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Classification Based on Species 3.2.2.1

Classification at the Community Level

The community type found at any given location depends not only on the local climate, but also on the physical and chemical properties of the soil, the topography and elevation, and previous site history. Species integrate all factors of the environment. The functions of all the species in a community combine to produce the function of the community. Therefore, classification based upon communities implies classification based on function. Communities frequently are named after the species that predominate. Richards (1952) described forest communities on the northern edge of the British Guiana forest as follows: "The five communities, in the order they are usually met with from the creek to the ridges, are the Mora consociation (Mora excelsa), the Morabukea consociation (dominant Mora gonggrijpii), the mixed forest association (many dominants), the Greenheart consociation (dominant Ocotea rodiaei) and the wallaba consociation (dominant Eperua falcata)." Communities along a soil and hydrologic gradient in the Rio Negro region of Venezuela have been given local names: igapo, caatinga, bana, and tierra firme (Fig. 3.5).

Classification based upon community or dominant species is useful in designing management strategies because managers are usually interested primarily in species, since markets demand trees of a given species. Community classification is useful too because such classifications reflect function. For

Caatinga Forest

Tierra Firme Forest

Species Dominant Mixed Species Tierra Firme Tierra Firme

High Caatinga Low Caatinga Baña High Caatinga

Species Dominant Mixed Species Tierra Firme Tierra Firme

High Caatinga Low Caatinga Baña High Caatinga

1000 meters

Fig. 3.5. Structure of tropical vegetation along a soil and hydrologie gradient in the Amazon lowlands. (Adapted from Jordan 1985, with permission of John Wiley and Sons Ltd., publisher)

1000 meters

Fig. 3.5. Structure of tropical vegetation along a soil and hydrologie gradient in the Amazon lowlands. (Adapted from Jordan 1985, with permission of John Wiley and Sons Ltd., publisher)

example, the caatinga and bana soils (Fig. 3.5) are low in nitrogen and thus nitrogen-conserving functions, such as production of sclerophyllous leaves, are common. The tierra firme soils are very low in available phosphorus, and thus phosphorus-mobilizing functions, such as secretion of organic acids from roots, may be common (Cuevas 2001).

The changes in species function along a gradient of soil fertility are reflected in a change in species diversity. Diversity is lowest in the bana, intermediate in the caatinga, and higher in the tierra firme. As on mountain slopes and moisture gradients, diversity reflects the environmental stress impinging on the community, except here, soil quality rather than climate governs the level of stress. Communities undergoing the greatest stress due to soil conditions have the lowest potential productivity (Cuevas 2001).

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