Part Seven Ecological Biogeography

Part Seven Ecological Biogeography
Chap.23 Terrestrial Ecosystems Chap.24 Aquatic Ecosystems Chap.25 Coastal and Wetland Ecosystems Chap.26 Large-scale Patterns of Biological Diversity Smith & Smith (2015) Elements of ecology. 9th. Ed. Pearson. Part Seven Ecological Biogeography 生態生物地理學鄭先祐 (Ayo) 教授生態科學與技術學系國立臺南大學環境與生態學院

The 19th century was the golden age of the naturalist-explorer and the tropics were the frontier. A. R. Wallace, H.W. Bates, J. Hooker, A. Von Humboldt, C. Darwin. Early naturalists were struck by The diversity of life The similar appearance and nature of organisms found in geographically different regions Link of form and function Similar types of plant communities in similar climates

Fig. 1 Convergent evolution of unrelated species showing similar relationships between form and function. A North American rodent, the flying squirrel, and an Australian marsupial. The sugar glider, both have a flat, bushy tail and an extension of the skin between the foreleg and the hindleg that enables them to glide down from one tree limb to another. Unit Opener: Figure 1

Fig. 2 Shrub desert ecosystems: (a) Karoo Desert in southern Africa; (b) western New South Wales, Australia. Both sites are characterized by low, seasonal precipitation. Unit Opener – Figure 2

Early naturalists were faced with two conflicting observations that each needed an explanation Diversity observed among plants and animals Similarity among different, unrelated species that exhibited similar behaviors or inhabited similar environments.

Darwin's theory of natural selection is a two-step process. The production of variation in some characteristic among individuals within the population. This characteristic results in differences among individuals in their survival and reproduction. This process can explain both diversity and similarity.

Convergent evolution is the independent evolution of a similar characteristic in two different species NOT derived from a recent, common ancestor. Environmental conditions guide the process of natural selection. A similar form and function can arise in unrelated (or distantly related) organisms exposed to similar environmental pressures.

Early plant geographers correlated the distribution of vegetation to climate. The world could be divided into zones representing broad categories (or formations) of vegetation based on physical appearance (physiognomy) (觀相術). Deserts Grasslands Coniferous, temperate, and tropical forests Regions of the globe occupied by a given type of vegetation formation were characterized by a similar climate.

Biogeography is the study of the spatial or geographical distribution of organisms, both past and present. Biogeography works to describe and understand the processes responsible for patterns in the distribution of species and larger taxonomic groups. Historical biogeography — origin, dispersal, extinction of groups Ecological biogeography — distribution of contemporary organisms

Chapter Opener Spectacular fall color is a hallmark of the eastern deciduous mixed hardwood forest.

Chap.23 Terrestrial Ecosystems
陸域生態體系 Smith & Smith (2015) Elements of ecology. 9th. Ed. Pearson. 鄭先祐 (Ayo) 教授生態科學與技術學系國立臺南大學環境與生態學院

Chapter 23 Terrestrial Ecosystems
Biomes are biotic units and are classified by predominant plant types. Developed by E.E. Clements and V.E. Shelford Eight major terrestrial biomes Tropical forest (熱帶林) Temperate forest (溫帶林) Conifer forest (taiga or boreal forest) (針葉林) Temperate grasslands (溫帶草原) Tropical savanna (熱帶疏林) Chaparral (shrublands) (灌叢林) Tundra (凍原) Desert (沙漠)

Forest ecosystems are characterized by a closed canopy of trees. Woodland and savanna(疏林) ecosystems are characterized by the codominance of grasses and trees (or shrubs). Desert is a general category used to describe the scarcity of plant cover.

Fig. 23.1 Major biomes and biogeographical realms of the world.
Figure 23.1 Fig Major biomes and biogeographical realms of the world.

Biome types form in response to a distinctive climatic pattern. This was first noted by R. Whittaker (Cornell University) Topography, soil type, and disturbance exposure can also influence the occurrence of a biome in a location.

Fog The pattern of terrestrial biomes in relation to temperature and moisture. Where the climate varies, soil can shift the balance between types. Figure 23.2

Mean annual precipitation declines with decreasing mean annual temperature. A decrease in the range of environmental conditions defined by moisture availability as one moves from the tropic to the temperature and arctic regions. Mean annual temperature decreases from the equator to the poles, while seasonal variation in temperature increases. Reflects the systematic latitudinal pattern of environmental conditions as a direct result from seasonal variations and solar radiation influx.

