A mathematical analysis of the changes in plant relative growth rates necessary to increase aboveground production following grazing was conducted. The equation derived gives an isoline where production of a grazed and ungrazed plant will be the same. The equation has four variables (mean shoot relative growth rate, change in relative growth rate after grazing, grazing intensity, and recovery time) and may be analyzed graphically in a number of ways.Under certain conditions, small increases in shoot relative growth rate following grazing will lead to increased aboveground production. Under other conditions, very large increases in relative growth rate after grazing can occur without production being increased over that of ungrazed plants. Plants growing at nearly their maximum potential relative growth rate have little opportunity to respond positively to grazing and potentially can sustain less grazing than plants with growth rates far below maximum. Plants with high relative growth rates at the time of grazing require large increases in growth rate while slow growing plants require only small increases. High grazing intensities are least likely to increase production and high grazing frequencies require greater responses than infrequent grazing events.

Production of tallgrass prairie vegetation was measured on experimental plots in which defoliation intensity and frequency were manipulated by mowing and using movable exclosures on areas chronically grazed by cattle. Defoliation history largely controlled whether or not defoliated plants overcompensated (exhibited enhanced production compared to undefoliated controls) for tissue removal. Plants on chronically grazed sites only compensated for foliage removed by grazers. Production on plots mowed prior to the year of measurement was similar to that on chronically grazed sites, while previously unmowed plots exhibited substantial aboveground overcompensation. Aboveground production was maximized by the most frequent mowing treatment and by intermediate mowing heights. Nitrogen and phosphorus concentrations and amounts in aboveground tissues were increased by mowing and grazing. Current mowing regime was more important than mowing history in determining nitrogen concentrations except very early in the growing season. Effects of grazing and mowing on belowground biomass were inconsistent, but frequent mowing appeared to limit accumulation of belowground N reserves and biomass. In North American grasslands, overcompensation is a nonequilibrium plant response to grazing. Photosynthate that would be stored as reserves and used for root growth and flower and seed production instead is used to replace lost leaf area, thereby resulting in higher foliage productivity. However, under chronic grazing or mowing, vegetation is prevented from maintaining high nutrient and water uptake capacity (large root biomass) and accumulating reserves that allow overcompensation responses.

Net photosynthesis (P), root respiration (R), and regrowth of Bouteloua gracilis (H.B.K.) Lag. were examined in the laboratory over a 10-day period following clipping to a 4-cm height to simulate grazing by large herbivores. Net photosynthesis rates of tissue remaining immediately following defoliation were only about 40% as great as preclipping rates. Three days after clipping, P rates of defoliated plants had increased to values about 21% greater (per unit leaf area) than those of unclipped controls and remained at that level through Day 10. No statistically significant changes in R occurred following defoliation. Biomass of unclipped plants nearly doubled during the 10-day study period, while that of defoliated plants increased 67%. Over half the new growth of defoliated plants was allocated to new leaf blades and only 18% to new roots, while only 33% of the new growth of control plants was allocated to new leaf blades but 29% went to new roots. As a consequence of increased P rates and increased carbon allocation to synthesis of additional photosynthetic tissue following defoliation, net CO uptake per plant increased from 9% to 80% of that of the controls from Day 0 through Day 10.

Population samples of an African C4 grass, Panicum coloratum L., were collected from two locations in the Serengeti Grasslands varying in grazing intensity, one a high—grazing location (GA = grazing—adapted), the other a low—grazing location (NGA = nongrazing—adapted). Plants were cloned, put in controlled environments mimicking the natural photo—thermoperiod, and subjected to light grazing pressure by a generalist feeding North American grasshopper, Melanoplus sanguinipes. Carbon assimilation and redistribution were measured in the short term with an infrared gas analyzer and 11C—labelled CO2, coupled with a three—compartment analytical model, and by harvesting whole plants at the end of a 12—wk regrowth experiment. Results documented several significant differences between the GA and NGA samples, suggesting the evolution of physiological traits related to C assimilation, translocation, and storage in response to previous grazing history. Pregrazing net C—fixation rates, translocation rates, and C—storage pools were identical for the two ecotypes. After grazing, overall C—fixation rates were 39% higher for the GA plants, and the regrowth data suggest they remained higher than NGA rates throughout the experiment. Removal of < 10% of initial green—leaf biomass by grazing at each grazing period produced major differences in carbon flux between the two samples. Throughout the experiment GA plants produced and stored more C in leaves, stored less C in stem sinks, had higher phloem activity, and translocated more of the labile C to roots where it was stored in higher quantities. This suggests that storage of labile C reserves in sinks or pools readily accessible to the plant, which allows rapid mobilization after grazing, is an important element of adaptation to grazing. Major storage of labile reserves in stems, characteristic of the NGA plants, may be advantageous in ungrazed habitats where there is vertical growth due to canopy closure and competition for light, but such storage makes those reserves accessible to grazers. GA ecotype plants in the regrowth experiment compensated completely for the eight weekly defoliation events by the time of the 12—wk harvest; yield of the NGA ecotype was reduced 21% by the moderate level of grasshopper grazing. Increased total yield (biomass harvested plus compensation) by the GA ecotype was expressed in both above— and belowground biomass, suggesting that suppression of the latter does not contribute to compensation by the former.