The Incidence of Herbivory in High-Elevation Populations of Ranunculus glacialis: A Re-Evaluation of Stress-Tolerance in Alpine Environments

Abstract

Growing conditions in the upper alpine zone are characterized by low temperature. low partial pressures of CO 2 and, in the temperate zone, a short growing period. The plants which have evolved under these conditions presumably share a number of characteristics that were ascribed to stress-tolerance, namely slow growth, extended longevity, resource limitation and low palatability to herbivores. Hence chronic biomass removals by herbivores should be a threat to plant persistence in alpine environments, as predicted by Grime's C-S-R theory. I tested this hypothesis on populations of an alpine buttercup. Ramunculus glacialis. A survey along an altitudinal transect in the Central Alps of Austria indicated that between 15 and 26% of the R. glacialis plants in each population examined exhibited signs of herbivory damage. Merely a small population, isolated by glaciers, at the highest site (3310 m a.s.l.) showed no traces of herbivory. At two sites (2600 m and 3180 m a.s.l.) twenty plants each were tagged and examined for a two-year period. Herbivory damage was considerable: on an average nearly 25% of a plant's total leaf area was removed in 1987. primarily by snow mice (Microtus nivalis). Inflorescences of 65-85% of all flowering plants were removed as well. At the lower site (2600 m. roughly 600 m above the treeline) up to 5 g dry matter and 140 mg nitrogen m -2 were consumed in one season. Despite the magnitude of these losses both reproductive investment and the number of leaves initiated per plant did not change appreciably in the subsequent year. Since populations of R. glacialis are able to support populations of herbivores at the altitudinal limits of plant growth without obvious reductions in vigor. these plants and other food species (e.g. Oxyria digyna) cannot fit the stress-tolerator scheme proposed by Grime. The widespread occurrence of herbivory at high elevations and plant traits challenge the concept of stress-tolerance as it is commonly applied to alpine environments.