Understanding the ecology, physiology and responses to global environmental change of the ~250,000 species of vascular plants in the world is an overwhelming challenge. One way to simplify this diversity is to categorize plants into groups based upon traits or functional attributes that may respond in similar ways to environmental or evolutionary stresses. This approach may be particularly useful in tropical dry forests, which are characterized by high species richness and diverse leaf flushing and shedding phenologies. While leaf habit may explain variation in traits among dry forest trees, an increasing number of studies are showing that both traits and stoichiometry are under strong phylogenetic control, i.e. taxa that are more closely related are more similar compared to distantly related taxa.
The goal of this study was to determine whether variation in key plant traits and leaf stoichiometry among tropical dry forest tree species is better correlated with functional strategy (i.e. leaf habit) or phylogenetic history. We measured specific leaf area (SLA), wood density, leaf water, foliar nitrogen and phosphorus concentrations, N:P, C:N, foliar delta13C (for water use efficiency) and delta15N (as indicator of nitrogen fixation in legumes) in sun leaves from multiple individuals of 87 dry forest species with different leaf habits in Guanacaste, Costa Rica. We predicted that deciduous species have traits that maximize carbon gain during the growing season, i.e. higher foliar N, SLA, P, and lower wood density, and that evergreen species would have traits consistent with nutrient conservation strategies, i.e. lower N, P, SLA, but higher wood density and WUE.
Results/Conclusions
Nested ANOVAs with leaf, tree and species as main effects showed that i) within-tree variation was not significant for all but one trait, and ii) there was significant variation within trees of the same species, but the magnitude of this variation was dwarfed by variation among species, which was strongly significant for every trait. Pairwise-correlations among traits were significant for 21 of 32 comparisons and were consistent with predictions from leaf economics theory of trade-off among traits. However, analyses of variance did not support the hypothesis that leaf habit correlates with traits; only one of the traits (leaf N) varied significantly among evergreen and deciduous trees. However, traits did vary among families, suggesting a role for phylogenetic history. Leaf habit may not be a good predictor of traits in tropical dry forests because species fall along a continuum of ecological strategies.
