LABORATORY-SIMULATED GRAZING - INTERACTIVE EFFECTS OF DEFOLIATION AND CANOPY CLOSURE ON SERENGETI GRASSES

A factorial experiment was performed to evaluate the relationships between defoliation and canopy closure, recognizing that a major effect of grazing in nature is to prevent that closure. Short (Eustachys paspaloides) and mid-height (Themeda triandra) grasses from the Serengeti National Park, Tanzania, were grown for 12 wk in a balanced factorial experiment. Treatments combined clipping and canopy closure, simulated by raising an opaque cardboard collar as plant height increased. At harvest, plants were separated into morphological parts, dried, weighed, and analyzed for nitrogen (N) and nonstructural carbohydrates (CHO). Simulated canopy closure of unclipped plants reduced photon flux density at the plant base by 12% perpendicular to the light source and 55% at a 45-degrees- angle from the lights. Multivariate analysis revealed that experimental treatments produced completely distinct results. Simulated canopy closure of undefoliated plants more than doubled plant height, reduced tiller number by 65% in the short grass and 90% in the mid-height grass, converted differentiation from production of many small tillers to a few very large tillers, changed allocation patterns toward more stem and sheath, and shifted the short grass from a CHO-dominated economy to a more N-balanced economy. The time course of yield from defoliation reached a decided. maximum at 9 wk in the short grass but was much lower and remained comparatively constant in the mid-height grass. Comparing clipped-unshaded plants (simulating grazing) with unclipped-shaded plants (simulating no grazing) revealed that, for both grasses, compensation for defoliation was complete in total yield, yield of live tissues, crown mass, root mass, and total number of leaves. Neither species compensated in residual leaf mass, total leaf area, leaf area per unit of plant biomass, or stem mass. Defoliation shifted leaves of both species to lower CHO and higher N concentrations. In general, the short grass was modified more by canopy closure, the mid-height species by clipping. Greater defoliation tolerance of the short-statured species was due to much greater tillering, significantly greater N accumulation, greater allocation to leaf blade and sheath, less allocation to crown and root, a greater foliage concentration per unit canopy volume, a lower rate of foliage death, and the ability to sustain a considerably higher leaf area per unit of plant mass. This experiment indicated that growing unclipped plants isolated in pots grossly distorts morphology, growth, and chemical properties compared to plants experiencing the canopy closure normally encountered in ungrazed grasslands. Therefore, failure to account for the self-limiting consequences of canopy closure can mislead conclusions from laboratory experiments designed to test the consequences of defoliation.