23.1 Terrestrial Ecosystems Reflect Adaptations of the Dominant Plant Life-Forms
The classification of terrestrial biomes reflects the relative contribution of plant life-forms (生活型). Trees (喬木) Shrubs (灌木) Grasses (草) Why are there consistent patterns in the distribution/abundance of the dominant plant life-forms that relate to climate and the physical environment? Related to the advantages and constraints of adaptations under different environmental conditions.

The allocation of carbon and morphology is unique to each plant life-form. Grasses Maintain a higher proportion of biomass in photosynthetic tissue because little energy is required for support. Shrubs and trees Shrubs invest fewer resources to stems and other supporting structures than do trees. Advantage of woody tissue is height and access to light.

Leaf form is a plant characteristic used to further classify forest and woodland ecosystems. Leaf longevity Deciduous leaves live for only a single year or growing season. Winter-deciduous leaves are lost in response to low temperatures. Drought-deciduous leaves are lost in response to dry conditions

Leaf longevity Evergreen leaves live beyond a year
Leaf longevity Evergreen leaves live beyond a year The broadleaf evergreen leaf is characteristic of environments with no distinct growing season, where growth continues year-round. The needle-leaf evergreen leaf is characteristic of environments with a very short growing season or nutrient limitation

Fig. 23.3 Examples of winter and drought-deciduous trees.
Figure 23.3a Fig Examples of winter and drought-deciduous trees. Temperate deciduous forest in central Virginia during (a) summer and (b) winter seasons.

Fig. 23.3 Semiarid savanna/woodland in Zimbabwe, Africa, during (c) rainy and (d) dry seasons.
Figure 23.3c

Fig. 23.4 Examples of evergreen trees.
(a) Broadleaf evergreen trees dominate the canopy of this tropical rain forest in Queensland, Australia. (b) Needle-leaf evergreen trees (foxtail pine) inhabit the high-altitude zone of the Sierra Nevada in western North America. Figure 23.4

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 will be a function of the rate of net photosynthesis (carbon gain). Under conditions of low net photosynthetic rate, the period of time required to pay back the cost of production will be longer. The needle-leaf evergreen is a plant adapted for survival in an environment with a distinct growing season. The plant has a limited ability to produce enough carbon during a single growing season to produce new leaves each year.

Plant life-forms and leaf type are distributed relative to climate (precipitation and temperature) High to low precipitation Broadleaf evergreen trees(常綠闊葉林) (tropical and subtropical rain forest) Drought-deciduous trees (落葉林) (seasonal tropical forests) Stature and density of trees decline and give rise to woodlands and savannas Trees can no longer be supported, giving rise to arid shrub lands

High to low temperature
23.1 Terrestrial Ecosystems Reflect Adaptations of the Dominant Plant Life-Forms High to low temperature Broadleaf evergreen trees (tropical and subtropical rain forest) Winter-deciduous trees (temperate forests) Areas with low precipitation cannot support trees and develop into grasslands Needle-leaf evergreen trees (conifer forest or taiga) Trees can no longer be supported, giving rise to tundra

Quantifying Ecology 23.1 Climate Diagrams
The distribution of terrestrial ecosystems is influenced by Mean annual temperature(年平均氣溫) and precipitation(降雨量) (Whittaker's graph) Temperature and precipitation seasonality Topography (地形) A climate diagram(氣候圖) describes the local climate at representative locations around the world.

Figure 1 Fig. 1 Climate diagram for Bulawayo, Zimbabwe. This city is in the Southern Hemisphere, where the cooler winter season occurs during the period May—August. Note the distinct dry season during the winter months, with the rainy season beginning in October (spring) and lasting through the summer months.

23.2 Tropical Forests Characterize the Equatorial Zone
The tropical rain forest is dominated by broadleaf evergreen plants Rain forests are restricted to the equatorial zone between 10° N and 10° S Temperatures are warm throughout the year and rainfall occurs almost daily Mean temperature >18°C Minimum monthly precipitation >60 mm

Figure 23.5 Fig Geographic distribution of Earth’s tropical forest ecosystems and associated climate diagrams.

