First-year Acacia seedlings are anisohydric ‘water-spenders’ but differ in their rates of water use

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 (Kplant) were measured while soil water potential (Ψsoil) declined. Trajectory of the Ψleaf/Ψsoil 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 vuln...