Abstract
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.
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Acknowledgements
We thank, the Serengeti Wildlife Research Centre (SWRC), the Tanzania Wildlife Research Institute (TAWIRI), the Commission for Science and Technology (COSTEC), the Tanzania National Parks (TANAPA) for permission to conduct research in Tanzania. We are grateful to Jeremia Sarakikya and Meshark Mwita for excellent assistance in the field, and to Wake Forest University undergraduate students Robyn Corapi, Noemie Kloucek and Aidan Galloway for their help in the greenhouse. Wake Forest University award to DMR and National Science Foundation grants to TMA supported this work.
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This study was funded by the department of Biology, Wake Forest University, through Vecellio award to DMR and by the National Science Foundation (NSF BCS 1461728) to TMA.
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DMR, STC and TMA designed the experiment, collected and performed data analysis. DMR wrote first draft of the manuscript, and all authors contributed critically to subsequent drafts.
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Rugemalila, D.M., Cory, S.T., Smith, W.K. et al. The role of microsite sunlight environment on growth, architecture, and resource allocation in dominant Acacia tree seedlings, in Serengeti, East Africa. Plant Ecol 221, 1187–1199 (2020). https://doi.org/10.1007/s11258-020-01074-5
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DOI: https://doi.org/10.1007/s11258-020-01074-5