The Amazon basin of South America is the largest and most continuous rain forest in the world. The second largest is in Southeast Asia. The third largest is in West Africa. Tropical rain forests account for more than 50 percent of biological diversity. A 10 km2 area of tropical rain forest may contain 1500 species of flowering plants and up to 750 species of trees. The lowland tropical forest of peninsular Malaysia contains nearly 7900 species! Nearly 90 percent of all nonhuman primate species live in the tropical rain forests.

Figure 23.6a Fug Tropical rain forests in (a) Amazon Basin (South America) and (b) Malaysia (Southeast Asia).

Tropical rain forests account for more than 50 percent of biological diversity. A 10 km2 area of tropical rain forest may contain 1500 species of flowering plants and up to 750 species of trees. The lowland tropical forest of peninsular Malaysia contains nearly 7900 species! Nearly 90 percent of all nonhuman primate species live in the tropical rain forests.

Figure 23.7 Fig Examples of primate species that inhabit the tropical rain forests of the world: the chimpanzee inhabits the tropical rai forests of Central African, and (b) the orangutan inhabits the tropical rain forests of Borneo (Southeast Asia).

Tropical rain forests can be divided into five vertical layers. Emergent trees Upper canopy Lower canopy Shrub understory Ground layer of herbs and ferns Other growth forms Lianas (藤本植物) Epiphytes (附生植物) Strangler figs (榕屬蔓藤類植物)

Fig. 23.8 Vertical stratification of a tropical rain forest.
Figure 23.8 Fig Vertical stratification of a tropical rain forest.

Figure 23.9 Fig Plank-like buttresses (板根) help to support tall rain forest trees. Many trees grow buttresses that function as prop roots in shallow soil that offers poor anchorage

Tropical rain forests have high rates of NPP and rates of litter input to forest floor Leaf litter accumulates because decomposers consume the dead organic matter almost as rapidly as it falls to the forest floor The average time for leaf litter to decompose = 24 weeks

In regions of the tropics characterized by greater seasonality(季節性) in precipitation, the broadleaf evergreen forests are replaced by the dry tropical forests. The dry tropical forest undergoes a dry season whose length is based on latitude and supports drought-deciduous trees and shrubs The farther from the equator, the longer the dry season During the dry season, drought-deciduous plants drop their leaves

Figure 23.10a Fig A tropical dry forest in Costa Rica during the (a) rain and (b) dry season.

23.3 Tropical Savannas Are Characteristic of Semiarid Regions with Seasonal Rainfall
A savanna describes a range of vegetation types in the drier tropics and subtropics characterized by a ground cover of grasses with scattered trees or shrubs The distribution of savanna vegetation (especially woody vegetation) is controlled by moisture Rainfall Soil texture, structure, water-holding capacity

Figure 23.11a Fig Savanna ecosystems, such as the (a) cerrano of South America and (b) mulga woodlands of central Australia are characterized by a ground cover of grasses with scattered shrubs or trees.

Warm continental climate with mean temperature usually >18°C Maximum temperatures occur at the end of the wet season. Distinct seasonality in precipitation and large interannual variation in total precipitation.

Figure 23.12 Fig Geographic distribution of Earth’s tropical savanna ecosystems and associated climate diagrams showing long-term patterns of monthly temperature and precipitation.

Savannas have a two-layer vertical structure
23.3 Tropical Savannas Are Characteristic of Semiarid Regions with Seasonal Rainfall Savannas have a two-layer vertical structure Grasses Trees or shrubs Characteristics of savanna Fire-adapted vegetation Woody vegetation is short-lived (less than several decades) Seasonal precipitation controls the yearly cycle of plant activity and productivity in tropical savannas.

Figure 23.13

Microenvironments of higher soil nutrients and moisture are found under tree canopies This influences species distribution and diversity Savannas support a large diversity of herbivores (invertebrate and vertebrate) that graze or browse the vegetation. The African savanna is dominated by 60 or more species of large ungulate species. A large number of insects, carnivores (e.g., lion), and scavengers (e.g., vulture) are also supported.

