The authors summarize a study on the responses to grazing pressure of a biennial forb, Ipomopsis arizonica. The forb was either ungrazed, grazed, or clipped for three growing seasons and also received a water, water and fertilizer, or no input treatment. In the population of treated forbs, the complete range, from under to overcompensation, was observed. Overcompensation occurred in areas where the forb was not competing with other plants and received nutrient supplements. Undercompensation occurred when in association with grasses or where nutrients were not added. Overall, the probability of compensation decreased with competition, decreased nutrients, and grazing later in the growing season.
In a 3-year study, Maschinski and Whitham compared the impact of herbivory and nutrient availability on the compensatory response of the forb Ipomopsis arizonica when associated with either ponderosa pine (Pinus ponderosa) or mountain muhly (Muhlenbergia montana), or in the open in northern Arizona. In response to natural herbivory, 80% of the Arizona ipomopsis compensated equally, fruiting similarly to ungrazed species. Plants clipped later in the season-- after May 30th-- were unable to compensate, suggesting that young tissue can be more easily replaced than older tissue. The authors demonstrate experimentally that the degree of herbivore impact depends on plant association, nutrient availability, and timing of grazing. From 1985-1987, the most common response to vertebrate herbivory was equal compensation, whereby grazed plants set numbers of fruits and seeds equal to controls within the same growing season. However, they also observed cases of significant overcompensation and undercompensation. In 1985 and 1987, overcompensation occurred in vertebrate grazed plants that were supplemented with nutrients and growing free of competition. These plants produced 33% to 120% more fruit than control, ungrazed plants. Grazed and clipped plants where they grew in association with grasses produced 28% to 82% as many fruits as did ungrazed controls. The authors conclude that the studies indicate the compensatory response of plants to grazing is probalisitc when three external factors are considered. The probability of compensation for herbivory decreases as competition and other plants increases, as nutrient levels decrease, and as the timing of herbivory comes later in the growing season.
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