin each section by relating their geographic distribution to the broad-scale constraints of regional climate, as outlined in Figure 23.2, as well as to associated patterns of seasonality in temperature and precipitation (see Quantifying Ecology 23.1) that function as constraints on the dominant plant life-forms and patterns of primary and secondary productivity. In our discussion, we emphasize the unique physical and biological characteristics defining these broad categories of terrestrial ecosystems (biomes).
To help understand the relationship between regional climate and the distribution of terrestrial ecosystems, for each biome discussed in this chapter (tropical forest, savanna, etc.), we present a map showing its global distribution. Accompanying the map is a series of climate diagrams. The diagrams describe the local climate at representative locations around the world where a particular biome type is found. See Figure 1 for a representative climate diagram, which we have labeled to help you interpret the information it presents. As you study the diagram, take particular note of the patterns of seasonality.
In Figure 23.12, what is the distinctive feature of the climate diagrams for these tropical savanna ecosystems? How do the patterns differ between sites in the Northern and Southern Hemispheres? What feature of Earth’s climate system discussed in Chapter 2 is responsible for these distinctive patterns?
In Figure 23.23, what feature of the climate is common to all mediterranean ecosystems?
23.2 Tropical Forests Characterize the Equatorial Zone
The tropical rain forests are restricted primarily to the equatorial zone between latitudes 10° N and 10° S (Figure 23.5), where the temperatures are warm throughout the year and rainfall occurs almost daily. The largest and most continuous region of rain forest in the world is in the Amazon basin of South America (Figure 23.6). The second largest is located in Southeast Asia, and the third largest is in West Africa around the Gulf of Guinea and in the Congo basin. Smaller rain forests occur along the northeastern coast of Australia, the windward side of the Hawaiian Islands, the South Pacific Islands, the east coast of Madagascar, northern South America, and southern Central America.
The climate of tropical rain forest regions varies geographically but is typically characterized by a mean temperature of all months exceeding 18°C and minimum monthly precipitation above 60 millimeters (mm; see climate diagrams for representative tropical rain forest sites in Figure 23.5). Within the lowland forest zone, mean annual temperatures typically exceed 25°C with an annual range less than 5°C.
Tropical rain forests have a high diversity of plant and animal life. Covering only 6 percent of the land surface, tropical rain forests account for more than 50 percent of all known plant and animal species. Tree species number in the thousands. A 10-km2 area of tropical rain forest may contain 1500 species of flowering plants and up to 750 species of trees. The richest area is the lowland tropical forest of peninsular Malaysia, which contains some 7900 species.
Nearly 90 percent of all nonhuman primate specie live in the tropical rain forests of the world (Figure 23.7). Sixty-four species of New World primates—small mammals with prehensile tails—live in the trees. The Indo-Malaysian forests are inhabited by a number of primates, many with a limited distribution within the region. The orangutan, an arboreal ape, is confined to the island of Borneo. Peninsular Malaysia has seven species of primates, including three gibbons, two langurs, and two macaques. The long-tailed macaque is common in disturbed or secondary forests, and the pig-tailed macaque is a terrestrial species adaptable to human settlements. The tropical rain forest of Africa is home to mountain gorillas and chimpanzees. The diminished rain forest of Madagascar holds 39 species of lemurs (see Chapter 9, and Ecological Issues & Applications and Figure 9.20).
Tropical rain forests may be divided into five vertical layers (Figure 23.8): emergent trees, upper canopy, lower canopy, shrub understory, and a ground layer of herbs and ferns. Conspicuous in the rain forest are lianas—climbing vines—growing upward into the canopy, epiphytes growing on the trunks and branches, and strangler figs (Ficus spp.) that grow downward from the canopy to the ground. Many large trees develop plank-like outgrowths called buttresses (Figure 23.9). They function as prop roots to support trees rooted in shallow soil that offers poor anchorage. The floor of a tropical rain forest is thickly laced with roots, both large and small, forming a dense mat on the ground.
The continually warm, moist conditions in rain forests promote strong chemical weathering and rapid leaching of soluble materials. The characteristic soils are oxisols, which are deeply weathered with no distinct horizons (see Chapter 4 for discussion and classification of soils). Ultisols may develop in areas with more seasonal precipitation regimes and are typically associated with forested regions that exhibit seasonal soil moisture deficits. Areas of volcanic activity in parts of Central and Southeast Asia, where recent ash deposits quickly weather, are characterized by andosols (see Figure 4.12).
The warmer, wetter conditions of the tropical rain forest result in high rates of net primary productivity and subsequent high annual rates of litter input to the forest floor. Little litter accumulates, however, because decomposers consume the dead organic matter almost as rapidly as it falls to the forest floor. Most of the nutrients available for uptake by plants are a result of the rapidly decomposed organic matter that is continuously falling to the soil surface. Growing plants, however, rapidly absorb these nutrients. The average time for leaf litter to decompose is 24 weeks.
Moving from the equatorial zone to the regions of the tropics that are characterized by greater seasonality in precipitation, the broadleaf evergreen forests are replaced by the dry tropical forests (Figure 23.10). Dry tropical forests undergo a dry season whose length is based on latitude. The more distant the forest is from the equator, the longer is the dry season—in some areas, up to eight months. During the dry season, the drought-deciduous trees and shrubs drop their leaves. Before the start of the rainy season, which may be much wetter than the wettest time in the rain forest, the trees begin to leaf. During the wet season, the landscape becomes uniformly green.
The largest proportion of tropical dry forest is found in Africa and South America, to the south of the zones dominated by rain forest. These regions are influenced by the seasonal migration of the Intertropical Convergence Zone (see Section 2.6, Figure 2.18). In addition, areas of Central America, northern Australia, India, and Southeast Asia are also classified as dry tropical forest. Much of the original forest, especially in Central America and India, has been converted to agricultural and grazing land.
23.3 Tropical Savannas Are Characteristic of Semiarid Regions with Seasonal Rainfall
The term savanna was originally used to describe the treeless areas of South America. Now it is generally applied to a range of vegetation types in the drier tropics and subtropics characterized by a ground cover of grasses with scattered shrubs or trees. Savanna includes an array of vegetation types representing a continuum of increasing cover of woody vegetation, from open grassland to widely spaced shrubs or trees and to woodland (Figure 23.11). In South America, the more densely wooded areas are referred to as cerrado. The campos and llano are characterized by a more open appearance (lower density of trees), and thorn scrub is the dominant cover of the caatinga. In Africa, the miombo, mopane, and Acacia woodlands can be distinguished from the more open and park-like bushveld. Scattered individuals of Acacia and Eucalyptus dominate the mulga and brigalow of Australia.
The physiognomic diversity of the savanna vegetation reflects the different climate conditions occurring throughout this widely distributed ecosystem (Figure 23.12). Moisture appears to control the density of woody vegetation, a function of both rainfall (amount and distribution) and soil—its texture, structure, and water-holding capacity (Figure 23.13; also see Chapter 4 ).