23.4 Grassland Ecosystems of the Temperate Zone Vary with Climate and Geography
Natural grasslands occupy regions where rainfall is between 25 and 80 cm/year. Many grasslands exist through the intervention of fire and human activity. Natural grasslands have shrunk to less than 12 percent of their original size. Grasslands occur in the midlatitudes in midcontinental regions where annual precipitation is reduced.

Figure 23.14 Fig Geographic distribution of Earth’s temperate grassland ecosystems and associated climate diagrams.

The temperate grassland experiences recurring drought. Plant diversity reflects differences in the amount and reliability of precipitation. Grasslands do the poorest where precipitation is lowest and the temperatures are high. Grasslands are tallest and most productive when mean annual precipitation is greater than 800 mm and mean annual temperature is above 15°C.

Native grasslands of North America
23.4 Grassland Ecosystems of the Temperate Zone Vary with Climate and Geography Native grasslands of North America Tallgrass prairie is dominated by big bluestem growing 1 m tall Mixed-grass prairie composed of needlegrass-grama grass (Great Plains) Shortgrass prairie dominated by sod-forming blue grama and buffalo grass Desert grassland replaces buffalo grass with three-awn grass Annual grassland is characterized by rainy winters and hot, dry summers (Mediterranean climate) Central Valley of California

Fig Map showing the original extent of shortgrass, mixed-grass, and tallgrass prairies in North America before the arrival of Europeans, Figure 23.15

Fig. 23.16 North American grasslands.
(a) A remnant tallgrass prairie in Iowa (b) The mixed-grass prairie has been called “daisy(雛菊)-land” for the diversity of its wildflowers (c) Shortgrass steppe(乾草原) in western Wyoming. Figure 23.16a

Eurasia Steppes (乾草原) are treeless and are divided into four belts of latitude. South America Pampas (彭巴斯草原) extend westward in a large semicircle from Buenos Aires to cover about 15 percent of Argentina Southern Africa Velds (大草原) occupy the eastern part of a high plateau

Australia Arid tussock (乾生草叢)— mean rainfall 20 to 50 cm/year (most falls in the summer) Hummock grasslands (山崗草地) — rainfall <20 cm/year Coastal grasslands (沿岸草地) — tropical summer rainfall region Subhumid grasslands (次濕草地) along coastal areas — mean rainfall 50 to 100 cm/year

Grasslands are dominated by herbivorous species Large grazing ungulates Burrowing mammals North American grasslands Bison, pronghorn antelope, prairie dogs

Fig. 23.17 North American grasslands were once dominated by
(a) large grazing ungulates, such as bison and (b) burrowing mammals such as the prairie dog. Figure 23.17

Grasslands are dominated by herbivorous species
23.4 Grassland Ecosystems of the Temperate Zone Vary with Climate and Geography Grasslands are dominated by herbivorous species Eurasian steppes and Argentine pampas lack herds of large ungulates. Pampas deer and guanaco (羊駝) African grassveld (大草原) Historically, wildebeests and zebras with their associated predators (e.g., lion) Replaced with sheep, cattle, and horses Australia Kangaroo species

Grasslands evolved under the selective pressure of grazing Grazing stimulates primary production The major consumers in grassland ecosystems are invertebrates. Heaviest consumption takes place belowground (by nematodes)

There are three strata in the grassland Tall, green, ephemeral (短暫的) herbaceous growth Ground layer (地面層) Belowground root layer (地下根層)

Grasslands accumulate a layer of mulch that retains moisture and adds organic matter to the mineral soil Soils of the grasslands Mollisols (軟土) — relatively thick, dark-brown/black surface horizon rich in organic matter Temperate grassland productivity is related to annual precipitation.

Figure 23.18 Fig Relationship between aboveground net primary production (NPP) and mean annual precipitation for 52 grassland sites around the world.

23.5 Deserts Represent a Diverse Group of Ecosystems
The arid regions of the world occupy 25 to 35 percent of the Earth's landmass It is the lack of precipitation that is the distinctive characteristic of all deserts Deserts lie between 15° and 30° latitude in response to the effect of the Intertropical Convergence Zone (ITCZ) Temperate deserts lie in the rain shadow of mountain barriers or are located far inland

Figure 23.19 Fig Geographic distribution of Earth’s desert ecosystems and associated climate diagrams.

