A trade-off between growth and mortality rates characterizes tree species in closed canopy forests. This trade-off is maintained by inherent differences among species and spatial variation in light availability caused by canopy-opening disturbances. We evaluated conditions under which the trade-off is expressed and relationships with four key functional traits for 103 tree species from Barro Colorado Island, Panama. The trade-off is strongest for saplings for growth rates of the fastest growing individuals and mortality rates of the slowest growing individuals (r2 = 0.69), intermediate for saplings for average growth rates and overall mortality rates (r2 = 0.46), and much weaker for large trees (r2 < or = 0.10). This parallels likely levels of spatial variation in light availability, which is greatest for fast- vs. slow-growing saplings and least for large trees with foliage in the forest canopy. Inherent attributes of species contributing to the trade-off include abilities to disperse, acquire resources, grow rapidly, and tolerate shade and other stresses. There is growing interest in the possibility that functional traits might provide insight into such ecological differences and a growing consensus that seed mass (SM), leaf mass per area (LMA), wood density (WD), and maximum height (H(max)) are key traits among forest trees. Seed mass, LMA, WD, and H(max) are predicted to be small for light-demanding species with rapid growth and mortality and large for shade-tolerant species with slow growth and mortality. Six of these trait-demographic rate predictions were realized for saplings; however, with the exception of WD, the relationships were weak (r2 < 0.1 for three and r2 < 0.2 for five of the six remaining relationships). The four traits together explained 43-44% of interspecific variation in species positions on the growth-mortality trade-off; however, WD alone accounted for > 80% of the explained variation and, after WD was included, LMA and H(max) made insignificant contributions. Virtually the full range of values of SM, LMA, and H(max) occurred at all positions on the growth-mortality trade-off. Although WD provides a promising start, a successful trait-based ecology of tropical forest trees will require consideration of additional traits.

Tropical forests vary substantially in the densities of trees of different sizes and thus in above-ground biomass and carbon stores. However, these tree size distributions show fundamental similarities suggestive of underlying general principles. The theory of metabolic ecology predicts that tree abundances will scale as the )2 power of diameter. Demographic equilibrium theory explains tree abundances in terms of the scaling of growth and mortality. We use demographic equilibrium theory to derive analytic predictions for tree size distributions corresponding to different growth and mortality functions. We test both sets of predictions using data from 14 large-scale tropical forest plots encompassing censuses of 473 ha and > 2 million trees. The data are uniformly inconsistent with the predictions of metabolic ecology. In most forests, size distributions are much closer to the predictions of demographic equilibrium, and thus, intersite variation in size distributions is explained partly by intersite variation in growth and mortality.

According to conventional wisdom, functional diversity is exclusively a consequence of species having evolved adaptations to fill different niches within a heterogeneous environment. This view anticipates only one optimal combination of trait values in a given environment, but it is also conceivable that alternative designs of equal fitness in the same environment might evolve. To investigate that possibility, we use a genetic algorithm to search for optimal combinations of 34 functional traits in a realistic model of tree seedling growth and survival. We show that separate lineages of seedlings evolving in identical environments result in many alternative functional designs of approximately equal fitness.

A trade-off between growth and mortality rates characterizes tree species in closed canopy forests. This trade-off is maintained by inherent differences among species and spatial variation in light availability caused by canopy-opening disturbances. We evaluated conditions under which the trade-off is expressed and relationships with four key functional traits for 103 tree species from Barro Colorado Island, Panama. The trade-off is strongest for saplings for growth rates of the fastest growing individuals and mortality rates of the slowest growing individuals (r 2 ¼ 0.69), intermediate for saplings for average growth rates and overall mortality rates (r 2 ¼ 0.46), and much weaker for large trees (r 2 0.10). This parallels likely levels of spatial variation in light availability, which is greatest for fast-vs. slow-growing saplings and least for large trees with foliage in the forest canopy. Inherent attributes of species contributing to the trade-off include abilities to disperse, acquire resources, grow rapidly, and tolerate shade and other stresses. There is growing interest in the possibility that functional traits might provide insight into such ecological differences and a growing consensus that seed mass (SM), leaf mass per area (LMA), wood density (WD), and maximum height (H max) are key traits among forest trees. Seed mass, LMA, WD, and H max are predicted to be small for light-demanding species with rapid growth and mortality and large for shade-tolerant species with slow growth and mortality. Six of these trait-demographic rate predictions were realized for saplings; however, with the exception of WD, the relationships were weak (r 2 , 0.1 for three and r 2 , 0.2 for five of the six remaining relationships). The four traits together explained 43-44% of interspecific variation in species positions on the growth-mortality trade-off; however, WD alone accounted for .80% of the explained variation and, after WD was included, LMA and H max made insignificant contributions. Virtually the full range of values of SM, LMA, and H max occurred at all positions on the growth-mortality trade-off. Although WD provides a promising start, a successful traitbased ecology of tropical forest trees will require consideration of additional traits.

