Functional trade-offs: exploring the effects of climate change and agricultural practices as drivers of field margin plant communities
Over the past decades, agricultural intensification and climate change have led to vegetation shifts. However, functional trade-offs linking traits responding to climate and farming practices are rarely analyzed, especially on large-scale empirical studies. Here we used a standardized yearly monitoring effort of agricultural field margin flora at the national scale to assess the temporal response of diversity and functional traits to variations in climate and intensity of agricultural practices. We examined temporal trends in climate (temperature, soil moisture), intensity of agricultural practices (herbicides, fertilization, margin management), plant species richness, and community-weighted means and variances of traits expected to vary both with climate and agricultural practices (e.g. seed mass, specific leaf area), across 555 sites in France between 2013 and 2021. We found clear temporal climatic trends (temperature increased while soil moisture decreased), whereas trends in agricultural practices were weak over the past decade, with only slight decreases in herbicides and margin management intensity. During the same period, functional changes in plant communities were significant, showing an increase of thermophilic species (including some Mediterranean species) with a conservative resource acquisition strategy (high stature, late and short flowering), mainly explained by climate change. The reduction in field margin management intensity (mainly mowing), also resulted in a vegetation shift towards a more conservative strategy. In contrast, there was no impact of the slight temporal changes of practices conducted within cultivated fields (herbicides, fertilization) on vegetation changes. Our findings suggest that species adapted to climate change (including Mediterranean and conservative species) have temporally increased in proportion. Importantly, we identified functional trade-offs indicating that these species are also the most vulnerable to intensive agricultural practices, as they are less adapted to high levels of resources and disturbances. We put these results into the conceptual framework of Grimes CSR triangle and revealed a temporal decline of competitive and ruderal species in favor of stress-tolerant species better adapted to climate change. Choosing less intensive management can broaden the functional spectrum of agricultural plant communities, by maintaining the ability of stress-tolerant species selected by climate change to colonize habitats largely dominated by ruderals. Put together, these results suggest that climate change and agricultural intensification could have synergistic negative impacts on plant diversity in field margins, highlighting the need to study several biodiversity drivers at the same time in anthropized landscapes.