Terrestrial Biomes

At the beginning of this course, we talked about the physical geography is mainly concerned with four spheres: atmosphere, hydrosphere, lithosphere and biosphere. Biosphere refers all the living organisms of the earth, including both plants and animals.

1 Concepts

The study of the interactions between life-forms and their environment is the science of ecology.

An ecosystem is a self-sustaining association of living plants and animals and their nonliving physical environment.

Ecosystems fall into two major groups- aquatic and terrestrial. Aquatic ecosystems include marine environments and the freshwater environments of the lands. Our study of physical geography will not include these aquatic environments. In stead, we will focus on the terrestrial ecosystems, which are dominated by land plants spread widely over the land surfaces of the continents.

Within terrestrial ecosystems, the largest recognizable subdivision is the biome. A biome is a large, stable terrestrial ecosystem characterized by specific plant and animal communities. Each biome usually named for its dominant vegetation. we can generalize Earth’s wide-ranging plant species into six broad biomes: forest, savanna, grassland, shrubland, desert, and tundra.

Forests consist of trees growing so close together that their individual leaf canopies generally overlap. This means that the ground is largely in shade. Forests require considerable annual precipitation and can survive in widely varying temperature zone.

Woodlands are tree-dominated plant associations in which the trees are spaced more widely apart than in forests and do not have interlacing canopies.

Shrublands are plant associations dominated by relatively short woody plantrs, generally called shrubs or bushes.

Grasslands may contain scattered trees and shrubs, but the landscape is dominated by grasses and forbs (broadleaf herbaceous plants.

Deserts are typified by widely scattered plants with much bare ground intersperded.

Tundra consists of a complex of very low plants, including grasses, forbs, dwarf shrubsm mossesm and lichens, but no trees.

Wetlands are characterized by shallow standing water all or most of the year, with vegetation rising above the water level.

With trees, whether or not a plant losses its leaves sometime during the year is an important distinguishing characteristic. An evergreen tree is one that sheds its leaves on a sporadic or successive basis but always appears to be fully leaved. A deciduous tree is one that experiences an annual period in which all leaves die and usually fall from the tree, due to either a cold season or a dry season.

Trees are also often described in terms of leaf shapes. Broadleaf trees have leaves that are flat and expansive in shape, whereas needleleaf trees are adorned with thin sliver of tough, leathery waxy needles rather than typical leaves.

2. major terrestrial biomes

Now we like to talk about the major biomes of the world.

(1). Tropical rainforest

You are probably familiar with the term tropical rainforest because it is used so often these days in the media.

The Amazon region is the largest tract of tropical rainforest. In addition, the margins of Madagascar and Southeast Asia, the Pacific coast of Ecuador and Colombia, the east coast of Central America.

The rainforset is probably the most complex of all terrestrial ecosystmes. It has a large number of species of trees. As many as 3000 species may be found in a few square kilometers.

It has a layered structure. The top layers are the forest giants- tall trees that often grow to 40 m. the second layer down from the top usually forms a complete canopy of interlaced branches that provides continuous shade to the forest floor. Beneath the canopy is an erratic third layer of lower trees able to survive in the shade. Sometimes still more layers of increasingly shade-tolerant trees grow at lower levels. The ground surface is covered only by a thin litter of leaves. Dead plant matter rapidly decomposes because of the warm temperatures and abundant moisture.

In this environment of continuous warmth and moisture, those plants never experience a seasonal leaf fall.

(2). Tropical Deciduous Forest

Tropical Deciduous forest develops in the wet-dry tropical climate.

This is different from tropical forest because it is deciduous, with most of the trees shedding their leaves during the dry season. Shedding of leaves results from the stress of a long dry season that occurs at the time of low sun and cooler temperature.

In the tropical deciduous forest, the canopy is less dense, the trees are somwhat shorter, and there are fewer layers. Because of less dense canopy, light easily reaches the lower layers of the forest. As a result, the lower layers are better developed than in the rainforest.

Also, the diversity of tree species is not as great as tropical forest.

(3). Tropical Savanna

Tropical Savanna is large expanses of grassland, interrupted by trees and shrubs.

Savanna biomes range from woodland to grassland.

In savanna woodland, the trees are spaced rather widely apart, because soil moisture during the dry season is not sufficient to support a full tree cover. the trees are of medium height. Tree crowns are flattened or umbrella-shaped, and the trunks have thick, rough bark. In Africa, the savanna woodland grades into a belt of thorntree-tall-grass savanna. The trees are largely of thorny species.

The open spacing permits development of a dense lower layer, which usually consists of grasses.

The savanna biome has a very pronounced seasonal rhythm. During the wet season, the grass grows tall, green, and luxuriant. At the onset of the dry season, the grass begins to wither, and before long the above ground portion is dead and brown. At this time,many of the trees and shrubs shed their leaves. After then, a number of wild fires may occur.

