Sheep grazing and dynamics of alpine plants in a landscape-scale experiment

Abstract

Large herbivores such as sheep affect plants directly and indirectly through selective foraging, trampling, urination and defecation, thereby modifying intra- and interspecific interactions and altering resource availability for plants. Tolerance and resistance are the main means by which plants cope with herbivores. How herbivores change the relative abundance of tolerant and resistant species has large impact on long-term ecosystem productivity, and the possibility of using plant functional groups to make general predictions is currently a key topic. The ability to express tolerance and resistance will depend on herbivore selectivity and on grazing frequency and intensity, factors that are all dependent on herbivore density. Most studies of grazing focus on effects on the plant community composition, and either compare sites of contrasting herbivore densities or use small-scale exclosure experiments, thus comparing only two levels of grazing despite theories often predicting non-linear responses of plants to increasing grazing pressure. There is a need to compare more than two herbivore density levels, to link herbivore density to grazing pressure, and furthermore, to link plant responses to grazing with plant traits that can explain the mechanisms underlying changes in plant community composition. This thesis presents studies of effects of sheep grazing on plants within the framework of a landscape-scale experiment covering 2.7 km2 in a productive alpine ecosystem in southern Norway. The experiment was established in 2001, and the impact of three levels of sheep densities (high, low and zero), and more specifically of changing grazing regime (enhancement and cessation of grazing) on alpine herb species was investigated.<br> The experimental densities of sheep corresponded to low (low sheep density) and moderate (high sheep density) grazing pressure. Selected herb species, as determined by a combination of biomass and nutrient quality of the plants, did not in general respond negatively to enhanced grazing pressure. Rather, grazing response was related to both plant height and root/shoot ratio, i.e. avoidance through low stature combined with capacity for regrowth was the main mechanism behind a positive response to enhanced grazing pressure.<br> I studied the detailed life history of two herb species, the tall, grazed Geranium sylvaticum and the low stature, non-selected Viola biflora, to explore how they coped with the three grazing levels. Contrasting responses to high sheep density and no sheep were predicted for the two species, the tall G. sylvaticum expected to benefit from cessation of grazing and the low stature V. biflora predicted to benefit from presence of herbivores. By following individual plants from 2002 through 2007, population-level consequences of different sheep densities were evaluated. Grazing increased clonal reproduction rates of G. sylvaticum, but at the same time reduced growth compared to ungrazed plants. However, the proportion of the populations that was grazed was low in all years (6−14% and 2−6% in the high and low sheep density treatments, respectively), and no differences in population growth rate (ë) between sheep grazing treatments were detected. The V. biflora populations that were exposed to sheep appeared to have more favourable conditions for growth than the population protected from sheep, but no effect of sheep grazing treatment was found on ë of this species either. However, in years with high levels of grazing by sheep and rodents, ë of V. biflora was suppressed. Between-year variations in ë were large for both species, and the pattern of variation was similar, strongly suggesting that external factors other than sheep grazing controlled between-year variation in vegetation. <br> The experimental sheep densities did not substantially change plant community or population dynamics on the six-year time scale included in this study, suggesting no negative impact on ecosystem productivity. The long history of grazing in Scandinavian alpine ecosystems, combined with long-lived plant species and moderate grazing pressure, is likely to explain the small changes, and variable climatic conditions seemed to overrule effects of grazing. Furthermore, effects of sheep on other herbivores in the ecosystem (rodents, invertebrates) may contribute to the lack of consistent results.