Savannas are associated with a warm continental climate with distinct seasonality in precipitation and a large interannual (year to year) variation in total precipitation (see climate diagrams for representative savanna sites in Figure 23.12). Mean monthly temperatures typically do not fall below 18°C, although during the coldest months in highland areas, temperatures can be considerably lower. There is seasonality in temperatures, and maximum temperatures occur at the end of the wet season. The nature of the vegetation cover, however, is more closely determined by the amount and seasonality of precipitation than by temperature.
(Adapted from Archibold 1995.)
Savannas, despite their differences in vegetation, exhibit a certain set of characteristics. Savannas occur on land surfaces of little relief—often on old plateaus, interrupted by escarpments and dissected by rivers. Continuous weathering in these regions has produced nutrient-poor oxisols, which are particularly deficient in phosphorus. Alfisols are common in the drier savannas, whereas entisols are associated with the driest savannas (see Figure 4.12). Subject to recurrent fires, the dominant vegetation is fire adapted. Grass cover with or without woody vegetation is always present, and the woody component is short-lived—individuals seldom survive for more than several decades. Savannas are characterized by a two-layer vertical structure because of the ground cover of grasses and the presence of shrubs or trees (see Figure 16.12b).
The yearly cycle of plant activity and subsequent productivity in tropical savannas is largely controlled by the markedly seasonal precipitation and corresponding changes in available soil moisture. Most leaf litter is decomposed during the wet season, and most woody debris is consumed by termites during the dry season.
The microenvironments associated with tree canopies can influence species distribution, productivity, and soil characteristics. Stem flow and associated litter accumulation result in higher soil nutrients and moisture under tree canopies, often encouraging increased productivity and the establishment of species adapted to the more shaded environments.
Savannas can support a large and varied assemblage of herbivores—invertebrate and vertebrate, grazing and browsing. The African savanna, visually at least, is dominated by a large and diverse ungulate fauna of at least 60 species that share the vegetative resources. Some species, such as the wildebeest and zebra, are migratory during the dry season (see Figure 7.10 for example).
Savanna vegetation supports an incredible number of insects: flies, grasshoppers, locusts, crickets, carabid beetles, ants, and detritus-feeding dung beetles and termites. Mound-building termites excavate and move tons of soil, mixing mineral soil with organic matter. Some species construct extensive subterranean galleries and others accumulate organic matter.
Preying on the ungulate fauna is an array of carnivores including the lion, leopard, cheetah, hyena, and wild dog. Scavengers, including vultures and jackals, subsist on the remains of prey killed by carnivores.
23.4 Grassland Ecosystems of the Temperate Zone Vary with Climate and Geography
Natural grasslands occupy regions where rainfall is between 25 and 80 centimeters (cm) a year, but they are not exclusively climatic. Many exist through the intervention of fire and human activity. Conversions of forests into agricultural lands and the planting of hay and pasturelands extended grasslands into once forested regions. Formerly covering about 42 percent of the land surface of Earth, natural grasslands have shrunk to less than 12 percent of their original size because of conversion to cropland and grazing lands.
The natural grasslands of the world occur in the midlatitudes in midcontinental regions, where annual precipitation declines as air masses move inward from the coastal environments (Figure 23.14; see Section 2.7 for discussion of continental patterns of precipitation). In the Northern Hemisphere, these regions include the prairies of North America and the steppes of central Eurasia. In the Southern Hemisphere, grasslands are represented by the pampas of Argentina and the veld of the high plateaus of southern Africa. Smaller areas occur in southeastern Australia and the drier parts of New Zealand.
The temperate grassland climate is one of recurring drought, and much of the diversity of vegetation cover reflects differences in the amount and reliability of precipitation. Grasslands do the least well where precipitation is lowest and the temperatures are high. They are tallest in stature and the most productive where mean annual precipitation is greater than 800 mm and mean annual temperature is above 15°C. Thus, native grasslands of North America, influenced by declining precipitation from east to west, consist of three main types distinguished by the height of the dominant species: tallgrass, mixed-grass, and shortgrass prairie (Figure 23.15). Tallgrass prairie (Figure 23.16a) is dominated by big bluestem (Andropogon gerardi), growing 1 meter (m) tall with flowering stalks 1 to 3.5 m tall. Mixed-grass prairie (Figure 23.16b), typical of the Great Plains, is composed largely of needlegrass–grama grass (Bouteloua–Stipa). South and west of the mixed prairie and grading into the desert regions is the shortgrass prairie (Figure 23.16c), dominated by sod-forming blue grama (Bouteloua gracilis) and buffalo grass (Buchloe dactyloides), which has remained somewhat intact, and desert grasslands. From southeastern Texas to southern Arizona and south into Mexico lies the desert grassland, similar in many respects to the shortgrass plains, except that three-awn grass (Aristida spp.) replaces buffalo grass. Confined largely to the Central Valley of California is annual grassland. It is associated with a mediterranean climate (see Section 23.6) characterized by rainy winters and hot, dry summers. Growth occurs during early spring, and most plants are dormant in summer, turning the hills a dry tan color accented by the deep green foliage of scattered California oaks.
At one time, the great grasslands of the Eurasian continent extended from eastern Europe to western Siberia south to Kazakhstan. These steppes, treeless, except for ribbons and patches of forest, are divided into four belts of latitude, from the mesic meadow steppes in the north to semiarid grasslands in the south.
In the Southern Hemisphere, the major grasslands exist in southern Africa and southern South America. Known as pampas, the South American grasslands extend westward in a large semicircle from Buenos Aires and cover about 15 percent of Argentina. These pampas have been modified by the introduction of European forage grasses and alfalfa (Medicago sativa), and the eastern tallgrass pampas have been converted to wheat and corn. In Patagonia, where annual rainfall averages about 25 cm, the pampas change to open steppe.
The velds of southern Africa (not to be confused with savanna) occupy the eastern part of a high plateau 1500 to 2000 m above sea level in the Transvaal and the Orange Free State.
Australia has four types of grasslands: arid tussock grassland in the northern part of the continent, where the rainfall averages between 20 and 50 cm, mostly in the summer; arid hummock grasslands in areas with less than 20 cm rainfall; coastal grasslands in the tropical summer rainfall region; and subhumid grasslands along coastal areas where annual rainfall is between 50 and 100 cm. However, the introduction of fertilizers, nonnative grasses, legumes, and sheep grazing have changed most of these grasslands.
Grasslands support a diversity of animal life dominated by herbivorous species, both invertebrate and vertebrate. Large grazing ungulates and burrowing mammals are the most conspicuous vertebrates (Figure 23.17). The North American grasslands were once dominated by huge migratory herds of millions of bison (Bison bison) and the forb-consuming pronghorn antelope (Antilocarpa americana). The most common burrowing rodent was the prairie dog (Cynomys spp.), which along with gophers (Thomomys and Geomys spp.) and the mound-building harvester ants (Pogonomyrex spp.), appeared to be instrumental in developing and maintaining the ecological structure of the shortgrass prairie.
The Eurasian steppes and the Argentine pampas lack herds of large ungulates. On the pampas, the two major large herbivores are the pampas deer (Ozotoceros bezoarticus), and, farther south, the guanaco (Lama guanicoe), a small relative of the camel. These species, however, are greatly reduced in number compared with the past.
