The impact of shade on whole-plant carbon allocation in an 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 favor
investment in storage belowground to support regrowth aboveground. It is
unclear if carbon (C) limitation from grass shading similarly favors
allocation belowground. Further, investigating how light reduction changes
allocation by juvenile trees to above- and belowground biomass, storage,
and defense 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.