To improve projections how global-change will affect vegetation, and which species will win or lose under conditions of changing climate and land-use, we urgently need to improve our understanding of the mechanisms underlying plant responses to drought and identify traits that allow to predict them. The problem is especially acute in the Caatinga, an extensive seasonally dry tropical forest biome in northeastern Brazil, which is threatened by desertification through decreasing rainfall and through anthropogenic disturbance.

Turgor-loss point (TLP), the water potential at which leaves lose turgor, has been identified to be a key trait of drought resistance in evergreen woody species, with species with a lower TLP being more drought resistant. In this study we addressed, if TLP is also related to drought resistance in drought deciduous tropical dry forest species.

We quantified for 21 deciduous woody species occurring in the Caatinga the turgor loss point, as well as the minimum leaf conductance (gmin) and bark conductance (gbark) to water vapor. We then analyzed how the community weighted mean (CWM) of these traits changes across a pronounced rainfall gradient. We hypothesized that the CWM of all three traits increases with rainfall.

While we found the expected pattern for gmin and gbark, the CWM of TLP decreased with rainfall, i.e. the relation was opposite to the one expected from evergreen species. These results are consistent with a strategy of minimizing water loss through early stomatal closure, which is associated with high TLP, and an effective cuticle and bark. They indicate that avoiding desiccation is an important mechanism for deciduous woody plants in this system.

More importantly, these results highlight the need to assess the relation of functional traits to drought resistance in different plant life forms and systems, before using traits to predict plant drought responses.