The African grassveld once supported great migratory herds of wildebeest (Connochaetes taurinus) and zebra (Equus spp.) along with the associated carnivores, the lion (Panthera leo), leopard (Panthera pardus), and hyena (Crocuta crocuta). The great ungulate herds have been destroyed and replaced with sheep, cattle, and horses.
Many forms of Australian marsupial mammals evolved that are the ecological equivalents of placental grassland mammals. The dominant grazing animals are several kangaroo species, especially the red kangaroo (Macropus rufus) and the gray kangaroo (Macropus giganteus).
Grasslands evolved under the selective pressure of grazing. Thus, up to a point, grazing stimulates primary production. Although the most conspicuous grazers are large herbivores, the major consumers in grassland ecosystems are invertebrates. The heaviest consumption takes place belowground, where the dominant herbivores are nematodes.
The most visible feature of grassland is the tall, green, ephemeral herbaceous growth that develops in spring and dies back in autumn. One of the three strata in the grassland, it arises from the crowns, nodes, and rosettes of plants hugging the soil. The ground layer and the belowground root layer are the other two major strata of grasslands. The highly developed root layer can make up more than half the total plant biomass and typically extends fairly deep into the soil.
Depending on their history of fire and degree of grazing and mowing, grasslands accumulate a layer of mulch that retains moisture and, with continuous turnover of fine roots, adds organic matter to the mineral soil. Dominant soils of the grasslands are mollisols with a relatively thick, dark-brown to black surface horizon that is rich in organic matter (see Figure 4.12). Soils typically become thinner and paler in the drier regions because less organic material is incorporated into the surface horizon.
The productivity of temperate grassland ecosystems is primarily related to annual precipitation (Figure 23.18), yet temperature can complicate this relationship. Increasing temperatures have a positive effect on photosynthesis but can actually reduce productivity by increasing the demand for water.
23.5 Deserts Represent a Diverse Group of Ecosystems
The arid regions of the world occupy from 25 to 35 percent of Earth’s landmass (Figure 23.19). The wide range reflects the various approaches used to define desert ecosystems based on climate conditions and vegetation types. Much of this land lies between 15° and 30° latitude, where the air that is carried aloft along the Intertropical Convergence Zone subsides to form the semipermanent high-pressure cells that dominate the climate of tropical deserts (see Figure 2.17). The warming of the air as it descends in addition to cloudless skies result in intense radiation heat during the summer months.
Temperate deserts lie in the rain shadow of mountain barriers or are located far inland, where moist maritime air rarely penetrates. Here, temperatures are high during the summer but can drop to below freezing during the winter months. Thus, the lack of precipitation, rather than continually high temperature, is the distinctive characteristic of all deserts.
Most of the arid environments are found in the Northern Hemisphere. The Sahara, the world’s largest desert, covers approximately 9 million km2 of North Africa. It extends the breadth of the African continent to the deserts of the Arabian Peninsula, continuing eastward to Afghanistan and Pakistan and finally terminating in the Thar Desert of northwest India. The temperate deserts of Central Asia lie to the north. The most westerly of these is the Kara Kum desert region of Turkmenistan. Eastward lie the high-elevation deserts of western China and the high plateau of the Gobi Desert.
A similar transition to temperate desert occurs in western North America. Here, the Sierra Nevada effectively blocks the passage of moist air into the interior of the Southwest. Mountain ranges run parallel to the Sierras throughout the northern part of this region, and desert basins occur on the eastern sides of these ranges.
Apart from the drier parts of southern Argentina, the deserts of the Southern Hemisphere all lie within the subtropical high-pressure belt that mirrors that of the Northern Hemisphere (see preceding discussion). Cold ocean currents also contribute to the development of arid coastal regions (see Section 2.4). Drought conditions are severe along a narrow strip of the coast that includes Chile and Peru. The drier parts of Argentina lie in the rain shadow of the Andes.
The deserts of southern Africa include three regions. The Namib Desert occupies a narrow strip of land that runs along the west coast of Africa from southern Angola to the border of the cape region of South Africa. This region continues south and east across South Africa as the Karoo, which merges with the Kalahari Desert to the north in Botswana. The most extensive region of arid land in the Southern Hemisphere is found in Australia, where more than 40 percent of the land is classified as desert.
Deserts are not the same everywhere. Differences in moisture, temperature, soil drainage, topography, alkalinity, and salinity create variations in vegetation cover, dominant plants, and groups of associated species. There are hot deserts and cold deserts, extreme deserts and semideserts, ones with enough moisture to verge on being grasslands or shrublands, and gradations between those extremes within continental deserts.
Cool deserts—including the Great Basin of North America, the Gobi, Takla Makan, and Turkestan deserts of Asia—and high elevations of hot deserts are dominated by Artemisia and chenopod shrubs (Figure 23.20). They may be considered shrub steppes or desert scrub. In the Great Basin of North America, the northern, cool, arid region lying west of the Rocky Mountains is the northern desert scrub. The climate is continental, with warm summers and prolonged cold winters. The vegetation falls into two main associations: one is sagebrush, dominated by Artemisia tridentata, which often forms pure stands; the other is shadscale (Atriplex confertifolia), a C4 species, and other chenopods (halophytes—tolerant of saline soils).
A similar type of desert scrub exists in the semiarid inland of southwestern Australia. Many chenopod species, particularly the saltbushes of the genera Atriplex and Maireana, form extensive low shrublands on low riverine plains.
The hot deserts range from those lacking vegetation to ones with some combination of chenopods, dwarf shrubs, and succulents (Figure 23.21). Creosote bush (Larrea divaricata) and bur sage (Franseria spp.) dominate the deserts of southwestern North America—the Mojave, the Sonoran, and the Chihuahuan. Areas of favorable moisture support tall growths of Acacia spp., saguaro (Cereus giganteus), palo verde (Cercidium spp.), ocotillo (Fouquieria spp.), yucca (Yucca spp.), and ephemeral plants.
Both plants and animals adapt to the scarcity of water by either drought evasion or drought resistance. Drought-evading plants flower only when moisture is present. They persist as seeds during drought periods, ready to sprout, flower, and produce seeds when moisture and temperature are favorable. If no rains come, these ephemeral species do not germinate and grow.
Drought-evading animals, like their plant counterparts, adopt an annual cycle of activities or go into estivation or some other dormant stage during the dry season. For example, the spadefoot toad (Scaphiopus; Figure 23.22) remains underground in a gel-lined underground cell, making brief reproductive appearances during periods of winter and summer rains. If extreme drought develops during the breeding season, many animals such as lizards and birds do not reproduce.
Desert plants may be deep-rooted woody shrubs, such as mesquite (Prosopis spp.) and Tamarix, whose taproots reach the water table, rendering them independent of water supplied by rainfall. Some plants, such as Larrea and Atriplex, are deep-rooted perennials with superficial laterals that extend as far as 15 to 30 m from the stems. Other perennials, such as the various species of cactus, have shallow roots that often extend no more than a few centimeters below the surface.
