Ten year re-assessment after introduction of forb communities into species poor temperate grassland

Floristic diversification of grasslands through introduction of forb communities is a potential method for supporting biodiversity. We re-assessed community development within a 10 year field trial in Denmark in which forb communities had been transplanted into bare subsoil on previously-farmed land in open gaps in 2010. The randomized block experiment included un-planted vegetation free control plots on subsoil. Experimental forbs are common to the northwest European flora. In 2020 the community development was re-evaluated. All plots had developed into forb dominated, dense communities in which it was clear that species richness had declined primarily due to loss of annual and biennial species including increased density of the vegetation. On average, the colonization of graminoids was higher in the control plots compared to the treatment plots. Across the experiment, patches of grass e.g. Festuca rubra had colonized transplanted plots and defeated some forbs. The transplanted forbs Achillea millefolium, Agrimonia eupatoria, and Galium mollugo were widespread in all plots. Control plots had been colonized by species from the surroundings including the treatment plots. Our long-term results show that forb introduced communities were highly resistant to grass colonization for many years. Exposed subsoil can develop into forb dominated spots provided that a propagule source is close by. Top soil stripping combined with introduction of forb community hotspots is an effective tool for limiting germination and establishment of aggressive competitor species from soil seed bank and surroundings. Winter-cutting is a potential, low-input management technique for diversification, however further studies are needed to investigate whether the technique in itself is sufficient to diversify species-poor temperate grasslands.