Litter and graminoid biomass accumulation suppresses weedy forbs in grassland restoration

Accumulated biomass of sown species and litter plays an important role in success of restoration projects. We studied the effects of litter and graminoid biomass on species richness and biomass of early colonising forbs in former alfalfa fields sown with seed mixtures containing seeds of native grass species (Festuca pseudovina, Festuca rupicola, Poa angustifolia, Bromus inermis, 2005). The amount of litter, forbs and graminoids was measured in the first 3 years after sowing (2006-2008). Ten aboveground biomass samples (20 cm x 20 cm) per field were collected in June every year. We found significantly lower forb biomass in the second and third year, than in the first year after sowing. Litter and biomass of graminoids increased significantly during the study, and correlated negatively with the biomass and species richness of forbs. Mean scores of litter and graminoid biomass were two to three times higher in sown fields than in native grasslands. Our results suggest that the accumulation of litter and graminoid biomass is beneficial in suppression of weedy forbs, but in the long run it might also hamper the immigration of target species.