A mechanistic simulation analysis of water use, leaf angles, and grazing in East African graminoids

Abstract

A simulation model was constructed in order to interrelate the effects of defoliation by large mammalian herbivores with growth, photosynthesis, water use and light interception of representative graminoid vegetation from East Africa. Abiotic submodels describing water flows, light penetration and interception, and energy balance relationships were interfaced with a graminoid growth model that was specifically designed to examine grazing responses. Defoliation reduced rainfall interception and reduced transpiration, thereby increasing soil and plant water levels. Morphological adaptations made compensatory shoot growth possible, but grazing effects on plant water contents largely determined the degree of compensatory growth. Aboveground productivities were greatest at moderate grazing intensities while belowground productivities were greatest at lower grazing intensities. The success of a given water use strategy depended on simultaneous consideration of the abiotic environment and grazing. Under frequent grazing soil water conservation and reduced shoot/root ratio counteracted the depletion effect of high stomatal conductance on plant water content. Consequently photosynthesis under frequent grazing was maximized by higher stomatal conductance. Canopy geometry was adaptive for maximal light harvesting subject to the constraints imposed by transpiration, water availability, and herbivory. Total photosynthesis was increased greatly by erect leaf angles for all but not short grasses. For frequently grazed short-grasses the benefits of prostrate leaves associated with escape from grazing exceeded the associated costs of reduced photosynthate.