Long-term Effects of Shifts in Grazing Pressure on Alpine Plant Species along an Elevational Gradient

摘要

Sheep (Ovis aries) are the main large herbivore in Norwegian mountains during summer and these herbivores have shaped alpine plant communities over a long land-use history. Sheep are selective feeders, and increased livestock densities increases the pressure on some plant species and growth form groups while others benefit from grazing. Cessation of grazing also alters the competitive relationship among plants, but how plant species respond to shifts in grazing pressure along an elevational gradient in the long term is less known. I used a large-scale enclosure experiment, located in the low- to mid-alpine zone, with fixed sheep densities (0, 25 (maintained density) and 80 sheep km-2). Frequency changes of plant species (n = 37) and growth form groups were studied in permanent vegetation plots during 12 years of experimental grazing. Few effects were detected in response to changes in grazing intensity in general, except for the tall herbs, which increased in response to cessation of grazing. More responses appeared, both on growth form- and species level, when considering the effects along the elevational gradient, where low herbs increased at low elevations in response to enhanced grazing and dwarf shrubs moved upward along the elevational gradient in response to cessation of grazing. This study demonstrates that elevation is an important factor when assessing responses of plant species to changes in grazing pressure, and that varying responses are found among species with same growth form. These results are valuable in terms of understanding indirect effects of the current climate- and land-use driven changes in sub-alpine and alpine ecosystems.