Field margins are major habitats for biodiversity conservation and ecosystem functioning in agricultural landscapes, but biotic homogenization of plant communities threatens their ecological and agronomic functions. Our objective is to assess the effects of field margin structure and long-term management of herbaceous layer over 20 years (1995–2015) on the taxonomic and functional α- and β-diversity, and the functional composition of herbaceous plant communities. In 2015, we surveyed 302 field margins in bocage landscapes of Brittany (France). Results were very similar between taxonomic and functional diversity but revealed discrepancies between α- and β-diversity. Deep ditches, mowing, and grazing increased α-diversity but did not affect β-diversity. Denser hedgerows had lower α-diversity than other field margins but contributed to β-diversity by harboring more unique sets of species or life strategies. Long-term herbicide spraying in field margins and cropping intensity in adjacent habitats did not affect α-diversity, but had more complex effects on β-diversity and selected for common weeds. Preservation of dense hedgerows and abandonment of herbicide spraying are key measures to prevent the establishment of common weeds and biotic homogenization in field margins. Above all, our study shows how important it is to go beyond α-diversity to make robust conservation and restoration decisions.

We conducted the study in the Long-Term Socio-Ecological Research site “Zone Atelier Armorique” in Brittany, north-western France (48° 36' N, 1° 32' W).

We surveyed 302 field margins, from herbaceous to woody margins with or without shrub or tree layer (Fig. S2), across the three contrasting landscapes of the Zone Atelier Armorique (106 in landscape A, 103 in landscape B, and 93 in landscape C) (Fig. 1). We defined a field margin as one side of the entire field boundary (Alignier, 2018) (Fig. S3). Thus, within a boundary, we distinguished two field margins for vegetation sampling, each adjoining a specific habitat (e.g., arable field, woodland, road, water). From May to July 2015, we sampled herbaceous plants in 25 m long plots (one plot per field margin) located in the middle of the field margin to avoid edge effects. We sampled the full width of field margins (varying from 0.15 m to 3.8 m) to account for local heterogeneity in field margin structure (e.g., presence of banks and ditches). We visually estimated species percent cover as a proxy for abundance using an ordinal scale from one to five (Le Coeur et al., 1997). We used the middle of cover classes for subsequent analyses (one: 5%, two: 14.5%, three: 37.5%, four: 62.5% and five:87.5%).

We collected functional attributes and ecological indicator values in the following databases: Ecoflora (Fitter and Peat, 1994), Baseflor (Julve, 1998), and LEDA (Kleyer et al., 2008).

Field margin structure was defined using seven variables measured in 2015, namely canopy height, margin width, tree cover, shrub cover, woody species richness, bank height, and ditch depth. We visually estimated tree and shrub cover (%). Canopy width was also measured but removed from further analyses given its high positive correlation with canopy height (r = 0.82; Fig. S3). On the other hand, long-term management of herbaceous plant communities in field margins was defined using four variables measured all year round from 1995 to 2015 (Fig. S1), namely the occurrence of herbicide spraying, mowing, grazing, and tillage disturbances (intentional or not). An indicator of management intensity (%) for each practice was calculated as the number of occurring events divided by the total number of observations for each field margin. The number of observations per field margin over 20 years ranged from 77 to 98, with a mean of 95 ± 7 observations (i.e., generally four observations per field margin per year). In addition, we calculated cropping intensity (%) in habitats adjoining field margins, measured as the number of years of cropping (i.e. excluding grasslands and fallows) divided by the total number of years. Cropping intensity indicates not only the nature of adjacent habitats (non-arable vs arable) but also provides an estimate of the accumulation of agrochemical drift events (all arable fields' adjoining margins were under conventional farming).

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