Close to the edge: spatial variation in plant diversity, biomass and floral resources in conventional and agri-environment cereal fields.
Abstract
Non-crop (segetal) plants in arable systems are commonly perceived simply as 'weeds', that is, harmful at worst and undesirable at best. The increase in management intensity in European arable systems has vastly reduced the populations of all but the most disturbance-tolerant plant species, negatively impacting the whole agricultural food web. In recent years, efforts have been made to promote agricultural biodiversity through measures such as flower strips and unsprayed field margins. However, studies of their impacts on the arable flora have rarely considered their spatial variation within the crop field. We investigated the spatial distribution of vascular plant species richness and their contribution to the food web via biomass and flower units in conventional and agri-environment cereal fields in six regions of Germany. We studied two types of in-crop measures (extensive cereals without pesticides or fertiliser, and with or without intercropping with flowering species) and one adjacent measure (neighbouring flower strip), recording at 1-m intervals from the field edge to the interior. These results were then extrapolated to illustrate the effects of these measures on resource provision at the field scale. Species richness and plant biomass dropped off sharply after the first metre in the conventional treatments, regardless of the adjacent habitat. The 'extensive' treatments maintained a much higher level of diversity and resource provision in the field interior. At the field level, this can mean more than a 60-fold difference in the provision of flowering resources between conventional management (1900 flower units/ha) and agri-environment measures (AEMs) (127,000 units/ha for extensive cereals). Synthesis and applications. The strong edge effects we found in conventional cultivation support the premise that reducing field sizes could play a role in promoting in-crop biodiversity. However, incorporating extensive field margins as an AEM would be more efficient at maximising the diversity of generalists whilst maintaining high yields.