Despite the increasing importance of functional traits for the study of plant ecology, we do not know how variation in a given trait changes across ecological scales, which prevents us from assessing potential scale-dependent aspects of trait variation. To address this deficiency, we partitioned the variance in two key functional traits (leaf mass area and leaf dry matter content) across six nested ecological scales (site, plot, species, tree, strata and leaf) in lowland tropical rainforests. In both traits, the plot level shows virtually no variance despite high species turnover among plots and the size of within-species variation (leaf + strata + tree) is comparable with that of species level variation. The lack of variance at the plot level brings substantial support to the idea that trait-based environmental filtering plays a central role in plant community assembly. These results and the finding that the amount of within-species variation is comparable with interspecific variation support a shift of focus from species-based to trait-based ecology.

A fundamental goal of ecology is to understand what controls the distribution and abundance of species. Both environmental niches and trade-offs among species in dispersal and competitive ability have traditionally been cited as determinants of plant community composition. More recently, neutral models have shown that communities of species with identical life-history characteristics and no adaptation to environmental niches can form spatial distribution patterns similar to those found in nature, so long as the species have a limited dispersal distance. If there is a strong correlation between geographic distance and change in environmental conditions, however, such spatial patterns can arise through either neutral or niche-based processes. To test these competing theories, we developed a sampling design that decoupled distance and environment in the understory plant communities of an old-growth, temperate forest. We found strong evidence of niche-structuring but almost no support for neutral predictions. Dispersal limitation acted in conjunction with environmental gradients to determine species' distributions, and both functional and phylogenetic constraints appear to contribute to the niche differentiation that structures community assembly. Our results indicate that testing a neutral hypothesis without accounting for environmental gradients will at best cause unexplained variation in plant distributions and may well provide misleading support for neutrality because of a correlation between geographic distance and environment. N eutral theory (1-3) demonstrates that the compounded effects of dispersal limitation, speciation, and the role of chance through time can cause ecologically identical species to form patterns of distribution and abundance similar to those found in nature. The assumption of ecological equivalence among species in neutral theory challenges contemporary views on the importance of evolution and ecological adaptation in determining patterns of distribution and abundance, and thus current approaches to species conservation (2, 4). Although some assumptions of neutral theory have been questioned (5, 6), the value of the theory lies in the degree to which it can predict or refute the importance of species-specific traits in determining distribution and abundance. Aspects of neutral theory can be tested by model fitting (6), but a stronger test lies in directly assessing the degree to which species distributions are explained by environmental heterogeneity versus neutral processes operating on natural landscapes (4, 6, 7).Because of the spatial effects of dispersal limitation, neutral theory predicts that the compositional similarity between plant communities will decrease as the distance between two points increases (3,8). In contrast, niche theory predicts that community composition will change as a result of species-specific differences in evolved adaptive responses along environmental gradients (9, 10). These hypotheses are hard to distinguish in natural ecosystems, because a change in envi...

Recent work has identified a worldwide "economic" spectrum of correlated leaf traits that affects global patterns of nutrient cycling and primary productivity and that is used to calibrate vegetation-climate models. The correlation patterns are displayed by species from the arctic to the tropics and are largely independent of growth form or phylogeny. This generality suggests that unidentified fundamental constraints control the return of photosynthates on investments of nutrients and dry mass in leaves. Using novel graph theoretic methods and structural equation modeling, we show that the relationships among these variables can best be explained by assuming (1) a necessary trade-off between allocation to structural tissues versus liquid phase processes and (2) an evolutionary tradeoff between leaf photosynthetic rates, construction costs, and leaf longevity.

No abstract

No abstract