Most of the arid environments are found in the Northern Hemisphere The Sahara in North Africa is the world's largest desert (9 million km2) Temperate deserts of Central Asia (e.g., Gobi Desert) Temperate deserts of North America Most of the deserts in the Southern Hemisphere lie within the subtropical high-pressure belt (ITCZ) The deserts of southern Africa include three regions: Namib Desert, Karoo, Kalahari Desert 40 percent of Australia's land is classified as desert.

The vegetation cover, dominant plants, and groups of associated species differ in response to differences in: Moisture Temperature Soil drainage (土壤排水性) Topography (地形) Alkalinity (鹼濃度) Salinitity (鹽度)

Cold deserts and high elevation hot deserts (e.g., Great Basin of North America, Gobi) Can be considered shrub steppes or desert scrub Dominated by Artemisia and chenopod (藜科植物) shrubs

Fig. 23.20 Two examples of desert scrub.
Northern desert shrubland in Wyoming is dominated by sagebrush (蒿屬植物) (Artemisia). Saltbrush shrubland in Victoria, Australia, is dominated by Atriplex and is an ecological equivalent of the Great Basin shrublands in North America. Figure 23.20

Hot deserts range from those lacking vegetation to ones with some combination of chenopods (藜科植物), dwarf shrubs, and succulents. Southwestern North America (Mojave, Sonoran, Chihuahuan) Bur sage and creosote bush Acacia, saguaro, palo verde, ocotillo, and so forth

Fig. 23.21 Two examples of hot deserts.
Figure 23.21a Fig Two examples of hot deserts. The Chihuahuan Desert in NuevoLeon, Mexico. Dunes in the Saudi Arabian desert near Riyadh.

Plants and animals adapt to the scarcity of water Drought evasion Drought-evading plants flower only when moisture is present and persist as seeds during drought periods Drought-evading animals adopt an annual cycle or go into a dormant state (e.g., estivation) Spadefoot toad (Scaphiopus)

Figure 23.22 Fig A spadefoot toad (鏟足蛙) emerges from its desert burrow to breed when the rains come.

Plants and animals adapt to the scarcity of water Drought evasion Drought resistance Desert plants may be deep-rooted woody shrubs (e.g., mesquite) whose taproots reach the water table Other plants may have shallow roots that extend no more than a few centimeters below the surface (e.g., cacti), enabling a quick uptake of water when it is available. Deserts have low primary productivity.

Desert ecosystems support a diversity of animal life Insects: beetles, ants, locusts Reptiles: lizards, snakes Birds Mammals: grazing and seed-eating herbivores, generalist carnivores

23.6 Mediterranean Climates Support Temperate Shrublands
A shrubland is a plant community in which the shrub growth is dominant or codominant A shrub is a plant with multiple woody, persistent stems and a height from 4.5 to 8m. Mediterranean ecosystems are found along the western margins of the continents between 30° and 40° latitude Dominated by evergreen shrubs and sclerophyllous trees Five regions Western North America Borders of the Mediterranean Sea Central Chile Cape region of South Africa Southwestern/southern Australia

Figure 23.23 Fig Geographic distribution of Earth’s Mediterranean ecosystem and associated climate diagrams.

A Mediterranean climate Hot, dry summers with at least 1 month of protracted drought Cool, moist winters Mediterranean climates support similar-looking plant communities Xeric broadleaf evergreen shrubs Dwarf trees Herbaceous understory

Sclerophyllous vegetation has adaptations that reduce water loss during the summer period Small leaves, thickened cuticles, glandular hairs, sunken stomata Shrubland vegetation also shares adaptations to fire and low nutrient levels in the soil. The largest area of Mediterranean ecosystem forms a discontinuous belt around the Mediterranean Sea

Fig Sclerophyllous leaves of some tree and shrub species inhabiting Mediterranean shrubland (chaparral) of California. chamise scrub oak (橡樹) chinquapin (板栗) Figure 23.24a

Fig. 23.25 Mediterranean vegetation (fynbos) of the Western Cape region of South Africa.
Figure 23.25

Fig. 23.26 Chaparral (叢林) is the dominant Mediterranean shrub vegetation of southern California
Figure 23.26

For the most part, Mediterranean shrublands lack an understory and ground litter and are highly inflammable. Many species have seeds that require the heat and scarring action of fire to induce germination. As chaparral grows taller and denser, a large fuel load builds up and sets the stage for fire. A similarity in habitat structure of the Mediterranean shrublands has resulted in pronounced parallel and convergent evolution among bird and some lizard species This is especially pronounced in the California chaparral and Chilean mattoral.

