Linking physiological traits to species distributions along environmental gradients is a central aim of ecology, particularly in the face of imminent global climate changes.  Recent studies suggest that species distributions at local and regional scales along rainfall gradients in the tropics are related to drought sensitivity, yet the physiological mechanisms underlying drought sensitivity remain unclear.  Some evidence suggests that stem hydraulic conductance and tolerance to low leaf water potentials could provide a mechanistic basis for understanding drought sensitivity among tropical seedlings.  In this study we asked two questions: (1) Are local and regional species distributions explained by seedling hydraulic traits? (2)  Do these hydraulic traits constrain net photosynthetic carbon gain?

Results/Conclusions

Here we expand on previous studies by attempting to link leaf and stem hydraulic traits to local species distributions within the 50-ha Barro Colorado Island Forest Dynamics Plot and to regional distributions along a rainfall gradient on the Isthmus of Panama.  We found that leaf water status (tolerance to low leaf water potentials) was a significant predictor of regional but not local species distributions, whereas stem hydraulic conductance failed to correlate with species distributions at either spatial scale.  These results suggest that considering differences in tolerance to low leaf water status may explain drought sensitivity and thus species distributions.  Furthermore, we found that stem hydraulic conductance and leaf water status correlate with net carbon assimilation.  Taken together, these results suggest that environmental filtering limits species distributions based on their tolerance to low leaf water potentials, which, along with hydraulic supply, influences carbon gain.