Distribution of leaf shape variability in a tropical forest

Background/Question/Methods

Leaf functional traits are important because they determine the ability of a plant to capture light, one of the most limiting resources in closed canopy forests. The distribution of leaf functional traits can give insight into how species coexist. This study examined the distribution of leaf shape morphology to better understand its ecological importance. Specifically, we asked: 1) Does leaf shape vary based on the functional group to which a species belongs? 2) Are there strong species-level differences? 3) Within species, are there differences based on height and therefore, light availability? Moving beyond traditional methods of measuring and describing shape, this study used geometric morphometrics to quantify leaf shape and to produce variables that describe shape as a whole. Variation in leaf shape was analyzed for 291 specimens from eight tropical tree species co-occurring in a forest in Belize using a relative warps analysis. A multivariate analysis of variance (MANOVA) was conducted with the first five principal components as the dependent variables and the independent variables being specimens grouped by species identity, functional group, canopy position/ontogenetic stage, and individual. This was used to determine the relative importance of each grouping in explaining overall shape variation.

Results/Conclusions

The relative warps analysis revealed that a majority of the leaf shape variation was due to differences in the length: width ratio. With the specimens grouped by species, individual tree, functional group, and canopy position, species had the largest explanatory power with regards to the variation in leaf shape, but there was still a large amount of interspecific overlap between the species. Functional group and canopy position also explained some of the variation, most interestingly with their relation to the second principal component that was associated with the shape of the apex. This has some interesting implications as to functional role that the distribution of leaf shape could have in the heterogeneous light environments of a tropical forest. The methods used here to quantify and analyze shape helped us to better understand how leaf shape variation is distributed in a tropical forest, and helped us to visualize what aspect of shape is responsible for that variation which enabled us to propose functional explanations for the observed patterns in variation. Geometric morphometrics shows promise in further understanding shape differences and their link to community processes.