High cover of forest increases the abundance of most grassland butterflies in boreal farmland MARJAANA TOIVONEN, 1,2 AINO PELTONEN, 3 IRINA HERZON, 1 JANNE HELI € OL € A, 2 NIKO LEIKOLA 2 and MIKKO KUUSSAARI 2 1 Department of Agricultural Sciences, University of Helsinki, Helsinki, Finland, 2 Finnish Environment Institute, Natural Environment Centre, Helsinki, Finland and 3 Lammi Biological Station, University of Helsinki, Lammi, Finland Abstract. 1. High cover of forest in the landscape matrix has been shown to weaken the negative effects of habitat fragmentation on grassland butterflies. No studies have however focused on examining species-specific responses of grassland butterflies to forest. 2. The data from 3 years of butterfly monitoring in Southern Finland were used to test whether the amount of forest cover in the surrounding landscape affected the abundance of grassland butterfly species in semi-natural grasslands, field margins, and forest edges. 3. More than half of the studied species benefitted from high cover of forest. Species with the strongest preference for forested landscapes were Lycaena vir- gaureae, Argynnis adippe, Argynnis aglaja, and Boloria selene, which probably find suitable resources in herbaceous habitats at forest edges and clearings. 4. Several small-sized species were positively affected by surrounding forest cover in field margins but not in the other habitat types. Although field margins are suboptimal habitats for grassland butterflies, they provide important corri- dors for dispersal. High cover of forest in the landscape matrix may enhance butterfly dispersal along field margins by reducing windiness, which is likely to be most important for small-sized species with poor dispersal capacity. 5. The most abundant grassland species showed little or no preference for forested landscapes. 6. Our results suggest that high cover of forest enhances the persistence of most grassland butterflies, including declining species, in boreal agricultural landscapes. The responses to forest are however strongly dependent on species- specific properties and habitat types. Key words. Agricultural landscape, biodiversity, butterfly, field margin, forest edge, habitat type, landscape composition, semi-natural grassland, species- specific effects, surrounding landscape matrix. Introduction The long-term decline of semi-natural grasslands due to changes in agricultural practices threatens grassland biota throughout Europe (Polus et al., 2007; Cousins et al., 2015). Large areas of semi-natural grasslands that were once traditionally managed with extensive grazing and hay making have become either intensively cultivated, reforested, or abandoned (Stoate et al., 2009; Cousins et al., 2015). Grassland butterflies are one of the species groups that have seriously suffered from the loss and frag- mentation of semi-natural grasslands (Wenzel et al., 2006; Kuussaari et al., 2007a; Br€ uckmann et al., 2010; € Ockinger et al., 2012a; Nilsson et al., 2013). Even during the last two decades only, grassland butterfly populations in Eur- ope have generally declined by almost 50% (van Swaay et al., 2015). Correspondence: Marjaana Toivonen, Finnish Environment Institute, Natural Environment Centre, P.O. Box 140, FI-00251 Helsinki, Finland. E-mail: marjaana.toivonen@ymparisto.fi Ó 2017 The Royal Entomological Society 1 Insect Conservation and Diversity (2017) doi: 10.1111/icad.12226