A latitudinal gradient in the distribution of variance in plant functional traits

Background/Question/Methods The latitudinal gradient in species richness remains a poorly explained but widespread phenomenon in ecology. Many hypotheses depend on fine-scale niche differentiation in the tropics, and, in particular, fine-scale partitioning of an increased array of light levels has been posited as a driver of tropical diversity. Meanwhile, others have posited that diffuse selection pressures among tropical trees could actually lead to a shared generalist strategy among coexisting species. We address these competing hypotheses using data on leaf functional traits of coexisting tree species from three forests along a latitudinal and diversity gradient (ESGR, Michigan, Thompson Mills, Georgia, and Barro Colorado Island, Panama). For specific leaf area, leaf nitrogen content, leaf dry matter content, leaf area, and leaf shape index, we calculate measures of niche differentiation based on these traits. First, we determine the source of variation in traits among species, canopy position/light environment, and individual tree. Then, we measure trait convergence, differentiation, and overlap, taking into account both ontogenetic and individual level variation in those traits. Results/Conclusions Species identity explains progressively more trait variability as species diversity increases and latitude decreases. Canopy position/light environment explains similar levels of variation at sites, while the explanatory power of individual-level variation increases with latitude. Focusing on SLA as a measure of position on the leaf economics spectrum, we found a strong increase in trait range as we move toward the tropics. There was less variance around trait means as species richness increased; this was especially noticeable in sun leaves. However, trait means show similar evenness of spacing along the gradient, as measured by the standard deviation of nearest neighbor distances along trait axis. The greatest overall trait overlap occurs in the species-poor Michigan forest (0.439-0.729), particularly notable within the shaded sapling layer (0.583-0.729). Overlaps range from 0.324-0.504 in the Georgia forest, while BCI trait overlaps ranged from 0.318-0.567. Combined, these results are largely consistent with the idea of tropical tree species being more specifically adapted to a wider range of light environments demonstrated by a greater range in SLA in the tropics.