The application of functional traits to predict and explain plant species’ distributions and growth and mortality rates has been a major direction in functional ecology for decades, yet numerous physiological traits have not been incorporated into the approach. For Hawaiian montane wet forest (MWF) and lowland dry forest (LDF), we measured an extensive suite of traits related to water transport, gas exchange, and resource economics, including leaf vein, stomatal, and wilting traits, and diameter- and biomass-based relative growth rates (RGR) and mortality rate (m) in four-hectare forest plots. We hypothesized that (1) wet and dry forest species would differ in a wide range of traits as expected from contrasting adaptation; (2) trait values would be more convergent among dry than wet forest species due to the stronger environmental filtering; (3) traits would be inter-correlated within “modules”, since selection would lead to trait-trait correlations within organs and functional categories due to common developmental pathway or function or benefit; (4) RGR and m would correlate with a number of specific traits, given their mechanistically direct contribution to vital rates, with (5) stronger relationships when controlling for tree size, and thus, (6) RGR and m can be strongly explained from trait-based models.
Results/Conclusions
Species from the MWF and LDF varied strongly in their traits. On average, the MWF species’ traits were associated with adaptation to high soil moisture and nutrient supply and greater shade tolerance whereas the LDF species’ traits were associated with drought tolerance. Thus, MWF species achieved greater heights than LDF species and lower wood density, larger stomata and epidermal cells, higher maximum stomatal conductance and CO2 assimilation rate, high stomatal conductance to nitrogen ratio, high carbon discrimination rate, higher saturated water content, lower dry matter content, higher phosphorus concentration, lower nitrogen to phosphorus ratio, high chlorophyll to nitrogen ratio, lower vein lengths per area, less negative turgor loss point, and greater leaf shrinkage. As hypothesized, the functional traits were more variable in wet than dry forest, were correlated within functional modules, and predicted species’ RGRs and m across forests, with stronger relationships when controlling for tree size. Models based on multiple traits could strongly predict RGR and m across forests (R2= 0.70-0.72; P< 0.01). We conclude that extensive functional traits provide strong power to resolve ecological patterns, an especially promising step for comparative ecology in combination with increasingly mechanistic approaches.