The role of microsite sunlight environment on growth, architecture, and resource allocation in dominant Acacia tree seedlings, in Serengeti, East Africa

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.