Despite their aridity, desert ecosystems support a surprising diversity of animal life, including a wide assortment of beetles, ants, locusts, lizards, snakes, birds, and mammals. The mammals are mostly herbivorous species. Grazing herbivores of the desert tend to be generalists and opportunists in their mode of feeding. They consume a wide range of species, plant types, and parts. Desert rodents—particularly the family Heteromyidae—and ants feed largely on seeds and are important in the dynamics of desert ecosystems. Seed-eating herbivores can eat up to 90 percent of the available seeds. That consumption can distinctly affect plant composition and plant populations. Desert carnivores, such as foxes and coyotes, have mixed diets that include leaves and fruits; even insectivorous birds and rodents eat some plant material. Omnivory, rather than carnivory and complex food webs, seems to be the rule in desert ecosystems.
The infrequent rainfall coupled with high rates of evaporation limit the availability of water to plants, so primary productivity is low. Most desert soils are poorly developed aridisols and entisols, and the sparse cover of arid lands limits the ability of vegetation to heavily modify the soil environment (see Figure 4.12). Underneath established plants, however, “islands of fertility” can develop because of higher litter input and the enrichment by wastes from animals that seek shade, particularly under shrubs.
23.6 Mediterranean Climates Support Temperate Shrublands
Shrublands—plant communities where the shrub growth form is either dominant or codominant—are difficult types of ecosystems to categorize, largely because of the difficulty in characterizing the term shrub itself. In general, a shrub is a plant with multiple woody, persistent stems but no central trunk and a height from 4.5 to 8 m. However, under severe environmental conditions, even many trees do not exceed that size. Some trees—particularly individuals that coppice (resprout from the stump) after destruction of the aboveground tissues by fire, browsing, or cutting—are multistemmed, and some shrubs can have large, single stems. In addition, the shrub growth form can be a dominant component of a variety of tropical and temperate ecosystems, including the tropical savannas and scrub desert communities (see Section 23.3, respectively). However, in five widely disjunct regions along the western margins of the continents, between 30° and 40° latitude, are found the mediterranean ecosystems dominated by evergreen shrubs and sclerophyllous trees that have adapted to the distinctive climate of summer drought and cool, moist winters.
The five regions of mediterranean ecosystems include the semiarid regions of western North America, the regions bordering the Mediterranean Sea, central Chile, the cape region of South Africa, and southwestern and southern Australia (Figure 23.23). The mediterranean climate has hot, dry summers, with at least one month of protracted drought, and cool, moist winters (see representative climate diagrams in Figure 23.23). About 65 percent of the annual precipitation falls during the winter months. Winter temperatures typically average 10–12°C with a risk of frost. The hot, dry summer climates of the mediterranean regions arise from the seasonal change in the semipermanent high-pressure zones that are centered over the tropical deserts at about 20° N and 20° S (see discussion in Section 23.5). The persistent flow of dry air out of these regions during the summer brings several months of hot, dry weather. Fire is a frequent hazard during these periods.
All five regions support similar-looking communities of xeric broadleaf evergreen shrubs and dwarf trees known as sclerophyllous (scleros, “hard”; phyll, “leaf”) vegetation with a herbaceous understory. Sclerophyllous vegetation possesses small leaves, thickened cuticles, glandular hairs, and sunken stomata—all characteristics that function to reduce water loss during the hot, dry summer period (Figure 23.24). Vegetation in each of the mediterranean systems also shares adaptations to fire and to low nutrient levels in the soil.
The largest area of mediterranean ecosystem forms a discontinuous belt around the Mediterranean Sea in southern Europe and North Africa. Much of the area is currently or was once dominated by mixed evergreen woodland supporting species such as holm oak (Quercus ilex) and cork oak (Quercus suber). Often, these two species grow in mixed stands in association with strawberry tree (Arbutus unedo) and various species of shrubs. The easternmost limit of these ecosystems is in the coastal areas of Syria, Lebanon, and Israel, where they grade into the arid lands of the Middle East. Here, deciduous oak species are more abundant. Desert vegetation extends across North Africa as far as Tunisia, with mediterranean shrub and woodland extending through the northern coastal areas of Algeria and Morocco.
The mediterranean zone in southern Africa is restricted to the mountainous region of the Cape Province, where the vegetation is known as fynbos. The vegetation is composed primarily of broadleaf proteoid (Proteaceae) and ericoid (Ericaceae) shrubs that grow to a height of 1.5 to 2.5 m (Figure 23.25). In southwest Australia, the mediterranean shrub community known as mallee is dominated by low-growing Eucalyptus, 5 to 8 m in height, with broad sclerophyllous leaves.
In North America, the sclerophyllous shrub community is known as chaparral, a word of Spanish origin meaning a thicket of shrubby evergreen oaks (Figure 23.26). California chaparral, dominated by scrub oak (Quercus berberidifolia) and chamise (Adenostoma fasciculatum), is evergreen, winter-active, and summer-dormant. Another shrub type, also designated as chaparral, is found in the Rocky Mountain foothills. Dominated by Gambel oak (Quercus gambelii), it is winter-deciduous.
The matorral shrub communities of central Chile occur in the coastal lowlands and on the west-facing slopes of the Andes. Most of the matorral species are evergreen shrubs 1 to 3 m in height with small sclerophyllous leaves, although drought-deciduous shrubs are also found.
For the most part, mediterranean shrublands lack an understory and ground litter and are highly flammable. Many species have seeds that require the heat and scarring action of fire to induce germination. Without fire, chaparral grows taller and denser, building up large fuel loads of leaves and twigs on the ground. In the dry season the shrubs, even though alive, nearly explode when ignited.
Shrub communities have a complex of animal life that varies with the region. In the mediterranean shrublands, similarity in habitat structure has resulted in pronounced parallel and convergent evolution among bird species and some lizard species, especially between the Chilean mattoral and the California chaparral. In North America, chaparral and sagebrush communities support mule deer (Odocoileus hemionus), coyotes (Canis latrans), a variety of rodents, jackrabbits (Lepus spp.), and sage grouse (Centrocercus urophasianus). The Australian mallee is rich in birds, including the endemic mallee fowl (Leipoa ocellata), which incubates its eggs in a large mound. Among the mammalian life are the gray kangaroo (Macropus giganteus) and various species of wallaby (Macropodidae).
The diverse topography and geology of the mediterranean environments give rise to a diversity of soil conditions, but soils are typically classified as alfisols (see Figure 4.12). The soils of the regions are generally deficient in nutrients, and litter decomposition is limited by low temperatures during the winter and low soil moisture during the summer months. These ecosystems vary in productivity depending on the annual precipitation and the severity of summer drought.
23.7 Forest Ecosystems Dominate the Wetter Regions of the Temperate Zone
Climatic conditions in the humid midlatitude regions give rise to the development of forests dominated by broadleaf deciduous trees (Figure 23.27). But in the mild, moist climates of the Southern Hemisphere, temperate evergreen forests become predominant. Deciduous forest once covered large areas of Europe and China, parts of North and South America, and the highlands of Central America. The deciduous forests of Europe and Asia, however, have largely disappeared, cleared over the centuries for agriculture. In eastern North America, the deciduous forest consists of several forest types or associations (Figure 23.28), including the mixed mesophytic forest of the unglaciated Appalachian plateau, the beech–maple and northern hardwood forests (with pine and hemlock) in northern regions that eventually grade into the boreal forest (see Section 24.8), the maple–basswood forests of the Great Lakes states, the oak–chestnut (now oak since the die-off of the American chestnut) or central hardwood forests, which cover most of the Appalachian Mountains, the magnolia–oak forests of the Gulf Coast states, and the oak–hickory forests of the Ozarks. In North America, temperate deciduous forests reach their greatest development in the mesic forests of the central Appalachians, where the number of tree species is unsurpassed by any other temperate area in the world.
