Abstract
Most tropical tree species do not produce distinct growth rings, and the causes of this phenomenon have not received sufficient quantitative study. It has been shown that rainfall seasonality influences the formation of growth rings in some deciduous taxa. However, the numerous exceptions observed call for an examination of additional drivers of the phenomenon. We therefore hypothesized that in addition to seasonal climatic stress, functional and phylogenetic constraints may determine growth-ring distinctness. Ten potentially influential factors were examined in 38 Indian tropical tree species. Distinctness of growth rings was quantitatively assessed based on both subjective visual criteria and objective measures of anatomical characters. Multivariate and phylogenetically constrained analyses were used to test for functional, environmental,and phylogenetic effects. First, subjective scores of growth ring distinctness correlated with objective anatomical measurements of vessel size and porosity related to water conductance, but also with additional anatomical characteristics unrelated to water dynamics. Second, ring distinctness variables were primarily related to deciduousness and species maximum height, and also weakly influenced by the topographic slope. A phylogenetic signal was detected in wood specific gravity values, the climatic variable of dry season rainfall, and the subjective distinctness score of growth rings, but accounting for phylogenetic structure did not significantly improve the prediction of ring distinctness. Thus, there was no evidence of an evolutionary constraint on the relationship in our sample of species. Our study thus demonstrates how distinctness of growth rings in tropical trees can be objectively represented on a continuous scale, and provides a quantitative explanation for its variability.