Time matters:effect of time since transition to organic farming on hedgerow ground vegetation

Summary

Agri-environment schemes such as organic farming has been implemented to counteract declines in farmland biodiversity but comparisons to conventional farming have produced variable outcomes depending on both taxa and landscape factors. However, changes in land-use may not always result in rapid changes in species distributions and diversity. The importance of such a time-lag between transition to organic farming and species responses have received little attention.
We investigated the effect of time since transition from conventional to organic practice on the biodiversity of hedgerow ground vegetation in 30 hedgerows within two landscapes of different heterogeneity and land-use intensity.
We found that time since transition of the neighbouring fields (3 to 30 years) significantly affected the species richness of hedgerow ground vegetation. Transition time up to thirty years did not result in saturation of the species curve and short term organic management resulted in relatively small positive effects on plant species richness. Species richness was higher although not significantly in hedgerows in the heterogeneous landscape.
Mainly herbs showed increased species richness in organic hedgerows whereas the number of grass species was unaffected. Generally, the flora was richer in hedgerows on loamy soils compared to hedgerows on sandy soils. Despite management practice and soil type a few tall-growing and highly competitive species such as Elytrigia repens, Dactylis glomerata, Urtica dioca, Anthriscus sylvestris and Cirsium arvense dominated the hedgerow ground vegetation and made up 70-100% of the plant cover.
The results have implications for the way effective incentives to support biodiversity can be formulated in the form of subsidy schemes to organic farmers, and the on-going revisions of agro-environmental policies, such as the planned greening of the Common European Agricultural Policy (2014-2020). Fluctuating market prices and short-term economic gains have become increasingly important as incentive for conversion from conventional to organic management and transition forth and back between practices has become more frequent. For biodiversity protection, financial support would be spent much better on farmers who keep their fields organic for many years, instead of to those who convert to organic practice and maybe convert back to conventional practice after only a few years of organic practice.