Diversity comes at a cost: multifaceted diversity reduces plant community stability in peatlands

1. Understanding how ecological stability relates to diversity is of crucial importance under global change. Greater biodiversity is expected to stabilize aggregate community properties through compensatory dynamics, as species fluctuate asynchronously and offset one another. Yet, diversity-stability relationships are not straightforward and can vary across and within ecosystems, particularly in wetlands where strong abiotic filters shape community assembly and temporal dynamics. 2. We examined how multiple facets of diversity (taxonomic, functional, and phylogenetic) and functional trait identity relate to temporal stability (invariability) and species asynchrony in peatland vegetation. We used a 17-year field experiment in a montane peatland complex spanning a bog and a transitional poor fen, combining open-top chamber (OTC) passive warming with natural hydrological contrasts. 3. Water table depth was the dominant environmental filter of plant communities, explaining 46 % of total compositional variance, whereas OTC-induced warming had no detectable effect. Community temporal stability and species asynchrony were higher under drier conditions (deeper water table), consistent with moisture-driven constraints on peatland vegetation dynamics. 4. Contrary to insurance hypothesis predictions, temporal stability decreased with multiple biodiversity facets, particularly phylogenetic diversity and species richness, but increased with deeper-rooted plant strategies, after controlling for experimental conditions. Species asynchrony was largely unrelated to biodiversity, except for functional redundancy, which was associated with lower asynchrony but showed no association with temporal stability. The stability-asynchrony association weakened substantially after controlling for hydrology. 5. Synthesis. Our results reveal that in peatlands, hydrology simultaneously structures biodiversity patterns, temporal stability and species asynchrony, yielding negative diversity-stability relationships that contradict classical insurance hypothesis predictions. These findings suggest that in peatlands, stability arises primarily from hydrological constraints, with limited contribution from compensatory dynamics among plant species. In strongly constrained, species-poor ecosystems, conservation may therefore prioritize maintaining or restoring the key abiotic conditions that favor functionally adapted communities over increasing diversity to sustain stable ecosystem functioning under global change.