Plant Functional Types

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Biomes or ecosystem types have long been recognized as occupying domains in an environmental space defined by temperature and moisture gradients. Within particular biomes, the factors governing structure and dynamics may vary considerably. For example, species composition and age structure in plant communities are largely controlled by asymmetric competition for an above-ground resource (light) and symmetric competition for below-ground resources (moisture and nutrients). In mesic forests, the principal constraint seems to be the availability of light. As a forest environment tends from mesic to xeric conditions, the effective constraint shifts from above-ground to below-ground (Tilman 1988; Smith and Huston 1989). In still drier environs, the forest yields to grassland, where the principal constraint is below-ground. This suggests that there are general trends or patterns in the relative influence of environmental constraints in structuring ecosystems.