Warming Leads to Biomass Increase, Leaf Nitrogen Decline, and Community Turnover in Mediterranean Nardus stricta...

Nardus stricta L. grasslands are designated a priority habitat under the European Union Habitats Directive, reflecting their role in biodiversity conservation and soil and water regulation. However, these grasslands are listed as vulnerable in the European Red List of Habitats, primarily due to threats associated with global change. Despite their conservation importance, subalpine Nardus grasslands in the Iberian mountains remain poorly studied, and responses to climate warming are largely unknown. We experimentally tested the effects of warming in five semi-natural N. stricta grasslands (1545-1875 m a.s.l.) in Serra da Estrela, Portugal, located at the transition between Mediterranean and Atlantic climates. In 2020, five paired open-top chambers (OTCs) and control plots were established at each site. Community composition and aboveground biomass were monitored annually from 2021 to 2023, and leaf nutrient content of N. stricta was analyzed in 2022. Warming increased mean air temperature by ~1.9°C across sites, reduced freezing exposure, and enhanced thermal accumulation during the growing season. These changes were associated with higher aboveground biomass and increased species richness, accompanied by moderate turnover. The dominant grass N. stricta increased in cover across site × year combinations, whereas the snow-associated endemic Festuca henriquesii, restricted to the high-elevation site COB, declined under warming. Warming also reduced leaf nitrogen concentration in N. stricta, altering the nutrient balance of the dominant grass and potentially leading to cascading ecological effects. Responses were site-dependent, with weaker biomass increases at drier, lower-elevation sites, suggesting baseline water availability modulated warming effects. Our results demonstrate a consistent thermal signal but context-dependent ecological responses. Although warming enhanced productivity and richness, it also altered species composition and reduced leaf nitrogen content, suggesting early restructuring of Iberian subalpine grasslands under climate change. These findings highlight the importance of incorporating site-specific environmental context, particularly soil moisture, into conservation strategies for Mediterranean mountain ecosystems.