23.7 Forest Ecosystems Dominate the Wetter Regions of the Temperate Zone
Climatic conditions of the humid mid-latitude regions give rise to the forests dominated by broadleaf deciduous trees. In the mild, moist climates of the Southern Hemisphere, temperate evergreen forests are dominant. Deciduous forests in most parts of the Earth have largely disappeared, cleared for agriculture.

Figure 23.27 Fig Geographic distribution of Earth’s temperate forest ecosystems and associated climate diagrams.

Eastern North America deciduous forest Mixed mesophytic forest (Appalachian plateau) Beech(山毛櫸)–maple and northern hardwood forests (northern regions) Maple(槭樹)–basswood(菩提樹)forests (Great Lakes states) Oak(橡樹)–chestnut(栗木)or central hardwood forests (Appalachian Mountains) Magnolia(木蘭)–oak forests (Gulf Coast states) Oak–hickory (山胡桃木) forests (Ozarks)

山胡桃木 Figure 23.28 菩提樹鐵杉 Fig Large-scale distribution of temperate forest communities in the eastern United States, derived from contemporary data.

Asiatic broadleaf forest (eastern China, Japan, Taiwan, and Korea)
23.7 Forest Ecosystems Dominate the Wetter Regions of the Temperate Zone Asiatic broadleaf forest (eastern China, Japan, Taiwan, and Korea) Similar to the North American deciduous forests but with more broadleaf evergreen species. Southern Hemisphere Southern Andes — deciduous forests Southern Chile — broadleaf evergreen rain forests Tasmania and New Zealand — evergreen forests The end of the growing season in the broadleaf deciduous forests is marked by the autumn colors

Fig. 23.29 A temperate forest of the Appalachian region:
Figure 23.29 Fig A temperate forest of the Appalachian region: The canopy during autumn, and Interior of the forest during spring.

There are four vertical layers in deciduous forests that are highly developed and unevenly aged Upper canopy — dominant tree species Lower tree canopy or understory Shrub layer Ground layer of herbs, ferns, and mosses A diversity of animal life is associated with this vertical stratification and the growth forms of plants Highest diversity in the forest occurs on and just below the ground layer.

23.8 Conifer Forests Dominate the Cool Temperate and Boreal Zones
Conifer forests are found primarily in a broad circumpolar belt across the Northern Hemisphere and on mountain ranges. Low temperatures limit the growing season to a few months each year. Dominated by needle-leaf evergreen trees.

Figure 23.30 Fig Geographic distribution of Earth’s conifer forest ecosystems and associated climate diagrams.

The variable composition and structure of these forests reflect the wide range of climatic conditions in which they grow. Central Europe Dominated by Norway spruce (雲杉) North America Rocky Mountains Subalpine forest — Engelmann spruce, subalpine fir (冷杉) Middle elevations — Douglas fir (冷杉) Low elevations — ponderosa pine and lodgepole pine North America California Sierra Giant sequoia (加州紅杉) North America Pacific Northwest Highly productive coastal forest

Fig. 23.31 Two coniferous forest types.
A Norway spruce (雲杉) in the Tarvisio region of Italy A montane coniferous forest in the Rocky Mountains. The dry, lower slopes support ponderosa pine, the upper slopes are cloaked with Douglas fir (冷杉). Figure 23.31

The boreal forest (or taiga) is the largest expanse of conifer forest. This is the largest vegetation formation on Earth. The taiga encompasses the high latitudes of the Northern Hemisphere and covers 11 percent of the Earth's terrestrial surface. The taiga primarily occupies formerly glaciated land and is a region of cold lakes, bogs, rivers, and alder thickets.

The taiga is divided into three vegetation zones Forest-tundra ecotone Open lichen woodland Main boreal forest — this grades into the temperate forest

Fig 23.32 Major subdivisions of the North American boreal forest.
Figure 23.32 Fig Major subdivisions of the North American boreal forest.

Figure 23.33 Fig Black spruce (雲杉) is a dominant conifer in the North American taiga.