Contrary to the conventional theory of optimal stomatal control, there is substantial transpiration at night in many tree species, but the functional significance of this phenomenon remains uncertain. To investigate the possible roles of nocturnal transpiration, we compared and contrasted the correlations of both nocturnal and diurnal sap flow with a range of traits in 21 temperate deciduous tree species. These traits included soil water affinity, shade tolerance, cold hardiness, nitrogen concentration of tissues, minimum transpiration rate of excised leaves, growth rate, photosynthetic capacity, stomatal length and density, and the water potential and relative water content of leaves at the wilting point. Nocturnal sap flow was higher in species with higher leaf nitrogen concentrations, higher rates of extension growth and lower shade tolerances. Diurnal sap flow was higher in species with higher leaf nitrogen concentrations and photosynthetic capacities on a leaf area basis. Because leaf metabolism and dark respiration, in particular, are strongly related to leaf nitrogen concentration, our findings suggest that nocturnal transpiration functions to sustain carbohydrate export and other processes driven by dark respiration, and that this function is most important in fast- growing shade-intolerant tree species.

A key aspect of biodiversity is the great quantitative variation in functional traits observed among species. One perspective assertsthat trait values should converge on a single optimum value in a particular selective environment, and consequently trait variation would reflect differences in selective environment, and evolutionary outcomes would be predictable. An alternative perspective asserts that there are likely multiple alternative optima within a particular selective environment, and consequently different lineages would evolve toward different optima due to chance. Because there is evidence for both of these perspectives, there is a long-standing controversy over the relative importance of convergence due to environmental selection versus divergence due to chance in shaping trait variation. Here, I use a model of tree seedling growth and survival to distinguish trait variation associated with multiple alternative optima from variation associated with environmental differences. I show that variation in whole plant traits is best explained by environmental differences, whereas in organ level traits variation is more affected by alternative optima.Consequently, I predict that in nature variation in organ level traits is most closely related to phylogeny, whereas variation in whole plant traits is most closely related to ecology.

Does variation in environmental harshness explain local and regional species diversity gradients? We hypothesise that for a given life form like trees, greater harshness leads to a smaller range of traits that are viable and thereby also to lower species diversity. On the basis of a strong dependence of maximum tree height on site productivity and other measures of site quality, we propose maximum tree height as an inverse measure of environmental harshness for trees. Our results show that tree species richness is strongly positively correlated with maximum tree height across multiple spatial scales in forests of both eastern and western North America. Maximum tree height co-varied with species richness along gradients from benign to harsh environmental conditions, which supports the hypothesis that harshness may be a general mechanism limiting local diversity and explaining diversity gradients within a biogeographic region.

Shipley et al. (Reports, 3 November 2006, p. 812) predicted plant community composition and relative abundances with a high level of accuracy by maximizing Shannon's index of information entropy (species diversity), subject to constraints on plant trait averages. We show that the entropy maximization assumption is relatively unimportant and that the high accuracy is due largely to a statistical effect. S hipley et al.(1) combined a trait-based framework and the assumption of maximum entropy to predict relative abundances of plant species at sites varying in successional age. Specifically, they chose relative abundances such that Shannon's index of information entropy (i.e., Shannon's diversity index) was maximized, subject to the constraint of reproducing the average values of traits at different sites. Both their use of traits and their maximization of entropy are interesting. The maximum entropy assumption is particularly novel and intriguing as this assumption essentially maximizes biodiversity, which could be interpreted in terms of minimizing harm due to specialized natural enemies (2) and/or maximizing niche differentiation with respect to unmeasured traits, thereby gaining hypothesized benefits of high diversity such as resilience against environmental fluctuations (3, 4).A key question raised by the results in Shipley et al.(1) is the relative importance of the entropy assumption versus the trait constraints in achieving good predictions of abundances. We tested the importance of the maximum entropy criterion by repeating the analysis under the opposite assumption; specifically, we chose relative abundances to minimize entropy (5), while maintaining the same trait constraints as in (1). Predictions of relative abundances of species in individual plots were almost as good when entropy was minimized (r 2 = 0.86) as when it was maximized (r 2 = 0.94), which suggests that this assumption is relatively unimportant (6). In contrast, when we tested the importance of the trait constraints by repeating the analysis using only four traits instead of eight traits as constraints, the accuracy of the predictions dropped substantially (Table 1). Thus, although the maximum entropy assumption consistently improves predictions, the constraints on trait averages are clearly much more important.The primacy of the trait-based assumption for obtaining good predictions of abundances leads to the further question of the degree to which this assumption is successful because of strong convergence in traits at a given successional stage or merely because the predictions are being made for the same sites from which the trait averages were calculated. To assess this question, we used cross-validation analysis. Specifically, we fit the trait patterns across successional ages based on data from all but one site and then predicted abundances at the remaining site using the same constrained entropy maximization approach as before. The key difference between the crossvalidation and the original analysis is that under cross-validation, t...