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.

由于资源可用性的变化，幼苗的建立是稀树草原招募的重要生命史阶段。尽管树水争夺水是树苗死亡率的重要推动因素，但尽管稀树草原生物量因草冠生物量的异质性而变化，但在稀树草原生态系统中，阳光照射对树苗性能的重要性几乎没有引起注意。我们研究了两个主要的东非树种（季节性阳光微环境相思 =  Vachellia）罗布斯塔（伯奇）和A. tortilis田间自然条件下（Forssk）。在坦桑尼亚的塞伦盖蒂国家公园，罗布斯塔A.树木生长在北部的高草原（阴凉处），而南部的短草草原（阴影较少）则有A. tortilis。我们还设计了温室实验，以量化阳光对幼苗生长，建筑和资源分配特征的影响。在现场，A.罗布斯塔幼苗用低下层阳光在雨季相关联相比A. tortilis，与在旱季这一趋势切换。在温室试验，低日照（25％的辐射）下，A.罗布斯塔获得了高度的速度比A. tortilis中，冠层叶片之间的自遮蔽了明显A. tortilis但不是A.罗布斯塔。在不同的光照环境下，物种之间对叶，茎和根的生物量分配不同，这表明对可变光照的响应具有表型可塑性。我们的研究表明，应该将微地点光的可变性纳入热带稀树草原募集的时空可变性模型中。