Growth and Physiological Responses of Tundra Plants to Defoliation

In addition to extreme abiotic conditions, biotic factors such as grazing influence the growth of tundra plants. Strategies of carbon assimilation, accumulation, and utilization must not only satisfy the requirements of a rigorous physical environment but also must simultaneously adjust to the impacts associated with herbivory. Representatives of four growth forms found in northern Alaska (a deciduous shrub, an evergreen shrub, a singleshooted graminoid, and a tussock-forming graminoid) were subjected to various defoliation regimes and their physiological and morphological responses were documented. Results from this study support the hypotheses that (1) carbon allocation patterns are a function of growth form and dictate plant responses to defoliation; (2) maximum photosynthetic rates are a function of growth form and are inversely related to leaf longevity; (3) the impact of grazing is reduced in plant species with rapid leaf turnover and little supportive tissue; (4) defoliation results in an immediate adjustment in carbon allocation pattern within the defoliated plant; and (5) carbon allocation to maintenance tissue or longlived productive tissue is accompanied by mechanisms that reduce grazing.