Spatial scale or amplitude of predictors as determinants of the relative importance of environmental factors to plant community structure

Abstract

The literature on tropical rain forest plant-community relationships with environmental factors usually does not recognize that the relative importance of environmental factors recorded in each study might be due to their amplitude of variation within sites. Geographic scale, however, is recognized as an important modulator of this relative importance. To disentangle the effects of scale and environmental amplitude, ferns and trees in two landscapes of the same size (each 25 km2) with different soil-fertility amplitudes but similar soil-texture range were sampled in central Amazonia. We found that major determinants of community structure were the same for ferns and trees. Texture was the main predictor of community structure in the site with homogeneous soil fertility, while availability of exchangeable cations was the main predictor in the site with a wider fertility range. When both sites were analyzed together, soil fertility was the main predictor of community structure and soil texture segregated floristic subgroups within certain ranges of the soil-fertility gradient. We conclude that: (1) floristic patterns for trees and ferns are congruent; (2) floristic variation depends on the amplitude of the studied gradients, more than on geographical scale; (3) limiting factors are not necessarily the most important predictors of compositional patterns; and (4) communities are structured hierarchically. Therefore, landscape structure (meaning which combinations of environmental factors, their amplitude of variation and which part of the gradient is found within the landscape) affect our perception of the relative importance that environmental factors will have as predictors of species composition. © 2012 The Author(s) Journal compilation © 2012 by The Association for Tropical Biology and Conservation.