Two decades of change in a field margin vegetation metacommunity as a result of field margin structure and management practice changes

Field margins have considerable ecological significance in agricultural landscapes by providing habitat and resources for farmland biodiversity. Few attempts have been made to examine the long term dynamics of multiple vegetation communities in field margins, at the landscape scale. In addition, drivers and processes governing such long term vegetation dynamics are poorly investigated. The aim of the study was to assess whether the diversity and composition of a field margin metacommunity have changed over the last two decades as a result of changes in both structural condition and management of field margins. In total, 309 field margins in north-western France, first surveyed in 1994, were resurveyed in 2015 by using the same protocol. Total species richness (gamma diversity) was stable over time while mean species richness per field margin (alpha diversity) increased. No difference in species composition dissimilarity (beta diversity) was recorded indicating that no biotic homogenization had occurred between 1994 and 2015. Species composition has shifted over time, with a decline of hemicryptophytes and an increase of shade-tolerant species. No change in Ellenberg nitrogen value was observed. Analysis of drivers revealed a significant influence of changes in the structural condition of field margins, i.e. the development of the tree layer, on vegetation diversity. Dominant height of vegetation and tree cover were highest in 2015 compared with 1994. This has likely been driven by a decline in intensity of field margin management. Despite changes in alpha diversity, the field margin metacommunity had a stable Clementsian structure over time, suggesting grouped distribution of species along environmental gradients. This study provides a rare example of the long term vegetation change in field margins in relation with margin structure and management changes, using permanent plots and diachronic data. Results highlight the need to maintain a variety of differently structured and managed field margins to preserve high plant diversity. They underline the need to place conservation strategies in the context of regional-scale processes to ensure the long-term conservation of field margin metacommunities.