Plant quality, seasonality and sheep grazing in an alpine ecosystem

Large herbivores are affecting a suite of plant traits in many ecosystems, including plant quality. At northern latitudes, the phenological development of plants over the growing season is also regarded crucial for plant quality. The relative role of grazing and seasonality for quality of different plant functional groups has not been quantified in northern, alpine ecosystems, but are assumed to form a key role in the ecosystems due to the feedback on herbivore behaviour and performance. In an experimental setting (3 sheep density treatments replicated 3 times) and based on 1518 samples from 6 species of plants belonging to 3 functional groups collected over the entire growing season, we tested the relative role of grazing and seasonality for plant quality as indexed by N- and P-content and C:N and C:P ratios. We examined sheep diet composition and quality in faeces. There was a marked seasonal decline in specific N- and P-content for all plant species. N-content was higher for grass tissue collected at high sheep density late in the season relative to those from controls without sheep, but not for forb and dwarf shrub tissue. P-content in tissue peaked at low sheep density for Avenella flexuosa (late season), forbs and Vaccinium myrtillus. C:N and C:P ratios mainly followed the patterns for N- and P-content, respectively. The grass A. flexuosa increased while forbs and Salix sp. declined in proportion in the diet over the growing season. A. flexuosa were more consumed at high sheep density, while forbs were more consumed at low density. N- and P-content in sheep faeces declined markedly over the grazing season, but was not related to treatment. We conclude that the endogenous seasonal changes in plant quality over the growing season are stronger than grazing effects, but that grazing extends the season of access to high quality grasses suggesting potential for grazing facilitation.