The vulnerability of mesic fynbos species to plant water stress in the Cape Floristic Region

Background/Question/Methods Climate change predictions for the Cape Floristic Region of South Africa include longer periods of drought, likely to increase plant water stress. Which species are lost as a result of increased drought will depend on differences in drought sensitivity which is partially dependent on differences in the capacity of xylem to withstand high negative pressures. There has been very little research on the hydraulics of mesic fynbos species, even though much of the richness of the Cape Flora fynbos is at the wetter end of the moisture gradient. Mesic species may also be more sensitive to increases in plant water stress. I assessed the vulnerability of seven wetland species and four related dryland species to increases in plant water stress. Vulnerability curves and P₅₀ were measured, as well as xylem specific conductivity (K_s), minimum seasonal xylem pressure potential (P_min), xylem density and theoretical implosion limit (t/b)_h².
Results/Conclusions Species in the same genera had similar shaped vulnerability curves and similar P₅₀, regardless of whether or not they were wetland species. Each of the three genera sampled had vulnerability curves with a distinctive shape, suggesting strong phylogenetic constraints on xylem function.

All of the species included in this study appeared to be sensitive to drought stress. As the wetland species were hydraulically very similar to the dryland species, variation within the various measured physiological variables were small and correlations were poor. However, data from this study correlated with that reported in other studies with decreased xylem density being associated with a decrease in P₅₀ and a decrease in P_min. When P₅₀ was plotted against (t/b)_h², species from this study were further away from the theoretical implosion limit than those reported in other studies. The implication is that their vessels are able to withstand negative pressures much greater than those that cause cavitation. Species appear to have extra investment in structural support suggesting that they are subject to selective pressures other than just hydraulics. Unlike chaparral shrubs, the mesic fynbos species in this study show low tolerance of summer drought and may be very vulnerable to a predicted future of extended summer drought periods.