Savanna tree species show contrasting acclimation responses to elevated atmospheric CO2 concentration and drought

1. Woody plant expansion into historically grass-dominated savannas is reshaping ecosystem structure and function, with consequences for biodiversity, nutrient cycling and land use. This shift is thought to be driven by the physiological responses of certain C3 tree species to elevated atmospheric CO2 concentrations (eCO2), modulated by water availability, among other constraints. However, the species-specific mechanisms regulating photosynthetic acclimation under these conditions remain unclear. 2. Using an Open-Top Chamber system, we examined the responses of five southern African C3 tree seedlings to ambient (400 ppm) and elevated (550 ppm) CO2 concentrations under contrasting soil water availability. 3. Our results revealed that Vachellia karroo, V. tortilis and V. sieberiana exhibited clear photosynthetic upregulation under eCO2, with increased photosynthetic capacity (carboxylation capacity and/or electron transport rate), particularly under well-watered conditions when stomatal conductance was highest. These responses diminished under water limitation but remained higher than in V. robusta and Senegalia burkei, which downregulated under eCO2 at high water availability and showed only modest improvements under dry conditions. Importantly, photosynthetic upregulation did not consistently translate into above-ground growth gains, with significant enhancement observed only in V. sieberiana under high water supply. 4. These divergent acclimation strategies highlight why some species emerge as dominant encroachers under rising CO2, while others remain constrained by stronger physiological limitations.