DOI: 10.5061/dryad.79cnp5hz9
title: Discrepancies between the drivers of alpha and beta plant diversity in arable field margins

description: 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.
description: 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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