Discrepancies between the drivers of alpha and beta plant diversity in arable field margins

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 ha..., 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 ditch..., Microsoft Excel or OpenOffice