Impact of selective grazing on plant production and quality through floristic contrasts and current-year defoliation in a wet grassland

Grazing impacts the structure and functional properties of vegetation through floristic changes (i.e., long-term effect) and current defoliation (i.e., short-term effect). The aim of this study was to assess the relative importance of these two grazing effects on productivity (ANPP) and plant quality (C/N ratio) among plant patches submitted to a variety of grazing intensity for several years. Long-term grazing effect was measured by comparing ANPP and C/N ratio among plant patches with contrasting floristic composition. Short-term impact of grazing was measured by comparing ANPP and C/N in plant patches, with and without defoliation. Floristic contrasts led to a lower ANPP in highly grazed patches than in lightly grazed ones. This result may be related to the increasing proportion of grazing-tolerant and grazing-avoiding species with increasing grazing intensity. Vegetation C/N contrasts were recorded among grazed patches but did not linearly relate to grazing intensity. Short-term effect of current-year defoliation on ANPP was limited as vegetation compensated for biomass removal. No evidence for grazing-enhancement of ANPP was found even at moderate grazing intensity. Long-term floristic changes with grazing thus appeared to be the main driving factor of variations in ANPP. In contrast, C/N ratio showed no general and consistent variation along the grazing gradient but varied consistently depending on the community investigated, thus suggesting an effect of the species pool available.