The high rainfall seasonality that characterizes tropical dry forests means that vegetation must balance trade-offs between acquisitive and conservative strategies or between carbon gain and water conservation under periods of abundance and stress. The rich biodiversity of tree forms that inhabit these forests indicates that species employ a range of water use and conservation strategies that allow successful establishment and persistence. Central to understanding adaptations to water-stress and drought are the relationships between form and function. Previous studies have shown a strong relationship between wood density and phenology and thus, these traits have been used to define plant functional types (PFT). Although implied by the functional groupings, whether these traits actually manifest in unique defining ecophysiological functions has not be directly tested in a number of systems. The objective of this study was to assess whether or not PFTs predict water use strategies of common, co-occurring tree species in the Caatinga, a tropical dry forest occupying over 800,000 km2 of northeastern Brazil. Experienced water limitation was assessed with pre-dawn and mid-day (ΨPD and ΨMD) leaf water potential, soil water access was inferred via water isotope ratios (δ2H and δ18O), and water use efficiency and potential differences in respired CO2 re-fixation was assessed with leaf δ13C measurements. Data was collected bi-weekly from the wettest month of the rainy season to the middle of the dry season. PFTs were: deciduous low wood density (DLWD), evergreen high wood density (EV), and deciduous high wood density (DHWD).
Results/Conclusions
At the aggregate PFT level, we observed large differences in experienced water potentials well into the dry season, with the DLWD maintaining ΨPD and ΨMD above -1 MPa, while the HWD groups experienced values at or below -3 MPa. Differences in leaf phenology were also observed, consistent with prior classifications. All species appeared reliant on shallow soil water sources, with the exception of one of the two EV species that appeared to shift to a deeper source with the onset of the dry season. Our results support the use of Caatinga forest PFTs to predict function in the aggregate. However, we did observe remarkable exceptions at the species level, suggesting important limitations and the need for more explicit assessments of the relationship between form and function. Our work also highlights the importance of stem water storage as a key trait allowing the DLWD species to employ a drought avoidance strategy in this tropical dry forest.