Combining fertilisation and mowing as an effective practice to control Brachypodium rupestre encroachment in an abandoned grassland of the Alps

Abstract

In the Alps, grasslands have been the basis for European mountain farming systems for centuries, but nowadays agro-pastoral abandonment is among the major threats to their conservation. Grazing and mowing interruption favours the spread of coarse plants, such as the tall grass Brachypodium rupestre, which negatively impacts grassland agroecosystem functions and ultimately leads to grassland degradation. Practices such as nutrient addition (i.e. fertilisation) and biomass removal (i.e. mowing) have been successfully applied in several mountain environments to reverse the degradation process and restore the original species composition. However, in the Alps, experiments combining both practices have been scarce so far. We hypothesised that the benefits of fertilisation and mowing on the species composition of a B. rupestre encroached grassland could be maximised by coupling fertilisation (120 kg ha⁻¹ N – 80 kg ha⁻¹ P₂O₅ – 80 kg ha⁻¹ K₂O) and mowing. Treatments were carried out yearly over ten years and data were collected throughout the entire period to study the changes in agronomic performances (i.e. pastoral value and abundance of meso-eutrophic grassland species cover), plant diversity (i.e. species richness and effective number of species), and botanical composition. Fertiliser addition effectively enhanced meso-eutrophic grassland species after five years but did not affect either B. rupestre cover or the sward pastoral value. Instead, it slightly reduced the dry grassland species cover, which is considered of conservation interest, and the plant diversity. Mowing successfully reduced B. rupestre presence after five years while maintaining the initial dry grassland species cover and overall species diversity as well. However, it did not improve either meso-eutrophic grassland species cover or the pastoral value. The combination of fertilisation and mowing showed the most promising results. It was able to decrease B. rupestre cover (- 80 %) in the short term while increasing meso-eutrophic grassland species cover (+ 300 %) and the pastoral value (+ 6.5), without negatively impacting dry grassland species cover and plant diversity. According to our long-term study, combining biomass removal by mowing and nutrient addition by fertilisation can be a suitable strategy to achieve agronomic performances and habitat conservation targets, and successfully restore degraded mountain grasslands in the Alps.