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Fig. 3.4. Structure of tropical vegetation along an elevational gradient in continental regions. (Adapted from Jordan 1985, with permission of John Wiley and Sons Ltd., publisher)

with no evapotranspiration). For each combination of region and province, there is a characteristic vegetation type (Fig. 3.2).

Walter (1971) also used climate to delineate global vegetation units. Within the tropics, he recognized equatorial rain forest, tropical rain forests with orographic rains, tropical savannas, rain-green tropical deciduous forests, tropical thorn-steppes, and thorn forests. Figure 3.1 illustrates how forest structure is influenced by climate in lowland regions such as those of Africa, where vegetation formations are finely tuned to the moisture gradient. Beard (1949) classified forests on mountains of small tropical islands according to rainfall and elevation. Elevation of each forest type varies, depending upon the size of the island and the direction of wind relative to the mountains (Fig. 3.3). In continental regions, clouds and rain form at higher elevations than on islands and, as a consequence, the range of each forest type occurs at a higher elevation and covers a greater range (Fig. 3.4; see also Chap. 2).

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