Above- and below-ground allocation and functional trait response to soil water inputs and drying rates of two common savanna grasses

The study of functional trait plasticity and optimal allocation strategies in a water competition context may help to explain the mechanisms relating plant competition to ecological semi-arid patterns. We measured four functional plant traits –root to shoot ratio, superficial to deep roots ratio, root diameter, and root estimated surface area to shoot ratio– in wild populations of two C4 grasses in a Kenyan savanna to assess their responses to different water inputs (changing with latitude) and soil drying rates (driven by canopy cover). Root:shoot allocation was different between the two species. We show that to account for root architecture helps understanding root allocation strategies, especially in species with high plasticity in root geometry. Concretely, in our case, a higher root allocation in wetter conditions was driven by water storage and not water foraging in the species with high root diameter variability. Superficial root allocation responded to changes associated with drying rates but not to water input, and shallower root systems developed under higher water stress conditions, supporting rainfall intermittency models of savanna tree-grass competition. We also suggest self-shading as a facilitation mechanism that may affect spatial patterns according to the scale-dependent feedback hypothesis.