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

Abstract

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 determine the drivers of plant diversity in field margins for conservation and restoration purposes. To do so, we assessed the effects of field margin structure and long-term management 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, northwestern France. Results were very similar between taxonomic and functional diversity but revealed important discrepancies between the drivers of 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 strongly contributed to β-diversity by harbouring 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. All in all, preservation of dense hedgerows, abandonment of herbicide spraying, and protection against agrochemical drifts are key measures to prevent the establishment of common weeds and biotic homogenization of herbaceous plant communities in field margins. Above all, our study shows how important it is to go beyond α-diversity to make robust conservation and restoration decisions.

Keywords: agricultural intensification; biotic homogenization; functional diversity; hedgerow; long-term management; weed.

Figure 1.

Geographical position of the study area in the department of Ille-et-Vilaine, Brittany, France. Grey points indicate the location of sampled field margins (n = 302) in the three contrasting landscapes (a, b and c). Black lines in each landscape window represent the hedgerow network, based on the national reference layer of hedgerows in metropolitan France provided by the National Institute of Geographical and Forest Information.

Figure 2.

Field margin structure and long-term management both affected the functional α-diversity of herbaceous plant communities, based on the results of generalized additive models (GAMs). (a) Functional α-diversity decreased in an almost linear way with tree cover, whereas (b) ditch depth linearly increased functional α-diversity. Further, (c) long-term grazing and (d) mowing intensity both linearly increased functional α-diversity. Raw data are represented by the dots. Grey-shaded areas represent 95% confidence intervals.

Figure 3.

Effects of field margin structure and long-term management on functional β-diversity of plant communities in field margins, based on the results of generalized dissimilarity models (GDMs). Note that the y-axis does not represent β-diversity values, but the effect of each explanatory variable on β-diversity (while holding other covariates constant). Curve shapes describe the effect of each explanatory variable along its gradient (x-axis) on functional β-diversity. A steeper slope implies greater dissimilarity per unit change in the explanatory variable along the corresponding section of the gradient. Absolute curve height indicates the strength of each effect. (a) Functional β-diversity increased gradually with tree cover, especially above 25% tree cover, where the slope became steeper, indicating that dense hedgerows are not only highly dissimilar to herbaceous field margins, but also more dissimilar to each other. (b) Functional β-diversity was not affected by canopy height below 10 m but then increased very steeply with canopy height. (c) Field margins with very low values of herbicide spraying were highly dissimilar to each other, as indicated by the steep slope. Then, functional homogenization occurred with increasing herbicide spraying intensity, as indicated by the flattening of the curve around 2%, before β-diversity increased again for higher values of herbicide spraying intensity. (d) Cropping intensity had a weaker effect on functional β-diversity; field margins with intermediate values of cropping intensity are more dissimilar to each other than field margins at both ends of the gradient.

Figure 4.

RLQ analysis performed on herbaceous plant communities of field margins. (a) Scores of species, (b) coefficients of environmental variables, and (c) traits on the first two axes. Red labels correspond to common weed species, based on [46]. Projection of non-weedy species versus common weeds on RLQ axes can be seen in electronic supplementary material, figure S10. Basic statistics for traits and environmental variables are given in electronic supplementary material, table S3. Species names are abbreviated using EPPO codes—see electronic supplementary material table S4 or EPPO Global Database (https://gd.eppo.int).