Seed and (micro)site limitation in ditch banks: Germination, establishment and survival under different management regimes

Abstract

Summary Although maintenance of species-rich plant communities in ditch banks seems possible with agri-environment schemes, in general these schemes have not delivered an increase in species richness in these habitats. This could be related to an emphasis in the existing management guidelines on preventing extinction of desirable plant species as opposed to encouraging their colonisation. We investigated germination, establishment, survival and reproduction of nine target species on ditch banks and determined how differences in vegetation productivity (biomass) and management regime affect the outcome of these processes. Our results suggest that ditch bank target species are more limited by colonisation (seed and microsite limitation) than by extinction (site limitation). Since target species are often lacking from the soil seed bank, restoration of populations of these species in ditch banks should focus on improving colonisation opportunities and promoting dispersal from nearby sources. Introduction by the sowing of seeds could be considered, especially in instances where natural colonisation appears unlikely. Although microsite limitation is always a limiting factor, the relative importance of microsite limitation compared to seed and site limitations greater in high-productivity ditch banks. This provides support for the ‘shifting limitation hypothesis’ postulating that the major factor regulating species richness shifts from regional to local processes under increasing productivity. For effective ditch bank restoration, it is essential to improve the conditions for germination and establishment, particularly at high-productivity ditch banks. We conclude that extensive management strategies advocating late first cuts appear suboptimal, regardless of the productivity of ditch bank systems. Current management recommendations in agri-environment schemes need to be revised and defined separately for low- and high-productivity situations in order to achieve largest conservation gain. Continuous monitoring and adaptation of the implemented management to fit individual circumstances will be required.