The biome concept organizes large-scale ecological variation. Terrestrial biomes are distinguished primarily by their predominant vegetation, and are mainly determined by temperature and rainfall.
Differences in temperature or precipitation determine the types of plants that grow in a given area . Generally speaking, height, density, and species diversity decreases from warm, wet climates to cool, dry climates. One such system was based on the location of the perennating organ. These are tissues that give rise to new growth the following season, and are therefore sensitive to climatic conditions. The relative proportions of different life forms vary with climate. In fact, life form spectra are more alike in similar climates on different continents than they are in different climates on the same continent . Regions of similar climate and dominant plant types are called biomes. This chapter describes some of the major terrestrial biomes in the world; tropical forests, savannas, deserts, temperate grasslands, temperate deciduous forests, Mediterranean scrub, coniferous forests, and tundra.
Different Terrestrial Biomes
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Tropical forests are found in areas centered on the equator. Central and South America possess half of the world’s tropical forests. Climate in these biomes shows little seasonal variation , with high yearly rainfall and relatively constant, warm temperatures. The dominant plants are phanerophytes - trees, lianas, and epiphytes. Tropical rainforests have an emergent layer of tall trees over 40 m tall, an overstory of trees up to 30 m tall, a sub-canopy layer of trees and tall shrubs, and a ground layer of herbaceous vegetation.
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Located north and south of tropical forest biomes are savannas (Figure 4), with lower yearly rainfall and longer dry seasons (Figure 6). These biomes are dominated by a mix of grasses and small trees. Savannas cover 60% of Africa and represent a transition from tropical forests to deserts. Trees in savannas are usually drought deciduous. Several savanna types associated with differing rainfall patterns, height of the water table and soil depth can be distinguished by their relative abundance of trees and grass.
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Deserts generally occur in a band around the world between 15–30° N and S latitude. They cover between 26–35% of the land surface of the Earth. The climate of deserts is dominated by low precipitation, generally below 250 mm yr-1. However, there is a lot of variability in desert types, with hot deserts, cold deserts, high elevation deserts, and rain shadow deserts. Consequently, there is a great deal of variation in the biodiversity, productivity and organisms found in different types of desert.
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Grassland biomes occur primarily in the interiors of continents and are characterized by large seasonal temperature variations, with hot summers and cold winters . Precipitation varies, with a strong summer peak. The type of grassland community that develops, and the productivity of grasslands, depends strongly upon precipitation. Higher precipitation leads to tall grass prairie with a high biodiversity of grasses and fords. Lower precipitation leads to short grass prairies and arid grasslands.
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Temperature deciduous forests occur in mid-latitudes (Figure 4) where cool winters, warm summers, and high year round precipitation occurs (Figure 9). Net primary productivity ranges from 600–1500 g m-2 yr-1 with high litter production. Litter serves as a major pathway for nutrient recycling. This biome is named for the dominant trees that drop their leaves during the winter months. These forests may have an overstory of 20–30 m tall trees, an understory of 5–10 m trees and shrubs, a shrub layer around 1–2 m in height, and a ground layer of herbaceous plants. Biodiversity is relatively high in this biome due to the niche partitioning allowed by the multiple forest layers. More complex forests are associated with a greater number of animal species; for example, bird species diversity shows a positive correlation with forest height and number of layers.
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Located at higher latitudes is a biome dominated by needle-leaved, drought tolerant, evergreen trees , and a climate consisting of long, cold winters and short, cool summers. Biodiversity is low in this two-layered forest made up of an overstory of trees and a ground layer of herbs or mosses. The overstory in much of the boreal forest is made up of only one or two species. The low biodiversity is mirrored by low net primary productivity of 200–600 g m-2 yr-1. Productivity varies with precipitation, the length of the frost-free period, and local soil drainage. In flooded areas, sphagnum bogs may develop. The acidic tissue of sphagnum, and the anoxic, flooded conditions, slows decomposition, resulting in the production of peat bogs.
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At latitudes beyond the boreal forest tree line lies a marshy area where growing seasons are very short and temperatures are below zero degrees Celsius for much of the year . Because of these low temperatures and short growing seasons, net primary productivity is very low in the tundra, between 100–200 g m-2 yr-1. Productivity varies with snowfall depth and local drainage. Rocky fields and dry meadows will have lower productivity than moist, low-lying areas and wet meadows.