DOI: 10.5061/dryad.k0p2ngfbv
title: Weed communities are more diverse, but not more abundant, in dense and complex bocage landscapes

description: Bocage landscapes are characterized by a network of hedgerows that
 delimits arable fields. Such landscapes provide many ecosystem services,
 including biodiversity conservation, but their effects on weed communities
 remain largely unknown. Bocage landscapes could affect weed communities
 through two main processes: plant spillover from hedgerows and increased
 environmental heterogeneity in arable fields. These bocage effects are
 also likely to vary between farming systems (conventional vs organic), due
 to differences in management practices. We sampled weed communities more
 than 20 m from field margins in 74 arable fields (37 per farming system).
 Fields were located along two independent landscape gradients of total
 length of hedgerows (with or without a shrub layer) and organic farming
 cover, in Brittany (France). We analysed the effect of ‘bocage’ (i.e., the
 density and complexity of hedgerow networks) and farming systems at field
 and landscape scales on species and functional diversity of weed
 communities. Further, we used fidelity to non-crop habitats and Ellenberg
 indicator values to assess the ‘plant spillover’ and ‘environmental
 heterogeneity’ hypotheses, respectively. Weed communities were more
 diverse and more abundant in organic farming systems. In addition, weed
 communities were more diverse, but not more abundant, in denser and more
 complex bocage landscapes. ‘Bocage’ increased species diversity of weeds,
 but also community-weighted variance of specific leaf area, plant height
 and seed mass. Positive effects of ‘bocage’ on weed diversity were driven
 by increased environmental heterogeneity rather than spillover of
 transient species from hedgerows. ‘Bocage’ effects were independent of
 farming systems at field and landscape scales. Synthesis and applications.
 Maintaining diverse weed communities is key to agroecological weed
 management and biodiversity conservation in agricultural landscapes.
 Farmers are often concerned that hedgerows harbour competitive plants
 spreading into field edges, thereby increasing weed pressure. However, our
 study shows that dense and complex bocage landscapes promote weed
 diversity in field cores, most likely by increasing environmental
 heterogeneity. Thus, bocage landscapes could actually enhance ecosystem
 services provided by weed communities and reduce weed-crop competition.
description: This study was conducted in the southern part of the Zone Atelier
 Armorique, a Long-Term Socio-Ecological Research (LTSER) site in Brittany,
 northwestern France (47°59'35N, 1°45'12W). This region is
 characterized by dense hedgerow networks and crop-livestock farming
 systems. Agricultural lands are dominated by wheat and maize fields and
 temporary grasslands, and dominant soil types are Brunisols and Luvisols.
 Hedgerows are mostly old (i.e., planted at least before World War II), and
 generally composed of oak (Quercus robur) or chestnut (Castanea sativa)
 trees planted on a bank and pruned for firewood every 9–12 years. When
 present, the shrub layer is generally dominated by hazel (Corylus
 avellana), hawthorn (Crataegus monogyna), blackthorn (Prunus spinosa),
 spindle (Euonymus europaeus), broom (Cytisus scoparius) or gorse (Ulex
 europaeus). A more detailed description of hedgerow features in the study
 area, based on a sample of 40 hedgerows, is provided by Boinot and
 Alignier (2022). Observation sites (n = 37) are located along two
 independent landscape gradients (r = 0.15, p-value = 0.20): total length
 of hedgerows (ranging from 6.3km to 18km within a buffer radius of 1000m
 around focal fields) and organic farming cover (ranging from 0% to 37% of
 total area within a buffer radius of 1000m). Each site contains a pair of
 fields less than one kilometre apart and managed under conventional vs
 organic farming (CF vs OF). Among these fields, 62 were grown with winter
 cereals, whereas 12 OF fields were grown with winter cereal-legume
 mixtures (we resorted to mixtures when we could not find any pure crop in
 the area). Management practices and landscape gradients were similar
 between pure crops and mixtures under OF. For each
 field, weeds were sampled on ten 1m² located in the field core, at least
 20 m from the field margin. We sampled 20 pairs of fields in 2019 and 34
 pairs of fields in 2020, in June and July before crop harvest. Pairs of
 fields sampled in 2019 were different from those sampled in 2020, but
 evenly located along the same range of landscape gradients. We visually
 estimated the cover of each species found in quadrats, with an accuracy of
 ± 5%. Ten quadrats per field were necessary to obtain a fairly accurate
 estimate of weed diversity, as indicated by species accumulation curves.
 Thus, we calculated the mean cover (%) of each species over the 10
 quadrats for each field. Then, we measured total weed cover (i.e., the
 cumulative cover of all species), and species diversity (Hill-Shannon
 index) for each field. Hill-Shannon diversity considers both the number of
 species within a community and their relative cover to calculate the mean
 rarity of species within a community. There was no weed in two CF fields,
 which were therefore assigned the lowest possible diversity value (zero).
 We described the functional structure of weed communities using the
 community-weighted mean (CWM) and community-weighted variance (CWV) of the
 three traits included in the leaf-height-seed scheme: specific leaf area
 (SLA), plant height and seed mass. Mean trait values for each species were
 collected in Ecoflora (Fitter and Peat, 1994) and LEDA (Kleyer et al.,
 2008) databases. Species with known trait values always represented more
 than 90% of the total weed cover for each field, thereby providing
 sufficient representativeness. To estimate plant
 spillover, we used the phi coefficient of association, which distinguishes
 ‘transient’ species that rely on regular re‐colonisation from neighbouring
 non-crop habitats, from ‘resident’ species typical of arable habitats and
 buffered by persistent seedbanks (Metcalfe et al., 2019). To avoid
 circular analysis, we used data from vegetation surveys at national scale.
