Chapter 6, part I: Terrestrial Biomes

Transcription

1 Chapter 6, part I: Terrestrial Biomes I. A biome is a large, relatively distinct terrestrial region with characteristic climate, soil, plants, and animals, regardless of where it occurs. A biome encompasses many interacting ecosystems. Temperature and precipitation are the primary abiotic factors that influence biome distribution. A. Tundra, the northernmost biome, is characterized by a permanently frozen layer of subsoil called the permafrost, by low-growing vegetation (Fig. 6.3) adapted to extreme cold, and by a very short growing season. 1. Compared to most other biomes, tundra has low species diversity, which is thought be many ecologist to illustrate the species-diversity-depressing effect of environmental instability, as was discussed previously. 2. The permafrost acts as a barrier to the infiltration of water which keeps the nearsurface soil wet to saturated. Despite what is says in the book, some of the wetter tundra soils build up high levels of organic matter in much the same way as bogs accumulate organic matter - because the microbes cannot efficiently mineralize the plant remains without O The permafrost also acts as a barrier to root penetration, which limits the size of plants that can grow there; no full-size trees. B. The taiga, or boreal (which means northern as in aurora borealis) forest, lies south of the tundra and is dominated by coniferous (i.e., cone-bearing) trees that are adapted to the cold winters, a short growing season, and acidic, mineral-poor soil. 1. The absence of permafrost permits tree-size vegetation to grow. 2. The water needed to support trees is also available at the latitudes where taiga occurs, because they wrap around that globe in a belt at just about the 60 N circle where Chapter 5 (Fig. 5.11) said it rains (and snows) plentifully. Actually, only about 20 in. of rain may fall in some areas of the taiga (because cold air can hold less water than warm air) but that is enough to support trees in regions where low temperatures keep the rate of water loss through evapotranspiration low. 3. Why needle-leafed trees instead of broad-leafed hardwoods? a. As a general rule of thumb, conifers tolerate harsh environmental conditions more resiliently than broad-leafed trees. b. Conifers also tolerate droughty, sandy soils better, which is why the pine barrens of New Jersey are PINE barrens. c. Because conifers can survive drought better than broad-leafed trees they can survive the long winter months of the taiga when the frozen soil prevents them from absorbing liquid water. They are more cold tolerant. d. Conifers also tolerate acidic and low nutrient soil conditions better than broad-leafed trees. In fact, conifers may be using their greater tolerance for these two soil conditions to their selective advantage in areas where they compete with broad-leafed trees, by fostering the

2 development of soils with high levels of acidity and low levels of many plant nutrients. Soil acidification by hemlocks is mentioned in the Alleeeffect discussion article. C. Temperate rain forest, such as occurs on the northwest coast of North America (Fig. 6.5), receives very high precipitation ( in./yr) and is dominated by large conifers. This biome is also found in southwestern Australia and along the west coast of southern Chile. 1. This biome occurs in areas that are the opposite of rain shadows, so the large examples are always near a coast. Thus, their temperatures are moderated by the nearby seawater. 2. The high rainfall keeps the soils soggy so high levels of organic matter develop in the soils under thick litter layers. 3. The predominance of conifers in the vegetation, again, results in the formation of acidic, nutrient poor soils. D. Temperate deciduous forest occurs where precipitation is relatively high (30 to 50 in./yr. Note: We get 40 in.) and soils are fairly rich in inorganic plant nutrients. Broad-leaf trees that lose their leaves seasonally dominate this biome. 1. Broad-leafed trees need soils that form in parent materials that are richer in nutrients than conifers need. The soils that form under these deciduous trees are quite rich in organic matter, but only in the top 2 to 4 inches. 2. Unlike conifers, deciduous trees are very good at intercepting calcium and magnesium ions (which are ingredients in lime) before they can leach out of the rooted depth of soil. The calcium and magnesium are then returned to the surface of the soil with the fallen leaves. In this way, deciduous temperate forests are self liming and do not allow their soils to become as acidic as taiga soils do. 3. Western Massachusetts is in a transition zone between this biome and taiga. E. Temperate grassland ( prairies in N.A. or steppes in Central Asia) typically possesses deep, mineral-rich soil and has moderate but uncertain precipitation (10 to 30 inches/yr). Temperate grassland is well suited to growing grain crops. 1. At temperate latitudes, about 30 in. of precipitation is needed to support a forest. A little drier and you get grasses instead. Around 25 inches/yr of rain will support tall-grass prairie (Fig. 6.7) composed of perennial grasses that can get taller than you are. Less than about 20 inches/yr of rain results in short-grass prairies. 2. Unlike trees, grasses shed a lot of biomass below ground in the form of dead roots instead of adding most of their dead biomass to the surface of the ground. Consequently, the very dark topsoil formed under tall grass prairie can be over 2 feet thick. 3. The low levels of precipitation in this biome are unable to leach out the nutrients that