(4). Subtropical evergreen forest

Subtropical evergreen forest is generally found in regions of moist subtropical climate, where winters are mild and there is ample rainfall throughout the year.

This forest occurs in two forms: broadleaf and needleleaf. In comparison with tropical rainforest, both tropical broadleaf evergreen forest and tropical rainforest are broadleaf evergreen types. But the former has relatively fewer species of trees. Trees are not as tall as in the tropical rainforest. Their leaves tend to be smaller, and the leaf canopy is less dense. As a result, it often has a well-developed lower layer of vegetation.

The subtropical needleleaf evergreen forest occurs only in the southeastern United States, it is pine forest.

(5). Desert Biome

Earth’s desert biomes cover more than one-third of its land area

Desert vegetation consists largely of drought-resisting plants. The plant cover is usually sparse, with considerable bare ground dotted by a scattering of individual plants. Typically the plants are shrubs, which occur in considerable variety, each with its own mechnisms to combat the stress of limited moisture.

(6). Mediterranean woodland and shrub

The mediterranean woodland and shrub biome is found in the areas with the pronounced dry summer and wet winter. The biome is dominated mostly by a dense growth of woody shrubs. A second significant plant association of mediterranean region is an open grassy woodland, in which the ground is almost completely covered but has a considerable scattering of trees as well.

The plant species vary from region to region. Oaks of various kinds are the most significant ones in Northern Hemisphere mediterranean lands. In all areas, the trees and shrubs are primarily broadleaf evergreens. Their leaves are mostly small and have a leathery texture or waxy coasting, which inhibits water loss during the long dry season. Moreover, most plants have deep roots.

(7). Midlatitude grassland

Vast grasslands occur widely in the midlatitudes of North America and Eurasia.

The vegetation typical of a grassland biome is a general response either to a lack of precpitation sufficient to support larger plant forms or to the frequency of fires that prevent the growth of tree or shrubs seedlings.

In the wetter areas of a grassland biome, the grasses grow tall and the term prairie is often applied. In dries regions, the grasses are shorter; such growht is often referred to as steppe.

Most of the grass species are perennials, lying dormant during the winter and sprouting anew the following summer. Trees are mostly restricted to riparian locations, whereas shrubs and bushes occur sporadically on rocky sites.

Grass fires are fairly common in summer, which helps to explain the relative scarcity of shrubs. tHe woody plants cannot tolerate fires and generally can survive only on dry slopes where there is little grass cover to fuel a fire.

(8). Midlatitude Deciduous Forest

Extensive areas on all Northern Hemisphere continents, as well as more limited tracts in theSouthern Hemisphere, were originally covered with a forest of largely broadleaf deciduous trees. Except in hilly country, a large porportion of this forest has been cleared for agriculture and other types of human use, so that very little of the original natural vegetation remains.

The forest is characterized by a fairly dense growth of tall broadleaf trees with interwoven branches that provide a complete canopy in summer. Some smaller trees and shrubs exist at lower levels, but for the most partm the forest floor is relatively barren of undergrowth. In winter the appearance of the forest changes dramatically owing to the seasonal fall of leaves.

Trees species vary considerably from region to region, though most are broadleaf and deciduous. An unusual situation in the southeastern US if extensive stands of pines rather than deciduous species.

(9). Boreal forest

One of the most extensive biomes is the boreal forest. The boreal forest occupies a vast expanse of northern North America and Eurasia. There is very close correlation between the location of the boreal forest biome and subarctic climatic type, with a similar correlation between the locations of the tundra climate and the tundra biome.

This great northern forest contains perhaps the simplest assemblage of plants of any biome. Most of the trees are conifers, nearly all needleleaf evergreens, the variety of species is limited to mostly pines, firs, and spruces extending broadly in homogeneous stands, in some places, the coniferous cover is interrupted by areas of deciduous trees, these deciduous stands are also of limited variety (mostly birch, poplar, and aspen).

The trees grow taller and more densely near the southern margins of this biome, where the summer growing season is longer and warmer. near the northern margens, the trees are spindly, short and more openly spaced.

Undergrowth is normally not dense beneath the forest canopy, but a layer of deciduous shrubs sometimes grows. The ground is usually covered with a complete growth of mosses and liches, with some grasses in the south and a considerable accumulation of decaying needles overall.

(10). Tundra

The tundra is essentially a cold desert or grassland in which moisture is scarce and summers so short and cool that trees are unable to survive.

This biome is distributed along the northern edge of the Northern Hemisphere continents. The plant cover consists of a considerable mixture of species, many of them in dwarf forms. Included are grasses, mosses, lichens, flowering hers, and a scattering of low shrubs. These plants often occur in a dense, ground-hugging arrangement. The plants complete their annual cycles quickly during the brief summer, when the ground is often moist and waterlogged.