Physiological Responses of Individual Plants to Grazing: Current Status and Ecological Significance

147 Individual plant respon~es to herbivory were initially investigated to establish prudent guidelines for livestock grazing on pastures and rangelands. A great deal has been learned concerning the effects of defoliation on the morphology, physiology and productivity of important forage and browse species. However, our perception and understanding of individual plant responses to herbivory have been substantially altered by research findings developed during the past 15 years. Information within this contemporary research perspective can be generalized into three _ broad categories. First, several traditional assumptions regarding plant responses to defoliation have proven to be over simplifications. For example, questions have arisen concerning the value of carbohydrate reserves as an indicator of plant growth following defoliation and t,he assumption that tillering is exclusively regulated by auxin produced in the apical meristem. Second, several novel hypotheses have been proposed to explain the potentially beneficial effects of herbivory. Increased photosynthetiC rates of residual foliage, accelerated growth rates, modified allocation patterns and enhanced nutrient absorption have been observed in various plant species following defoliation. !hird, the ability of biotic interactions and abiotic constraints to accentuate herbivore-mediated plant responses has only recently been fully appreciated. For example, herbivore-induced modifications of competitive interactions may pot~ntially constrain plant growth to a greater extent than the direct effects of biomass removal. Evidence from both the traditional and contemporary literature has been surveyed to assess the current status of our kllowledge and evaluate the ecol~gical significance of individual plant responses to grazing by large herbivores.