The taiga is dominated by a cold continental climate with strong seasonal variations Summers are short, cool, and moist Winters are long, harsh, and dry Interior Alaska and central Siberia experience some of the most extreme seasonal temperature fluctuations (as much as 100°C between min and max).

The permafrost (永凍土) is the perennially frozen subsurface that develops where the ground temperatures remain below 0°C Because the permafrost is impervious to water, it forces all water to remain and move above it The ground stays soggy(潮濕的) even though precipitation is low.

During periods of drought, fires can affect a large portion of the taiga. Boreal species, broadleaf trees and conifers, are well adapted to fire. Fire is a source of regeneration Many areas of boreal forest are being clear-cut because of global demand for timber and pulp.

The boreal forest has a unique animal community Herbivores include caribou(北美馴鹿), moose(麋鹿), snowshoe hare, red squirrel, and porcupine(豪豬) Predators include wolf, lynx(山貓), pine martin(紫岩燕), and owl (貓頭鷹) Nesting location of migratory neotropical birds Herbivorous insects (e.g., spruce budworm)

23.9 Low Precipitation and Cold Temperatures Define the Arctic Tundra
The tundra is a frozen plain that is located at the highest latitudes of the Northern Hemisphere Tundra has up to 100 percent plant cover and wet to moist soils. Polar desert has less than 5 percent plant cover and dry soil.

Figure 23.34 Fig Geographic distribution of Earth’s tundra ecosystems and associated climate diagrams.

Fig The plant cover that characterizes the wide expanse of the Arctic tundra in the Northwest Territories of Canada is in stark contrast to The polar desert that is characterized by dry soils and sparse plant cover. Figure 23.35

Three forces interact to form the conditions unique to the Arctic tundra Permanently frozen deep layer of permafrost Overlaying active layer of organic matter and mineral soil that thaws each summer and freezes the following winter. Vegetation that reduces warming and retards thawing in summer.

The alternate freezing and thawing of the upper layer of soil pushes stones and other material upward and outward from the mass to form a patterned surface Frost hummocks, frost boils, earth stripes, stone polygons Solifluction terraces or "flowing soil" form on sloping ground Creep, frost thrusting, downward flow of supersaturated soil over the permafrost

Fig. 23.36 Patterned landforms typical of the tundra region:
Frost hummocks (冰丘), and Polygons (多邊形). Alternate freezing and thawing of the upper layer of soil creates the symmetrically patterned landform Figure 23.36a

The vegetation structure of the tundra is simple — few species and slow growth Only species that can withstand constant soil disturbance, buffeting(猛擊) by the wind, and abrasion(擦傷) by wind-carried particles of soil and ice can survive. Arctic plants propagate by vegetative means.

Low ground vegetation Cotton grasses, sedges (莎草), and Sphagnum(水蘚) Well-drained sites Heath shrubs, dwarf willows (矮柳), birches(白樺), herbs, mosses(苔蘚), and lichens(地衣) Driest and most exposed sites Scattered heaths(石南植物), crustose and foliose lichen(地衣)

Tundra plants are photosynthetically active about three months out of the year They maximize use of the growing season and light by photosynthesizing during the 24-hour daylight period. Erect leaves allow for almost complete interception of solar radiation. Most of the photosynthate goes into the production of new growth. At the end of the season, resources are allocated to the roots.

Most of the tundra vegetation is underground Root-to-shoot ratios range from 3:1 to 10:1 Roots are concentrated in the upper soil that thaws during the summer.

Animal diversity in the tundra is low Invertebrates are concentrated near the surface Segmented whiteworms, collembolas, and flies Dominant vertebrates are herbivores Caribou, lemmings, Arctic hares, musk oxen Carnivore Wolf, Arctic fox, snowy owls, jaegers, and other birds

Fig. 23.37 Rocky Mountains alpine tundra.
Figure 23.37 Fig Rocky Mountains alpine tundra. Alpine tundra occurs in the higher mountains of the world.

Strong winds, snow, cold, and widely fluctuating temperatures dominate these mountainous areas Because the atmosphere is thin at high elevations, ultraviolet light is especially intense on clear days. Summer soil temperature ranges from 40°C to 0°C! These areas receive precipitation but the steep topography induces rapid runoff (and loss) of water.

Chap.23 Terrestrial Ecosystems
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