Selecting herb traits as indicators of sheep grazing in a Norwegian alpine habitat.

摘要

A common management aim is to keep large herbivore densities at sustainable levels. However, measuring grazing pressure is difficult. We seek combinations of herb species and morphological traits that serve as indicators of grazing pressure in alpine areas of Scandinavia. In a fully replicated, landscape-scale experiment with three replicates of each of three levels of sheep density (zero, low, and high), we measured grazing frequency (numbers of plants eaten, as a proportion), flowering frequency, and plant height for 17 common herb species for two consecutive grazing seasons. For eight species, grazing frequency varied significantly between treatments, and for four of these species there was additional strong between-year variation. The estimate for total grazing frequency of herbs decreased when more species of decreasing preference were included, and it was significantly lower in the second grazing season. There were large between-year differences in plant height probably related to weather patterns. A grazing treatment effect on plant height was found only for two species. Flowering frequency in selected species changed even at low sheep densities and was thus not considered suitable except for indicating ungrazed areas. Grazing frequency of three herbs, Saussurea virgaurea, Rumex acetosa, and Pyrola minor, provided a useful indicator of total grazing pressure, as these species were grazed only at high sheep density. Our study area is likely to be representative of alpine areas in southern Norway, but we advise careful calibration using monitoring procedures before similar management decisions on stocking rates can be fully implemented in other ecosystems..