The Asiatic broadleaf forest, found in eastern China, Japan, Taiwan, and Korea, is similar to the North American deciduous forest and contains several plant species of the same genera as those found in North America and western Europe. However, broadleaf evergreen species become increasingly present in Japan, South Korea, and southern China and in the wet foothills of the Himalayas. In southern Europe, their presence reflects the transition into the mediterranean region. Evergreen oaks and pines are also widely distributed in the southeastern United States, where they are usually associated with poorly developed sandy or swampy soils.
In the Southern Hemisphere, temperate deciduous forests are found only in the drier parts of the southern Andes. In southern Chile, broadleaf evergreen rain forests have developed in an oceanic climate that is virtually frost-free. Evergreen forests are also found in New Zealand, Tasmania, and parts of southeastern Australia where the winter temperatures are moderated by the coastal environment. Climate regions in these areas are similar to those of the Pacific Northwest of North America, but here the predominant species are conifers.
In the broadleaf deciduous forests of the temperate region, the end of the growing season is marked by the autumn colors of foliage shortly before the trees enter into their leafless winter period (Figure 23.29). The trees resume growth in the spring in response to increasing temperatures and longer day lengths. Many herbaceous species flower at this time before the developing canopy casts a heavy shade on the forest floor.
Highly developed, unevenly aged deciduous forests usually have four vertical layers or strata (see Figure 16.12a). The upper canopy consists of the dominant tree species, below which is the lower tree canopy, or understory. Next is the shrub layer, followed finally by the ground layer of herbs, ferns, and mosses. The diversity of animal life is associated with this vertical stratification and the growth forms of plants (see Figure 16.13). Some animals, particularly forest arthropods, spend most of their lives in a single stratum; others range over two or more strata. The greatest concentration and variety of life in the forest occurs on and just below the ground layer. Many animals—the soil and litter invertebrates in particular—remain in the subterranean stratum. Others, such as mice, shrews, ground squirrels, and forest salamanders, burrow into the soil or litter for shelter and food. Larger mammals live on the ground layer and feed on herbs, shrubs, and low trees. Birds move rather freely among several strata but typically favor one layer over another (see Figure 16.13).
Differences in climate, bedrock, and drainage are reflected in the variety of soil conditions present. Alfisols, inceptisols, and ultisols are the dominant soil types with alfisols typically associated with glacial materials in more northern regions (see Figure 4.12). Primary productivity varies geographically and is influenced largely by temperatures and the length of the growing season (see Section 20.3). Leaf fall in deciduous forests occurs over a short period in autumn, and the availability of nutrients is related to rates of decomposition and mineralization (see Chapter 21).
23.8 Conifer Forests Dominate the Cool Temperate and Boreal Zones
Conifer forests, dominated by needle-leaf evergreen trees, are found primarily in a broad circumpolar belt across the Northern Hemisphere and on mountain ranges, where low temperatures limit the growing season to a few months each year (Figure 23.30). The variable composition and structure of these forests reflect the wide range of climatic conditions in which they grow. In central Europe, extensive coniferous forests, dominated by Norway spruce (Picea abies), cover the slopes up to the subalpine zone in the Carpathian Mountains and the Alps (Figure 23.31a). In North America, several coniferous forests blanket the Rocky, Wasatch, Sierra Nevada, and Cascade mountains. At high elevations in the Rocky Mountains grows a subalpine forest dominated by Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa). Middle elevations have stands of Douglas fir, and lower elevations are dominated by open stands of ponderosa pine (Pinus ponderosa; Figure 23.31b) and dense stands of the early successional conifer, lodgepole pine (Pinus contorta). The largest tree of all, the giant sequoia (Sequoiadendron giganteum), grows in scattered groves on the western slopes of the California Sierra. In addition, the mild, moist climate of the Pacific Northwest supports a highly productive coastal forest extending along the coastal strip from Alaska to northern California.
The largest expanse of conifer forest—in fact, the largest vegetation formation on Earth—is the boreal forest, or taiga (Russian for “land of little sticks”). This belt of coniferous forest, encompassing the high latitudes of the Northern Hemisphere, covers about 11 percent of Earth’s terrestrial surface (see Figure 23.30). In North America, the boreal forest covers much of Alaska and Canada and spills into northern New England, with fingers extending down the western mountain ranges and into the Appalachians. In Eurasia, the boreal forest begins in Scotland and Scandinavia and extends across the continent, covering much of Siberia, to northern Japan.
Three major vegetation zones make up the taiga (Figure 23.32): (1) the forest–tundra ecotone with open stands of stunted spruce, lichens, and moss; (2) the open lichen woodland with stands of lichens and black spruce; and (3) the main boreal forest (Figure 23.33) with continuous stands of spruce and pine broken by poplar and birch on disturbed areas. This boreal–mixed forest grades into the temperate forest of southern Canada and the northern United States. Primarily occupying formerly glaciated land, the taiga is also a region of cold lakes, bogs, rivers, and alder thickets.
A cold continental climate with strong seasonal variation dominates the taiga. The summers are short, cool, and moist; the winters are long, harsh, and dry, with a prolonged period of snowfall. The driest winters and the greatest seasonal fluctuations are in interior Alaska and central Siberia, which experience seasonal temperature extremes (differences between minimum and maximum annual temperatures) of as much as 100°C.
Much of the taiga is under the controlling influence of permafrost, which impedes infiltration and maintains high soil moisture. Permafrost is the perennially frozen subsurface that may be hundreds of meters deep. It develops where the ground temperatures remain below 0°C for extended periods of time. Its upper layers may thaw in summer and refreeze in winter. Because the permafrost is impervious to water, it forces all water to remain and move above it. Thus, the ground stays soggy even though precipitation is low, enabling plants to exist in the driest parts of the Arctic.
Fires are recurring events in the taiga. During periods of drought, fires can sweep over hundreds of thousands of hectares. All of the boreal species, both broadleaf trees and conifers, are well adapted to fire. Unless too severe, fire provides a seedbed for regeneration of trees. Light surface burns favor early successional hardwood species. More severe fires eliminate hardwood competition and favor spruce and jack pine regeneration.
Because of the global demand for timber and pulp, vast areas of the boreal forest across North America and Siberia are being clear-cut with little concern for their future, see this chapter, Ecological Issues & Applications. This exploitation can alter the nature and threaten the survival of the boreal forest.
