Premise of the study: The first-year tree seedling (FYS) life stage 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 here, FYS water-use strategies may play a crucial role in FYS establishment and, ultimately, 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. Within a glasshouse experiment, gas exchange and whole-plant hydraulic conductance (K_plant) were measured while soil water potential (Ψ_soil) declined. Trajectory of the Ψ_leaf/Ψ_soil relationship during drought elucidated the degree of iso/anisohydry.

Key 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. A. 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 upon 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.