First‐year Acacia seedlings are anisohydric “water‐spenders” but differ in their rates of water use

PREMISE: First‐year seedlings (FYS) of tree species may be a critical demographic bottleneck in semi‐arid, seasonally dry ecosystems such as savannas. Given the highly variable water availability and potentially strong FYS–grass competition for water, FYS water‐use strategies may play a crucial role in FYS establishment in savannas and, ultimately, in tree–grass competition and coexistence. METHODS: We examined drought responses in FYS of two tree species that are dominant on opposite ends of an aridity gradient in Serengeti, Acacia (=Vachellia) tortilis and A. robusta. In a glasshouse experiment, gas exchange and whole‐plant hydraulic conductance (K (plant)) were measured as soil water potential (Ψ (soil)) declined. Trajectory of the Ψ (leaf)/Ψ (soil) relationship during drought elucidated the degree of iso/anisohydry. RESULTS: Both species were strongly anisohydric “water‐spenders,” allowing rapid wet‐season C gain after pulses of moisture availability. Despite being equally vulnerable to declines in K (plant) under severe drought, they differed in their rates of water use. Acacia tortilis, which occurs in the more arid regions, initially had greater K (max), transpiration (E), and photosynthesis (A (net)) than A. robusta. CONCLUSIONS: This work demonstrates an important mechanism of FYS establishment in savannas: Rather than investing in drought tolerance, savanna FYS maximize gas exchange during wet periods at the expense of desiccation during dry seasons. FYS establishment appears dependent on high C uptake during the pulses of water availability that characterize habitats dominated by these species. This study increases our understanding of species‐scale plant ecophysiology and ecosystem‐scale patterns of tree–grass coexistence.