Data from: First-year Acacia seedlings are anisohydric 'water-spenders' but differ in their rates of water use

<p><strong>Premise of the study:</strong> 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.</p> <p><strong>Methods:</strong> We examined drought responses in FYS of two tree species that are dominant on opposite ends of an aridity gradient in Serengeti, <em>Acacia</em> (<em>=Vachellia</em>) tortilis and <em>A. robusta</em>. Within a glasshouse experiment, gas exchange and whole-plant hydraulic conductance (<em>K</em>_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.</p> <p><strong>Key results:</strong> 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 <em>K</em>_plant under severe drought, they differed in their rates of water use. <em>A. tortili</em>s, which occurs in the more arid regions, initially had greater <em>K</em>_max, transpiration (<em>E</em>), and photosynthesis (<em>A</em>_net) than <em>A. robusta</em>.</p> <p><strong>Conclusions: </strong>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.  </p>