3 were originally present. Consequently, the soils stay nutrient rich, as well as near neutral in ph. F. Tropical grassland, called savanna, has widely scattered trees interspersed with grassy areas. It occurs in tropical areas with low or seasonal rainfall, including much of subsaharan Africa, south of the Amazon rainforest in Brazil and north of it in Venezuela, and a strip across northern Australia just south of that continent s rainforested peninsula. 1. Despite seemingly high rates of precipitation (34 to 60 in./yr) in this biome, trees are sparse because the hotter climate and more intense sunlight increase evapotranspirational demands for water, so the same amount of rainfall supports less total plant biomass here than in temperate grasslands or forests. Also, rainfall is strongly seasonal in many savannas, which makes it seem more limiting during the dry season. The same amount of rainfall in a tropical area can sustain more total plant biomass if that rainfall is spread out over the entire year than if it is concentrated into a few intense storms. 2. Savannah soils have been subject to leaching for millions of years and tend, as a result, to have low nutrient levels. Many of the common trees in savannahs are nitrogen-fixing legumes such as those in the genus Acacia (Fig. 6.10) 3. Grazing animals in Savannahs with strongly seasonal rainfall patterns follow the availability of edible plants in annual migrations. G. Thickets of small-leaf evergreen shrubs and trees and a climate of wet, mild winters and dry summers characterize the chaparral. 1. Chaparral occurs in climates termed Mediterranean, so we should not be surprised to find this biome along the European, Middle Eastern, and (at least northwest) African margins of that sea. 2. It can be found at similar latitudes on the west coast of North America (in southern California, Fig. 6.8) and at similar latitudes south in Australia and at the southernmost point in Africa. H. Desert, found in both temperate (cold desert, e.g., the Gobi desert or northern Afghanistan) and subtropical or tropical regions (warm desert, e.g., the Sahara and much of central Australia) with low levels of precipitation, contains organisms with specialized water-conserving adaptations. 1. At temperate latitudes, less than 10 in./yr of precipitation produces this biome. 2. The low levels of precipitation result in essentially no removal of minerals from the soil by leaching. As a result, these soils can be very salty (w/ not just NaCl, but also with calcium carbonate, gypsum, and even borax). The salts can build up to levels that are toxic for many plants. Deserts are also just about the only places where strongly alkaline soils can be found. 3. Deserts are NOT all covered in sand dunes or shifting sands. Far more surface area in deserts is covered by a pavement of rocks and pebbles that are large enough to resist erosion by wind. Any sand particles that started out at the surface have blown away long ago to become part of the shifting sands. 4. Most deserts have some plant and animal life. Only the worlds driest deserts, like

4 the Atacama desert that lies in the rainshadow of the Andes are almost devoid of life. Fig. 6.9 shows a comparatively moist desert. I. A tropical environment with mineral-poor soil and very high rainfall that is evenly distributed through the year characterizes tropical rain forest. Tropical rain forest has a high species diversity and high productivity. 1. The soils of tropical rainforests are subject to intense leaching by the 80 to 180 in. of annual rainfall required to support this biome. As a result, the soils have very low inherent fertility. The plant nutrients in these ecosystems are almost entirely stored in the plant biomass, rather than being contained in large reserves in the soil. 2. The decomposing activities of the microbes and the nutrient-scavenging action of the trees combine to recycle the nutrient elements from recently deceased plants and animals back into plant biomass with extraordinary efficiency. There is no thick litter layer in this kind of forest. 3. When the mineral nutrients are released from vegetation by slashing and burning portions of a rain forest, the minerals are released all at once into the soil with few tree roots under them to prevent their loss by leaching. Consequently, slashed and burned patches of rain forest only support crop production for two or three years before they wear out and must be abandoned for the forest to reclaim and enrich again. 4. Leaves of woody species in this biome need not be shed after a brief growing season, so they are built to last up to two years. They are also formed more slowly and are optimized for efficiency of photosynthetic activity, not for how fast they can be created. Accordingly, the odd, jaggy shapes that are common among temperate zone trees like oaks and maples are rarely seen in the tropical rain forests. 5. Except for occasional vines and lianas, little vegetation occurs in the understory of tropical rainforests, because only 2 to 3% of all that sunlight penetrates that far. Only in movie set jungles do you need a machete to cut your way through a rainforest. However, Tropical dry forests prove more of a challenge to ground level travelers. The river in Fig creates an opening in the forest canopy allowing light - and growing vegetation - to reach the forest floor 6. Much of the rain that falls on tropical rainforests is recycled, i.e., it formed from water vapor that was transpired by plant a little bit upwind in the same forest. Consequently, human-created changes in these communities that cause more of the rain to run off in streams will result in the loss of that water from several subsequent rainfalls. Therefore, many ecologists feel that if enough of the Amazon forest is clear cut, markedly less rain more fall on the forest that remains.

5 II. The temperature decreasing effects of increasing altitude can separate biomes along altitudinal gradients in a way that parallels latitudinal gradients. (Fig. 6.12) A. In moist, temperate regions, temperate deciduous forests at the base and on the lower slopes of a mountain range may give way to a band of taiga-like, conifer-dominated trees called subalpine forest. Beyond the tree line, the upper altitude where trees can be found, is alpine tundra, a biome with low-growing plants with adaptations remarkably similar to those found in the arctic. B. Near the equator, the subalpine forest is called cloudforest and often contains more conifers than occur at low elevations, but is usually not dominated by them. Above the cloud forest s tree line is the paramo, a cold misty biome dominated by grasses that are much taller than those of alpine tundra, which also occurs in the tropics if you travel even further upslope.