Relating functional traits within and across species to the stabilization of grassland productivity

Abstract Biodiversity loss threatens the stability of ecosystem functions such as productivity, yet how plant functional traits regulate the two key components of ecosystem stability, species stability and species asynchrony, remains unclear. Asynchrony is often attributed to trait differences among species along the slow–fast spectrum, reflecting contrasting responses to environmental fluctuations, but may also arise from competitive interactions among species with similar growth strategies, a mechanism that is difficult to detect without experimentally manipulating interspecific coexistence. In a five‐year grassland biodiversity experiment (2018–2022), we measured two functional traits central to the leaf economics spectrum, specific leaf area (SLA) and leaf dry matter content (LDMC) in 2022. Using more than 2000 leaf samples, we assessed how species' mean trait values, intraspecific trait variation and interspecific trait dissimilarity relate to species stability and asynchrony across a gradient of species richness. Species stability declined consistently with increasing mean SLA, indicating lower temporal stability in species with faster strategies, whereas mean LDMC was not related to stability. Greater intraspecific variation in SLA and LDMC reduced species stability in monocultures but not in polycultures. The relationship between interspecific trait dissimilarity and species asynchrony also depended on species richness. In monocultures, LDMC‐based trait dissimilarity was negatively associated with temporal correlations—that is, greater functional dissimilarity among species corresponded to higher asynchrony. However, in species‐rich mixtures, temporal correlations decreased as species interactions intensified, and the relationship shifted to positive, indicating that species with similar traits exhibited higher asynchrony. Synthesis. Our findings demonstrate that species' mean positions along the slow–fast spectrum strongly shape temporal stability, whereas the influences of within‐ and between‐species trait variation depend on the extent of interspecific coexistence. This highlights that trait‐based predictions of community stability must explicitly account for species richness and the balance between environment‐driven and interaction‐driven temporal dynamics.