The impact of shade on whole-plant carbon allocation in a dominant East African tree sapling

Plasticity in resource allocation can be beneficial for plants under stress. In savannas, tree-grass competition forces tree saplings growing in the grass layer to compete for water, nutrients, and light. Savanna tree saplings are also vulnerable to fire and herbivory, which may favour investment in storage belowground to support regrowth aboveground. It is unclear whether carbon (C) limitation from grass shading similarly favours allocation belowground. Further, investigating how light reduction changes allocation by juvenile trees to above- and belowground biomass, storage, and defence can help us understand juvenile tree allocation strategies during ubiquitous C limitation. Using a screenhouse experiment, we evaluated the effects of shade on carbon allocation and leaf physiology in saplings of a dominant ant-acacia, Acacia (Vachellia) drepanolobium. We hypothesized that shade would induce greater belowground allocation by saplings to root growth and storage. Indeed, we found that shaded saplings had higher root mass fractions and higher concentrations of starch in their roots than plants in full sunlight. Plants in full sunlight, meanwhile, invested more in aboveground growth, with higher stem mass fractions than shaded plants. Shade did not affect leaf mass fraction, but plants in the shade had a lower leaf mass per area, higher stomatal conductance, and a higher maximum photosynthetic rate, indicating leaf-level adjustments that increased carbon capture under light limitation. These responses are consistent with possible adaptive allocation strategies that buffer the impacts of fire and herbivory, underscoring the essential role of belowground reserves for regrowth.