 Weed frequency in arable fields was obtained from the ‘Biovigilance Flore’
 network 2002–2012 (Fried et al., 2008), whereas plant frequency in
 uncropped field margins came from the ‘500 ENI’ network (Fried et al.,
 2018). For each field, we computed the CWM and CWV of fidelity to estimate
 the contribution of spillover to plant diversity in arable fields. An
 increase in CWM Fidelity indicates greater spillover of transient species
 from neighbouring non-crop habitats. Further, an increase or a decrease in
 CWV Fidelity indicates co-existence or selection of fidelity values,
 respectively. Species with known fidelity values always represented more
 than 90% of the total weed cover, except for one field that was therefore
 excluded from analyses. To estimate environmental
 heterogeneity within arable fields, we used CWM and CWV of Ellenberg
 indicator values for temperature (EIV-T), soil moisture (EIV-F), light
 (EIV-L), and nutrients (EIV-N). An increase in CWV values indicates higher
 environmental heterogeneity. Further, a shift in CWM values suggest a
 change in mean environmental conditions. Ellenberg values were collected
 in the Baseflor database (Julve, 1998). Species with known Ellenberg
 values always represented more than 90% of the total weed cover for each
 field. Kermap (https://kermap.com/en/) generated
 hedgerow mapping, using Computer Assisted Photo-interpretation based on
 the IGN orthophotograph of 2017. We rasterized vector maps with a
 resolution of one pixel for 5 m × 5 m to compute the ‘bocage’ metric,
 i.e., the density and complexity of hedgerow networks, within circular
 buffer radii of 250m, 500m, 750m and 1000m around each field, using Chloe
 software (Boussard and Baudry, 2017). We assessed the
 effects of local farming systems (CF vs OF). OF systems were characterized
 by the absence of herbicide treatment, lower N-fertilization but higher
 soil disturbances in our study site. We considered both organic and
 mineral fertilization, converting organic fertilization into nitrogen
 amounts per hectare. Maize - winter wheat is by far the most common crop
 rotation in CF systems, whereas OF systems generally have more complex
 crop rotations, including polycultures and temporary grasslands. On
 average, surveyed fields have been under OF for 23 ± 6 years. Finally, we
 computed the total cover of OF systems (%OF) within circular buffer radii
 of 250m, 500m, 750m, and 1000m around each field, based on geographical
 data of CF vs OF systems in the Department of Ille-et-Vilaine in 2019
 (provided by the CartoBio team of the French Agency for the Development
 and Promotion of Organic Agriculture).
description: Microsoft Excel and Microsoft Word (or Open Office)
description: \############# BOTA DATA SET This data set provides the abundance of
 species in each sampled field field = field ID species code are
 abbreviated using EPPO codes () this data set
 gives the mean cover (%) of each species over the 10 quadrats for each
 field \############ ENV DATA SET This data set provides the environmental
 variables measured for each sampled field field = field ID farming =
 conventional farming (CF) or organic farming (OF) crop = current crop (P =
 pure crop, M = mixture) previous_crop = crop year n-1 cover_crop =
 presence absence of cover crop before current crop (0/1) nb_tillage =
 number of tillage events between previous crop and current crop
 nb_ploughing = number of ploughing events between previous crop and
 current crop ploughing = presence absence of ploughing event between
 previous crop and current crop (0/1) herbicide = herbicide Treatment
 Frequency Index between previous crop and current crop fertilization =
 nitrogen amount per hectare (including both mineral and organic
 fertilization) OF_cover.250 to 1000 = cover of organic farming in the
 landscape (%) within buffer radii of 250m to 1000m hedge_length.250 to
 1000 = total hedgerow length in the landscape (m) within buffer radii of
 250m to 1000m bocage.250 to 1000 = bocage index in the landscape (%)
 within buffer radii of 250m to 1000m SNH_cover.250 to 1000 = cover of
 seminatural habitats in the landscape (%) within buffer radii of 250m to
 1000m grassland_cover.250 to 1000 = cover of permanent grasslands in the
 landscape (%) within buffer radii of 250m to 1000m crop_div.250 to 1000 =
 Shannon crop diversity in the landscape within buffer radii of 250m to
 1000m field_size = focal field size (ha) year = year of sampling
 \############## TRAITS DATA SET This data set provides the functional
 traits, Ellenberg values and fidelity score for each species code =
 species EPPO codes () species = latin name of
 species family = family of species SLA = specific leaf area (mm/mg) plant
 height (cm) seed mass (mg) EIV_N = Ellenberg indicator value for nutrients
 EIV_L = Ellenberg indicator value for light EIV_F = Ellenberg indicator
 value for soil moisture EIV_T = Ellenberg indicator value for temperature
 phi = fidelity score to non-crop habitats \############## COORD DATA SET
 This data set provides the spatial coordinates of each sampled field field
 = field ID x = longitude (RGF93) y = latitude (RGF93)