Linking hydraulic strategy, drought response and carbon gain in co-existing savanna tree species.

Savannas cover a significant portion of the earth's land surface, yet how they will respond to increases in rainfall variability and drought frequency and intensity expected with climate change remains poorly understood. Studies of hydraulic-related traits of savanna trees are rare with most existing research focusing on temperate and tropical forest species. We measured growth, photosynthetic rates, monthly predawn and midday xylem pressure potentials, and eight traits relevant to xylem, leaf safety and water storage capacity, in six co-occurring Southern African semi-arid savanna species. The six species adopted different hydraulic strategies, ranging from drought tolerance (e.g., high wood density, low xylem vulnerability to cavitation and ${{\psi}}_{{tlp}}$) in Dichrostachys cinerea (L.) Wight & Arn. to drought avoidance (e.g., high capacitance and shoot saturated water content) in Terminalia sericea Burch. ex DC. Drought-avoiding species with high capacitance had higher growth and photosynthetic rates, while drought-tolerant species had slow growth and low photosynthetic rates, when soil water was not limiting. The different hydraulic strategies found in the six study species suggest that savanna tree species exploit different ecohydrological niches, likely contributing to their co-existence in an environment where rainfall and soil water availability are highly variable. All of the strategies allowed for survival during shorter-growing season droughts. Previous studies have shown that both drought avoiders and tolerators may be vulnerable to mortality during more extensive droughts in savannas. We suggest that access to deeper soil water combined with higher capacitance, as found in Sclerocarya birrea (A.Rich.) Hochst. appears to be the most successful strategy to survive extensive drought.