Climate change-related drought is expected to play an increasingly important role in plant communities. Therefore, to be able to forecast future plant assemblages, we need to understand how different species will respond to different drought scenarios (defined by the timing, severity and duration of the drought) as these will determine the cause of mortality, i.e., hydraulic failure or carbon starvation. Hydraulic failure is thought to occur primarily in short but severe droughts, while carbon starvation related mortality is usually linked to extended, less-severe drought periods. Here we measured drought response in seedlings of two co-occurring native tree species (Acer saccharum and Quercus rubra) combining information on xylem conductance loss and non-structural carbohydrate (NSC) concentrations along a soil water potential gradient. We then extrapolated these results to field-grown seedlings and evaluated the relative importance of each mechanism leading to mortality during these species’ natural recruitment dynamics.
Results/Conclusions
Results from the xylem conductance loss experiment confirm that Acer seedlings have relatively isohydric stomatal control and will likely experience higher mortality rates as a result of hydraulic failure. Quercus seedlings instead exhibit more anisohydric behavior and are therefore more susceptible to mortality from carbon starvation. Results from our field experiment show that Acer seedlings are active primarily in early spring, prior to canopy closure, and maintain very low photosynthetic activity in the summer. Consequentially, this species is likely to be most negatively affected by reduce water availability in spring than by summer droughts. Conversely, Quercus seedlings have a second peak of activity in summer that extends late into fall and are therefore susceptible to extended drought during summer that may cause early leaf senescence. Because of these differences, recruitment dynamics of these two species will likely be differently affected by climate change-related drought, potentially leading to changes in forest composition and biodiversity in the future.