Traits associated with drought survival in three Australian tropical rainforest seedlings

Drought affects the distribution of plant species in tropical forests and will likely increase under climate change. Future rainforest composition may be determined by species' abilities to withstand increased drought incidence, particularly at the vulnerable seedling stage. A greenhouse drought survival experiment was conducted on seedlings of three common evergreen tree species from Australia's Wet Tropics to assess species' drought survival. This was then related to five functional traits to evaluate the relative importance of desiccation tolerance (ability to persist through drought) and desiccation delay (ability to postpone drought stress) in drought survival. Among the three species examined, delay traits (leaf shedding, root-to-shoot ratio and stem saturated water content) corresponded with experimental drought survival better than tolerance traits (specific leaf area, stem density). Notably, we found differential leaf shedding among these evergreen species and a positive correlation between percent leaf loss and drought survival among individuals across all species (R-2 = 0.42). If this pattern holds with greater species replication, it suggests that desiccation delay, particularly via leaf shedding, is important for drought survival even in nominally evergreen species. We suggest that finer classifications of deciduousness such as percent leaf loss under drought stress may be useful in predicting species' responses to drought conditions.