Tolerance to low leaf water status of tropical tree seedlings is related to drought performance and distribution

Habitat specialization models predict that adaptations to environmental conditions explain species distributions. In tropical rainforests, the ability of the seedlings to survive during drought has been shown to be a key determinant of species distributions. We hypothesize that differences among species in their tolerance to low tissue water status is the mechanism underlying differences in performance during drought. To test this hypothesis we quantified tolerance to low leaf water status for over 20 species from central Panama in screenhouse experiments using two different experimental approaches. Results from both approaches were highly correlated with each other. We found that tolerance to low leaf water status correlated with species drought performance in the field and with their distribution across a gradient of dry season length, with the more desiccation-tolerant species having higher survival in dry relative to irrigated conditions, and occurring in drier areas. These results support the hypothesis that, in tropical forests, tolerance to low tissue water status governs seedling performance during drought, as well as being a determinant of species distribution patterns. Lower tolerance to low leaf water status was correlated with greater stem hydraulic conductance. In addition, all species tested, including both desiccation-sensitive and desiccation-resistant species, showed similar losses of xylem conductance, about 80%, when near death. These results suggest that a principal mechanism by which desiccation leads to plant mortality is the loss of xylem conductivity.