Planning grazing management enhances plant alpha-diversity, not beta, and sward pastoral value of high-elevation grasslands

Introduction: the goal of the research was to analyse the effects produced on botanical composition, plant diversity, Pastoral Value (PV), and soil nutrient content by a five-year implementation of a Grazing Management Plan (GMP) in an alpine summer pasture, previously managed under a continuous grazing system (CGS). The GMP are a policy and management tool aimed at enhancing farm productivity, while preserving plant diversity, soil and landscape (Lombardi et al., 2011). In the western Italian Alps, they are based on rotational grazing systems (RGS) with animal stocking rate adjustments to keep it balanced with grassland carrying capacity (Probo et al., 2014). It was hypothesized that GMP would enhance plant diversity and increase meso-eutrophic species number and cover. Furthermore, PV and soil nutrient content would increase and homogenize across vegetation communities. Materials and methods: a total of 199 vegetation surveys was carried out in summer 2011 and reiterated in 2016. Vegetation ecological groups were identified by means of a Hierarchical Cluster Analysis and plant diversity indices were computed (alpha: species richness, Shannon diversity; beta: Sørensen, Horn, and Morosita-Horn indices). The mean soil nutrient content was estimated through Landolt N indicator values (N index) for each transect. Paired-sample statistical tests were performed on the whole vegetation dataset, on vegetation ecological groups and considering functional pools of species. Results: on the whole, species richness, Shannon diversity, N index, and PV increased between 2011 and 2016, while beta-diversity remained stable. Species richness increased in almost all the ecological groups identified, with a peak in the mesotrophic one. A significant change in the botanical composition was recorded in oligotrophic, mesotrophic, and thermic groups. The number and cover of nitrogen-poor high-elevation species increased in all groups, likely boosted by livestock seed transportation and improved connectivity amongst different communities. The meso-eutrophic species number and cover increased in thermic, mesotrophic, and pre-forest groups, suggesting a greater use of such areas by livestock after GMP implementation than under CGS. In addition, an increase of PV was detected in the pre-forest group. Conclusion: GMP implementation, through RGS with stocking rates tuned on grassland carrying capacity, was an effective and a sustainable management tool to enhance the botanical composition and plant diversity of sub-alpine and alpine grasslands over five-year, and to improve their pastoral value as well.