Experimental extensification of mountain grasslands restores plant species richness but not species composition in the mid-term.
The traditional grasslands that characterize the cultural landscapes of the Palaeartic mountain massifs represent biodiversity hotspots. Yet, they are currently threatened by the intensification of farming practices, notably excesses in fertilization and irrigation. We experimentally investigated the passive restoration of montane and subalpine hay meadows after 6 years of management intensification, with different levels of fertilization and irrigation, followed by 5 years of release of intensive management, that is, extensification. More specifically, relying on a full randomized block-design replicated at 11 Swiss study sites constituted of extensively managed meadows, we exposed during 6 years (2010-2015) four 20 m diameter plots to three levels of intensification (low, medium and high inputs), while a fourth plot served as a control (no inputs). In the second phase of the experiment (2016-2020), all study meadows underwent farming extensification. We monitored total species richness and plant diversity (Simpson diversity), indicator plant species as well as the composition and variability of the plant communities based on Bray-Curtis dissimilarity distances. We found that total species richness decreased in the most intensified plots after 6 years of intensification, but all plots retrieved their baseline species richness after 5 years of re-extensification. In addition, we found no difference between the years in plant diversity (Simpson diversity) among the treatments. Yet, intensification led to different plants communities' compositions in all three levels of intensification in 2015 compared to the extensive plots, and this structural difference remained after 5 years of re-extensification. Synthesis and applications. Land-use intensification induces a rapid impoverishment of the flora of mountain meadows. Our results demonstrate the potential of mountain hay meadows to passively restore plant species richness after re-extensification; however, plants communities did not fully recover. We recommend maintaining fertilization inputs as low as possible and operating active restoration on grasslands formerly intensified.