Functional traits and tradeoffs determine drought tolerance and niche differentiation of tropical tree species

Background/Question/Methods

Plant functional traits affect species performance and shape species distribution along local and regional environmental gradients. Here we analyze the functional traits that underlie the drought tolerance strategies of tropical tree species, and their consequences for partitioning of local gradients in light and water availability, and regional gradients in rainfall. We first measured the hydraulics and stem and leaf traits of 13 co-existing dry forest tree species and then included them in a whole-plant growth model to predict their niche

Results/Conclusions

The species showed a strong trade-off between cavitation resistance (P50) and hydraulic conductivity. Cavitation resistance was positively correlated with the strength of stem and leaves (i.e., high stem wood density and leaf dry matter content), but surprisingly not with specific leaf area, which is generally thought to be a good indicator of the resource use strategy of a species. We then integrated biophysical principles in a plant model to understand how functional plant traits and trade-offs affect growth and survival, and how they cause niche separation along gradients of light and water availability for these 13 species. Model simulations show that functional traits result in a plant strategy spectrum, varying from acquisitive species with high growth rates at high resource levels to conservative species with high tolerances for both shade and drought. The model results indicate, therefore, a coupling of shade tolerance and drought tolerance. Most species grow more rapidly than other competing species at certain combinations of above- and belowground resource levels, and occupy therefore potentially a unique realized niche. The modeled resource niches were positively correlated with the observed light niches of species as measured in the field, but not with the water niches. Intriguingly, the functional divergence resulted in this dry forest only in light niche separation across species, but not in water niche separation. The functional traits were, however, correlated with the species distribution along the regional rainfall gradient.