Species richness and weed abundance in the vegetation of arable field boundaries

In the modem arable landscape, the vegetation of perennial field boundaries have important ecological functions such as providing a habitat for farmland wildlife, providing overwintering sites for predatory insects, providing movement corridors, reducing soil erosion and acting as an agrochemical buffer. In recent decades, plant diversity in these linear landscape structures has declined severely. The present study aims at identifying the most important factors that control botanical species richness in herbaceous arable field boundaries. The field boundary vegetation is usually managed by farmers who consider the boundary to be a source of weeds. Therefore, research concentrated on factors that simultaneously increase species richness and reduce weed abundance in the boundary vegetation.Species richness was primarily affected by an accumulation of nutrients in the field boundary. Nutrients may reach the boundary in two ways: (i) by misplacement of fertilizer and (ii) by capture of arable nutrient resources (through root growth into the field) by plants in field boundaries. The high nutrient levels in the boundary resulted in an increased productivity of the vegetation and a dominance of tall, competitive species. Subsequently, low statured species disappeared from the habitat and species richness declined. The most common type of boundary management practiced by farmers did not include removal of the cut material after mowing which strengthens the eutrophication of arable field boundaries. Herbicide drift had adverse effects on species richness but the effects were less severe and consistent compared to the effects of nutrients.The main factor promoting weed growth (in the study area primarily the clonal weeds Cirsium arvense and Elymus repens) was the presence of bare soil in the boundary. Bare soil may be created by cultivation activities of the farmer or by the smothering effects of cut material left lying in the boundary after mowing. Bare soil generally promotes the establishment of annual weed species. Furthermore, the perennial weed Elymus repens was found to be able to concentrate its biomass selectiveley in bare patches within the perennial vegetation. Additionally, bare soil was found to favour the establishment of tall competitive (early successional) species compared to later successional species which are indicative of more species rich plant communities.The results of this study suggest that boundaries that are not regularly disturbed and low to moderately productive combine species richness with low weed abundance. This may be achieved by a regular and consistent mowing regime of the boundary vegetation that includes removal of the cuttings. A boundary management approach is suggested which may be combined easily with other farming activities.