Long-term and year-to-year stability and its drivers in a Mediterranean grassland

Understanding the mechanisms underlying community stability has become an urgent need to protect ecosystems from global change and resulting biodiversity loss. While community stability can be influenced by species richness, synchrony in annual fluctuations of species, species stability and functional traits, the relative contributions of these drivers to stability are still unclear. In semi-natural grasslands, land-use changes such as fertilization might affect stability by decreasing richness and influencing year-to-year fluctuations. In addition, they can promote long-term directional trends, shifting community composition and influencing grassland maintenance. Thus, it is important to consider how species and community stability vary year-to-year but also in the long term. Using a 14-year vegetation time series of a species-rich semi-natural Mediterranean grassland, we studied the relative importance of richness, synchrony, species stability and functional traits on community stability. To assess land-use change effects on stability, we applied a fertilization treatment. To distinguish stability patterns produced by year-to-year fluctuations from those caused by long-term trends, we compared the results obtained using a detrending approach from those without detrending. Independently of the treatment and approach applied, the most stable communities were those composed of asynchronous species with low specific leaf area. Fertilization decreased year-to-year and long-term community stability by increasing community-weighted mean of specific leaf area, decreasing species stability or also reducing richness in the case of year-to-year stability. Additionally, traits such as seed mass had an indirect effect on stability through synchrony. Long-term trends appeared in control and fertilized plots (due to fertilization), decreasing community and species stability and leading to differences in the relationships found between community stability and some of its drivers. This reflects the importance of accounting for the effect of temporal trends on community and species stability using both a long-term and a year-to-year approach. Synthesis. Stability is influenced by richness, synchrony and functional traits. Fertilization decreases species and community stability by promoting long-term trends in species composition, favouring competitive species and decreasing richness. Studying stability at the community level and species level, and accounting for the effect of trends is essential to understand stability and its drivers more comprehensively.