Simulation study of east-African perennial graminoid responses to defoliation

Abstract

The responses of East African grasses to defoliation at specific heights and frequencies were simulated mathematically over a single optimal growing season. The simulation model used morphological and physiological features of graminoid growth that are known to affect responses to defoliation. Water and nutrient limitations were not considered. Except under the most severe defoliation regimes, the model simulated greater aboveground and belowground productivities in grazed plants than ungrazed plants. This simulation of productivity was maximal at moderate grazing intensities, declining at both high and low grazing intensities. Short, mid, and tall-grasses exhibited different responses to defoliation. Short-grasses gave greater yields with greater frequencies of defoliation than mid and tall-grasses. Since mid and tall-grasses maximized their production through increased shoot sizes, greater intervals between grazing events were necessary. Short-grasses maximized production through increased shoot numbers, hence maximum yields could be obtained with greater grazing frequencies. At greater frequencies and lower heights of defoliation the proportions of blade tissue increased, stems decreased, nitrogen contents increased and a dense, leafy concentration of live shoot biomass was induced by enhanced tillering, removal of culm meristems and prevention of senescence. The most important factors responsible for grazing stimulation were: stimulation of tillering through decreased self shading, continued activity of non-elevated tiller meristems which had escaped grazing removal, release from apical dominance and subsequent stimulation of tillering, and the presence of a high maximum photosynthetic rate which rapidly replenishes carbohydrate reserves used for the early regrowth. The three essential properties a plant must have in order to compensate for herbivory are: (a) the ability to protect and regenerate apical meristems; (b) the ability to store carbohydrates that can supply activity of these meristems following substantial leaf area reduction; and (c) a photosynthetic rate great enough to support continued shoot and root growth, and to replenish carbohydrate reserves. Stimulation of photosynthetic rate by defoliation contributes greatly to the latter requirement.