Maintaining plant diversity in field margins

While prairie strips represent one example of how creating linkages through agricultural fields can benefit biodiversity, field margins  – the semi-natural habitat that border agricultural lands – represent another opportunity to connect populations and support biodiversity. Field margins can be comprised of anything from exposed ditches to tree-covered hedgerows, providing habitat for a variety of plant communities. However, most studies on field margins focus only on local scale diversity, without considering how diversity is shaped at the landscape scale as well.

A new study looks at plant diversity along agricultural field margins, focusing on drivers of diversity across multiple scales. Over 300 field margins were sampled in northwestern France to determine how field margin structure (such as tree cover or margin width) and management practices (such as herbicide use or mowing) over 20 years impact plant communities.

Most importantly, results showed that drivers of diversity are not the same for local versus landscape scale diversity. Tree cover was the major factor in local scale diversity: greater tree cover led to field margins with lower taxonomic and functional local diversity. Ditch depth was also an important driver: deeper ditches supported more diverse plant communities. In terms of management practices, grazing was the most important driver of local scale diversity – field margins with higher grazing intensity over time had more diverse plant communities (although too much grazing could be problematic).

However, when it comes to landscape scale diversity, these drivers shifted. Tree cover was also the most important driver of landscape scale diversity; however, field margins with lower local diversity contributed the most to landscape level diversity because they were very dissimilar from each other. In terms of management, herbicide use and chemical drifts from other fields had the biggest impact on landscape level diversity, even though they had no significant effect on local diversity.

The key take-away is that most drivers affecting local scale diversity did not affect landscape scale diversity, and vice versa. By looking only at alpha diversity, it would be easy to think that dense hedgerows are not as good for plant communities as herbaceous margins, and that herbicides and chemical drifts from other fields have little effect on plant diversity. However, by also looking at landscape scale diversity, it would become apparent that dense hedgerows are actually helpful for supporting plant diversity, and that herbicides and chemical drifts have a significant impact on plant communities over time.

Field margins are key to protecting biodiversity at multiple scales, and at maintaining connectivity between plant populations across an agricultural landscape. Understanding the complete picture of how they contribute to community structure and composition can lead to more effective conservation and restoration measures.

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