The boreal forest has a unique animal community. Caribou (Rangifer tarandus), wide-ranging and feeding on grasses, sedges, and especially lichens, inhabit open spruce–lichen woodlands. Joining the caribou is the moose (Alces alces), called elk in Eurasia, the largest of all deer. It is a lowland mammal feeding on aquatic and emergent vegetation as well as alder and willow. Competing with moose for browse is the cyclic snowshoe hare (Lepus americanus). The arboreal red squirrel (Sciurus hudsonicus) inhabits the conifers and feeds on young pollen-bearing cones and seeds of spruce and fir, and the quill-bearing porcupine (Erethizon dorsatum) feeds on leaves, twigs, and the inner bark of trees. Preying on these is an assortment of predators including the wolf, lynx (Lynx canadensis and L. lynx), pine martin (Martes americana), and owls. The taiga is also the nesting ground of migratory neotropical birds and the habitat of northern seed-eating birds such as crossbills (Loxia spp.), grosbeaks (Coccothraustes spp.), and siskins (Carduelis spp.).
Of great ecological and economic importance are major herbivorous insects such as the spruce budworm (Choristoneura fumiferana). Although they are major food items for insectivorous summer birds, these insects experience periodic outbreaks during which they defoliate and kill large expanses of forest.
Compared to more temperate forests, boreal forests have generally low net primary productivity; they are limited by low nutrients, cooler temperatures, and the short growing season. Likewise, inputs of plant litter are low compared to the forests of the warmer temperate zone. However, rates of decomposition are slow under the cold, wet conditions, resulting in the accumulation of organic matter. Soils are primarily spodosols characterized by a thick organic layer (see Figure 4.12). The mineral soils beneath mature coniferous forests are comparatively infertile, and growth is often limited by the rate at which mineral nutrients are recycled through the ecosystem.
23.9 Low Precipitation and Cold Temperatures Define the Arctic Tundra
Encircling the top of the Northern Hemisphere is a frozen plain, clothed in sedges, heaths, and willows, dotted with lakes, and crossed by streams (Figure 23.34). Called tundra, its name comes from the Finnish tunturi, meaning “a treeless plain.” The arctic tundra falls into two broad types: tundra with up to 100 percent plant cover and wet to moist soil (Figure 23.35), and polar desert with less than 5 percent plant cover and dry soil.
Conditions unique to the Arctic tundra are a product of at least three interacting forces: (1) the permanently frozen deep layer of permafrost; (2) the overlying active layer of organic matter and mineral soil that thaws each summer and freezes the following winter; and (3) vegetation that reduces warming and retards thawing in summer. Permafrost chills the soil, retarding the general growth of plant parts both above- and belowground, limiting the activity of soil microorganisms, and diminishing the aeration and nutrient content of the soil.
Alternate freezing and thawing of the upper layer of soil creates the unique, symmetrically patterned landforms typical of the tundra (Figure 23.36). The frost pushes stones and other material upward and outward from the mass to form a patterned surface of frost hummocks, frost boils, earth stripes, and stone polygons. On sloping ground, creep, frost thrusting, and downward flow of supersaturated soil over the permafrost form solifluction terraces, or “flowing soil.” This gradual downward creep of soils and rocks eventually rounds off ridges and other irregularities in topography. Such molding of the landscape by frost action, called cryoplanation, is far more important than erosion in wearing down the Arctic landscape.
Structurally, the vegetation of the tundra is simple. The number of species tends to be low, and growth is slow. Only those species able to withstand constant disturbance of the soil, buffeting by the wind, and abrasion from wind-carried particles of soil and ice can survive. Low ground is covered with a complex of cotton grasses, sedges, and Sphagnum. Well-drained sites support heath shrubs, dwarf willows and birches, herbs, mosses, and lichens. The driest and most exposed sites support scattered heaths and crustose and foliose lichens growing on the rock. Arctic plants propagate themselves almost entirely by vegetative means, although viable seeds many hundreds of years old exist in the soil.
Plants are photosynthetically active on the Arctic tundra about three months out of the year. As snow cover disappears, plants commence photosynthetic activity. They maximize use of the growing season and light by photosynthesizing during the 24-hour daylight period, even at midnight when light is one-tenth that of noon. The nearly erect leaves of some Arctic plants permit almost complete interception of the low angle of the Arctic sun.
Much of the photosynthate goes into the production of new growth, but about one month before the growing season ends, plants cease to allocate photosynthate to aboveground biomass. They withdraw nutrients from the leaves and move them to roots and belowground biomass, sequestering 10 times the amount stored by temperate grasslands.
Structurally, most of the tundra vegetation is underground. Root-to-shoot ratios of vascular plants range from 3:1 to 10:1. Roots are concentrated in the upper soil that thaws during the summer, and aboveground parts seldom grow taller than 30 cm. It is not surprising, then, that the belowground net annual production is typically three times that of the aboveground productivity.
The tundra hosts fascinating animal life, even though the diversity of species is low. Invertebrates are concentrated near the surface, where there are abundant populations of segmented whiteworms (Enchytraeidae), collembolas, and flies (Diptera), chiefly crane flies. Summer in the Arctic tundra brings hordes of black flies (Simulium spp.), deer flies (Chrysops spp.), and mosquitoes.
Dominant vertebrates on the Arctic tundra are herbivores, including lemmings, Arctic hare, caribou, and musk ox (Ovibos moschatus). Although caribou have the greatest herbivore biomass, lemmings, which breed throughout the year, may reach densities as great as 125 to 250 per hectare; they consume three to six times as much forage as caribou do. Arctic hares (Lepus arcticus) that feed on willows disperse over the range in winter and congregate in more restricted areas in summer. Caribou are extensive grazers, spreading out over the tundra in summer to feed on sedges. Musk oxen are more intensive grazers, restricted to more localized areas where they feed on sedges, grasses, and dwarf willow. Herbivorous birds are few, dominated by ptarmigan and migratory geese.
The major Arctic carnivore is the wolf (Canus lupus), which preys on musk ox, caribou, and, when they are abundant, lemmings. Medium-sized to small predators include the Arctic fox (Alopex lagopus), which preys on Arctic hare, and several species of weasel, which prey on lemmings. Also feeding on lemmings are snowy owls (Nyctea scandiaca) and the hawk-like jaegers (Stercorarius spp.). Sandpipers (Tringa spp.), plovers (Pluvialis spp.), longspurs (Calcarius spp.), and waterfowl, which nest on the wide expanse of ponds and boggy ground, feed heavily on insects.
At lower latitudes, alpine tundra occurs in the higher mountains of the world. The alpine tundra is a severe environment of rock-strewn slopes, bogs, meadows, and shrubby thickets (Figure 23.37). It is a land of strong winds, snow, cold, and widely fluctuating temperatures. During summer, the temperature on the surface of the soil ranges from 40 to 0°C. The atmosphere is thin so light intensity, especially ultraviolet, is high on clear days. Alpine tundras have little permafrost, and it is confined mostly to very high elevations. Lacking permafrost, soils are drier. Only in alpine wet meadows and bogs do soil moisture conditions compare with those of the Arctic. Precipitation, especially snowfall and humidity, is higher in the alpine regions than in the Arctic tundra, but steep topography induces a rapid runoff of water.
