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We experimentally examined the influence of grass competition, grass species identity (taxa) and water availability on the seedling growth and survival of two dominant tree species (Vachellia (formerly Acacia) robusta and V. tortilis) of the Serengeti National Park in Tanzania. Despite being widely distributed, the species have an opposing overstorey dominance across a rainfall and productivity gradient, with V. robusta dominating the more productive mesic sites and V. tortilis in the lower productivity, drier sites. We investigated the role of different grass species, which vary in distribution and abundance across the rainfall gradient, in influencing the growth and survival of Vachellia seedlings. We found a significant effect of grass competition but no effect of grass species identity on the growth or survival of seedlings. Seedling survival was highest in the absence of grass competition, intermediate when grasses were defoliated to simulate grazing and lowest when grown with ungrazed grasses. Grass competition had a more negative effect on the stem diameter of V. tortilis than V. robusta. All V. tortilis seedlings grown under a combination of drought conditions and unclipped grasses died by the end of the experiment. However, reduced grass competition by simulated grazing improved V. tortilis seedling survival to comparable levels achieved by V. robusta species. Our study advances our understanding of tree and grass competition across environmental gradients and suggests that the presence of grass and soil moisture have species‐specific effects on tree seedling growth and survival in African savannas. [ABSTRACT FROM AUTHOR]

Seedling establishment is a critical life history stage for savanna tree recruitment due to variability in resource availability. While tree–grass competition for water is recognized as an important driver of tree seedling mortality, the importance of sunlight exposure on tree seedling performance has received little attention in savanna ecosystems despite variable seedling light environments caused by heterogeneity in biomass of the grass canopy. We studied the seasonal sunlight micro-environment for two dominant East African tree species (Acacia = Vachellia) robusta (Burch) and A. tortilis (Forssk) under natural field conditions. In the Serengeti National Park, Tanzania, A. robusta trees occur in tall grasslands of the north (shady) and A. tortilis in the southern short grasslands (less shaded). We also designed a greenhouse experiment to quantify sunlight effects on seedling growth, architecture, and resource allocation traits. In the field, A. robusta seedlings were associated with lower understorey sunlight during the wet season compared to A. tortilis, with this trend switching during the dry season. In the greenhouse experiments, under low sunlight (25% radiation), A. robusta gained height faster than A. tortilis and self-shading among canopy leaves was evident in A. tortilis but not A. robusta. Biomass allocation to leaves, stems, and roots differed between species under different light environments suggesting phenotypic plasticity in response to variable light availability. Our study suggests that microsite light variability should be incorporated in models of the spatial and temporal variability of savanna tree recruitment. [ABSTRACT FROM AUTHOR]

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