Hemiparasitic plants facilitate ecological restoration of encroached European grasslands

Species-rich grasslands of temperate Europe are threatened by the spread and increasing dominance of the rhizomatous grass Calamagrostis epigejos. Native hemiparasitic Rhinanthus species have been proposed as biocontrol to suppress C. epigejos, but experimental evidence is limited. We conducted a series of experiments at 21 grassland sites in Central Europe encroached by C. epigejos to test the effects of Rhinanthus on C. epigejos and on plant and arthropod communities and compare them to the effects of mowing applied once or twice a year. Rhinanthus suppressed Calamagrostis epigejos significantly more than mowing. Mowing frequency and Rhinanthus sowing had synergistic effects, leading to an average 75% reduction of C. epigejos biomass in Rhinanthus plots that were mown twice during the three-to four-year experimental period. The effect was more pronounced at sites in dry climates. Both Rhinanthus and mowing significantly increased plant diversity. Rhinanthus plots mown twice changed most rapidly towards the target grassland vegetation. The abundance and species richness of arachnids and Auchenorrhyncha were generally higher in abandoned plots than in plots with active management. Rhinanthus plots harboured fewer grass-feeding insects but more forb-associated herbivorous Auchenorrhyncha and Heteroptera species and several heliophilous spiders. Our experiment showed that Rhinanthus spp., in combination with mowing, is an efficient and widely applicable method to reverse the encroachment of C. epigejos into grasslands and restore the diversity of infested plant communities. Active management measures also trigger a species turnover of arthropod communities, reflecting changes in grassland vegetation and arthropod guild species pools.