Globally, about half of the forest that was present under modern (post-Pleistocene) climatic conditions—and before the spread of human influence—has disappeared largely through the impact of human activities (Figure 23.39). The spread of agriculture and animal husbandry, the harvesting of forests for timber and fuel, and the expansion of populated areas have all taken their toll on forests. The causes and timing of forest loss differ among regions and forest types, as do the current trends in change in forest cover. In the face of increasing demand and declining forest cover, the sustainable production of forest resources requires achieving a balance between net growth and harvest. To achieve this end, foresters have an array of silvicultural and harvesting techniques from clear-cutting to selection cutting.
Clear-cutting involves removing the forest and reverting it to an early stage of succession (Figure 23.40a). The area harvested can range from thousands of hectares to small patch cuts of a few hectares designed to create habitat for wildlife species that require an opening within the forest (see Section 19.4). Postharvest management varies widely for clear-cut areas. When natural forest stands are clear-cut, there is generally no follow-up management. Stands are left to regenerate naturally from existing seed and sprouts on the site and the input of seeds from adjacent forest stands. With no follow-up management, clear-cut areas can be badly disturbed by erosion that affects subsequent recovery of the site as well as adjacent aquatic communities.
Harvest by clear-cutting is the typical practice on forest plantations, but here intensive site management follows clearing. Plant materials that are not harvested (branches, leaves, and needles) are typically burned to clear the site for planting. After clearing, seedlings are planted and fertilizer applied to encourage plant growth. Herbicides are often used to discourage the growth of weedy plants that would compete with the seedlings for resources.
Interpreting Ecological Data
Q1. What does graph (a) imply about the change in average tree size (diameter or height) with stand age?
Q2. Assume that a decision is made to harvest the stand when it reaches the minimum salable wood volume (100 m3/ha). How many stems (trees) per m3 would be harvested?
The seed-tree, or shelterwood, system is a method of regenerating a new stand by removing all trees from an area except for a small number of seed-bearing trees (Figure 23.40b). The uncut trees are intended to be the main source of seed for establishing natural regeneration after harvest. Seed trees can be uniformly scattered or left in small clumps, and they may or may not be harvested later.
In many ways, the shelterwood system is similar to a clear-cut because generally not enough trees are left standing to affect the microclimate of the harvested area. The advantage of the shelterwood approach is that the seed source for natural regeneration is not limited to adjacent stands. This can result in improved distribution (or stocking) of seedlings as well as a more desirable mix of species.
Like any silviculture system, shelterwood harvesting requires careful planning to be effective. Trees left on the site must be strong enough to withstand winds and capable of producing adequate seed, seedbed conditions must be conducive to seedling establishment (this may require a preparatory treatment during or after harvest), and follow-up management may be required to fully establish the regeneration.
In selection cutting, mature single trees or groups of trees scattered through the forest are removed. Selection cutting produces only small openings or gaps in the forest canopy. Although this form of timber harvest can minimize the scale of disturbance within the forest caused by direct removal of trees, the network of trails and roads necessary to provide access can be a major source of disturbance (to both plants and soils). Selective cutting also can cause changes in species composition and diversity because only certain species are selectively removed.
Regardless of the differences in approach, some general principles apply if the harvesting of resources is to be sustainable. Forest trees function in the manner discussed for competition in plant populations (Chapter 11, Section 11.3). Whether a forest is planted as seedlings or grown by natural regeneration, its establishment begins with a population of small individuals (seedlings) that grow and compete for the essential resources of light, water, and nutrients. As biomass in the forest increases, the density of trees decreases and the average tree size increases as a result of self-thinning (Figure 23.41; also see Section 11.5, Figure 11.9 ). For a stand to be considered economically available for harvest (referred to as being in an operative state), minimum thresholds must be satisfied for the harvestable volume of timber per hectare and average tree size (see Figure 23.41); these thresholds vary depending on the species. In plantation forestry, for a given set of thresholds (timber volume and average tree size), the initial stand density (planting density) can be controlled to influence the timing of the stand’s availability for harvest (Figure 23.42).
After trees are harvested, a sufficient time must pass for the forest to regenerate. For sustained yield, the time between harvests must be sufficient for the forest to regain the level of biomass it had reached at the time of the previous harvest. Rotation time depends on a variety of factors related to the tree species, site conditions, type of management, and intended use of the trees being harvested. Wood for paper products (pulpwood), fence posts, and poles are harvested from fast-growing species, allowing a short rotation period (15–40 years). These species are often grown in highly managed plantations where trees can be spaced to reduce competition and fertilized to maximize growth rates. Trees harvested for timber (saw logs) require a much longer rotation period. Hardwood species used for furniture and cabinetry are typically slower growing and may have a rotation time of 80 to 120 years. Sustained forestry of these species works best in extensive areas where blocks of land can be maintained in different age classes.
Interpreting Ecological Data?
Q1. The analysis includes three initial planting densities: 500, 2000, and 4000 trees/ha. At which stand age does each of the three initial planting densities achieve the minimum constraint for average tree size?
Q2. Given the requirements of minimum wood volume and average tree size as defined, which of the initial planting densities meets these requirements at the earliest stand age (earliest operable window)?
As with agricultural crops, a significant amount of nutrients are lost from the forest when trees are harvested and removed (see Chapter 22, Ecological Issues & Applications). The loss of nutrients in plant biomass is often compounded by further losses from soil erosion and various postharvest management practices—particularly the use of fire. The reduction of nutrients reduces plant growth, requiring a longer rotation period for subsequent harvests or causing reduced forest yield if the rotation period is maintained. Forest managers often counter the loss of nutrients by using chemical fertilizers, which create other environmental problems for adjacent aquatic ecosystems (see Chapter 24, Ecological Issues & Applications).
In addition to the nutrients lost directly through biomass removal, logging can also result in the transport of nutrients from the ecosystem by altering processes involved in internal cycling. The removal of trees in clear-cutting and other forest management practices increases the amount of radiation (including direct sunlight) reaching the soil surface. The resulting increase in soil temperatures promotes decomposition of remaining soil organic matter and causes an increase in net mineralization rates (see Sections 21.4 and 21.5). This increase in nutrient availability in the soil occurs at the same time that demand for nutrients is low because plants have been removed and net primary productivity is low. As a result, there is a dramatic increase in the leaching of nutrients from the soil into ground and surface waters (Figure 23.43). This export of nutrients from the ecosystem results from decoupling the two processes of nutrient release in decomposition and nutrient uptake in net primary productivity.
Sustained yield is a key concept in forestry and is practiced to some degree by large timber companies and federal and state forestry agencies. But all too often, industrial forestry’s approach to sustained yield is to grow trees as a crop rather than maintaining a forest ecosystem. Their management approach represents a form of agriculture in which trees are grown as crops: trees are clear-cut, the site is sprayed with herbicides, planted or seeded with one species, then clear-cut and planted again. Clear-cutting practices in some national forests, especially in the Pacific Northwest and the Tongass National Forest in Alaska, hardly qualify as sustained-yield management. Even more extensive clear-cutting of forests is taking place in the northern forests of Canada, especially in British Columbia (Figure 23.44), and in large areas of Siberia.
The problem of sustained-yield forestry is its economic focus on the resource with little concern for the forest as a biological community. A carefully managed stand of trees, often reduced to one or two species, is not a forest in an ecological sense. Rarely will a naturally regenerated forest, and certainly not a planted one, support the diversity of life found in old-growth forests. By the time the trees reach economic or financial maturity—based on the type of rotation—they are cut again.
