Disturbance-dependent communities in wet meadows: the role of traditional management for the rare Marsilea quadrifolia.

Wet meadows are biodiversity-rich habitats in Europe, central to the Natura 2000 network, hosting specialised plant communities and several threatened species. Marsilea quadrifolia L., an aquatic pteridophyte of conservation concern, is a disturbance-dependent pioneer species of open semi-natural wetlands, threatened by changing agricultural practices. We assessed the effects of traditional management - seasonal grazing and annual mowing - implemented by an organic farm on M. quadrifolia and plant community composition in a wet Nature Reserve in Lombardy Region (N Italy), hosting the largest known extant wet meadow population in the country. Managed and unmanaged (exclusion) areas were compared through vegetation surveys over four years (2021-2024), assessing species cover, diversity indices, Grime’s functional strategies, Ellenberg indicator values, water table fluctuations and M. quadrifolia chlorophyll content as a stress indicator. Managed areas showed significantly higher abundance of M. quadrifolia, while overall plant diversity did not differ among treatments. However, unmanaged areas exhibited clear shifts in vegetation structure and functional composition, with a higher proportion of stress-tolerant and competitive strategies, a marked reduction in ruderal species, and lower community light demand. Interannual hydrological variability also influenced species performance, with drought limiting M. quadrifolia in 2022 and wetter conditions promoting its expansion in 2024. Chlorophyll measurements indicated no ecophysiological stress associated with exclusion. Low-intensity grazing combined with mowing maintains open patches, reduces competition, and supports threatened species such as M. quadrifolia. Traditional management within Natura 2000 sites remains essential for integrating conservation and agricultural use under changing water availability driven by climate change.