Leaf gas-exchange traits are independent of leaf economics traits: Implications for linking leaf functional traits with plant diversity

Background/Question/Methods

It is commonly assumed that leaf gas-exchange traits should be tightly correlated with traditional leaf economics (LE) traits. However, previous studies have shown that plants may be modularized and can combine different structures (modules) and function of different organs. Thus, it is possible that structures are also modularized even within a single leaf. Here we tested two alternative hypotheses regarding the correlation between gas-exchange and LE traits in leaves across species: 1) gas-exchange traits and leaf economics traits are correlated because they serve related leaf functions (i.e. coordination hypothesis), or 2) these two sets of traits are independent thus allowing multiple combinations of them (i.e. independence hypothesis). For that purpose, we measured leaf gas-exchange traits (stomatal density, stomatal length, minor vein density and minor vein diameter), and leaf economics traits (leaf mass-based nitrogen concentration [N_mass] and leaf mass per area [LMA]) across 85 woody angiosperm species from five tropical-subtropical forests in Southern China.

Results/Conclusions

Our data strongly support the independence hypothesis as the correlation was extremely low (r < 0.15 for all 8 bivariate correlations) between leaf gas-exchange traits and LE traits. Principal component analysis of all traits further revealed that the set of leaf gas-exchange traits is orthogonal to LE traits. By contrast, correlations were strong between traits within traditional LE traits with a bivariate correlation of r = -0.479. Correlations were also strong between traits within the gas-exchange traits with r = -0.469 and -0.697 for vein and stomata structures respectively (all P values < 0.001). Our results suggest that independence of leaf gas-exchange traits and LE traits can lead to multiple combinations of leaf functional traits, which in turn contribute to diverse plant ecological strategies. We conclude that in the tropical-subtropical biome of Southern China, gas-exchange structures and leaf economics structures are modularized and this organ-level modularity may have profound implications for linking plant functional traits with plant diversity and their responses to global environmental changes.