Summary
Ecosystem Distribution and Plant Adaptations 23.1
Terrestrial ecosystems can be grouped into broad categories called biomes. Biomes are classified according to the predominant plant types. There are at least eight major terrestrial biome types: tropical forest, temperate forest, conifer forest (taiga or boreal forest), tropical savanna, temperate grasslands, chaparral (shrublands), tundra, and desert. These broad categories reflect the relative contribution of three general plant life-forms: trees, shrubs, and grasses. Interaction between moisture and temperature is the primary factor limiting the nature and geographic distribution of terrestrial ecosystems.
Tropical Forests 23.2
Seasonality of rainfall determines the types of tropical forests. Rain forests, associated with high seasonal rainfall, are dominated by broadleaf evergreen trees. They are noted for their enormous diversity of plant and animal life. The vertical structure of the forest is divided into five general layers: emergent trees, high upper canopy, low tree stratum, shrub understory, and a ground layer of herbs and ferns. Conspicuous in the rain forest are the lianas or climbing vines, epiphytes growing up in the trees, and stranglers growing downward from the canopy to the ground. Many large trees develop buttresses for support. Nearly 90 percent of nonhuman primate species live in tropical rain forests.
Tropical rain forests support high levels of primary productivity. The high rainfall and consistently warm temperatures also result in high rates of decomposition and nutrient cycling.
Dry tropical forests undergo varying lengths of dry season, during which trees and shrubs drop their leaves (drought-deciduous). New leaves are grown at the onset of the rainy season. Most dry tropical forests have been lost to agriculture and grazing and other disturbances.
Tropical Savannas 23.3
Savannas are characterized by a codominance of grasses and woody plants. Such vegetation is characteristic of regions with alternating wet and dry seasons. Savannas range from grass with occasional trees to shrubs to communities where trees form an almost continuous canopy as a function of precipitation and soil texture. Productivity and decomposition in savanna ecosystems are closely tied to the seasonality of precipitation.
Savannas support a large and varied assemblage of both invertebrate and vertebrate herbivores. The African savanna is dominated by a large, diverse population of ungulate fauna and associated carnivores.
Temperate Grasslands 23.4
Natural grasslands occupy regions where rainfall is between 250 and 800 mm a year. Once covering extensive areas of the globe, natural grasslands have shrunk to a fraction of their original size because of conversion to cropland and grazing lands.
Grasslands vary with climate and geography. Native grasslands of North America, influenced by declining precipitation from east to west, consist of tallgrass prairie, mixed-grass prairie, shortgrass prairie, and desert grasslands. Eurasia has steppes; South America, the pampas; and southern Africa, the veld. Grassland consists of an ephemeral herbaceous layer that arises from crowns, nodes, and rosettes of plants hugging the ground. It also has a ground layer and a highly developed root layer. Depending on the history of fire and degree of grazing and mowing, grasslands accumulate a layer of mulch.
Grasslands support a diversity of animal life dominated by herbivorous species, both invertebrate and vertebrate. Grasslands once supported herds of large grazing ungulates such as bison in North America, migratory herds of wildebeest in Africa, and marsupial kangaroos in Australia. Grasslands evolved under the selective pressure of grazing. Although the most conspicuous grazers are large herbivores, the major consumers are invertebrates. The heaviest consumption takes place belowground, where the dominant herbivores are nematodes.
Deserts 23.5
Deserts occupy about one-seventh of Earth’s land surface and are largely confined to two worldwide belts between 15° N and 30° S latitude. Deserts result from dry descending air masses within these regions, the rain shadows of coastal mountain ranges, and remoteness from oceanic moisture. Two broad types of deserts exist: cool deserts, exemplified by the Great Basin of North America, and hot deserts, like the Sahara. Deserts are structurally simple—scattered shrubs, ephemeral plants, and open, stark topography. In this harsh environment, ways of circumventing aridity and high temperatures by either evading or resisting drought have evolved in plants and animals. Despite their aridity, deserts support a diversity of animal life, notably opportunistic herbivorous species and carnivores.
Shrubland 23.6
Shrubs have a densely branched, woody structure and low height. Shrublands are difficult to classify because of the variety of climates in which shrubs can be a dominant or codominant component of the plant community. But in five widely disjunct regions along the western margins of the continents between 30° and 40° latitude are found the mediterranean ecosystems. Dominated by evergreen shrubs and sclerophyll trees, these biomes have adapted to the distinctive climate of summer drought and cool, moist winters. These shrublands are fire adapted and highly flammable.
Temperate Forests 23.7
Broadleaf deciduous forests are found in the wetter environments of the warm temperate region. They once covered large areas of Europe and China, but their distribution has been reduced by human activity. In North America, deciduous forests are still widespread. They include various types such as beech–maple and oak–hickory forest; the greatest development is in the mixed mesophytic forest of the unglaciated Appalachians. Well-developed deciduous forests have four strata: upper canopy, lower canopy, shrub layer, and ground layer. Vertical structure influences the diversity and distribution of life in the forest. Certain species are associated with each stratum.
Conifer Forests 23.8
Coniferous forests of temperate regions include the montane pine forests and lower-elevation pine forests of Eurasia and North America and the temperate rain forests of the Pacific Northwest.
North of the temperate coniferous forest is the circumpolar taiga, or boreal forest, the largest biome on Earth. Characterized by a cold continental climate, the taiga consists of four major zones: the forest ecotone, open boreal woodland, main boreal forest, and boreal–mixed forest ecotone.
Permafrost, the maintenance of which is influenced by tree and ground cover, strongly influences the pattern of vegetation, as do recurring fires. Spruces and pines dominate boreal forest with successional communities of birch and poplar. Ground cover below spruce is mostly moss; in open spruce and pine stands, the cover is mostly lichen.
Major herbivores of the boreal region include caribou, moose, and snowshoe hare. Predators include the wolf, lynx, and pine martin.
Tundra 23.9
The Arctic tundra extends beyond the tree line at the far north of the Northern Hemisphere. It is characterized by low temperature, low precipitation, a short growing season, a perpetually frozen subsurface (the permafrost), and a frost-molded landscape. Plant species are few, growth forms are low, and growth rates are slow. Over much of the Arctic, the dominant vegetation is cotton grass, sedge, and dwarf heaths. These plants exploit the long days of summer by photosynthesizing during the 24-hour daylight period. Most plant growth occurs underground. The animal community is low in diversity but unique. Summer in the Arctic brings hordes of insects, providing a rich food source for shorebirds. Dominant vertebrates are lemming, Arctic hare, caribou, and musk ox. Major carnivores are the wolf, Arctic fox, and snowy owl.
Alpine tundras occur in the mountains of the world. They are characterized by widely fluctuating temperatures, strong winds, snow, and a thin atmosphere.
Forest Management Ecological Issues & Applications
More than 90 percent of global forest resources, which include fuel, building materials, and food, are harvested from native forests. Sustainable production of forest resources requires achieving a balance between net growth and harvest. To achieve this end, foresters have an array of silvicutural and harvesting techniques
