Forest and cone structure influence where crossbills forage in a managed Scots pine forest

Abstract

The structure of forests is a fundamental attribute that impacts biodiversity and ecosystem services and in turn is influenced by natural disturbances and management practices. Here, we examine spatial variation in the interaction between a specialized predispersal seed predator, common crossbills (Loxia curvirostra), and Scots pine (Pinus sylvestris) in relation to forest structure and tree features in a managed Mediterranean forest. Crossbills preferred to forage in pine stands that were more open, which correspond to older stands or regeneration stands with scattered old seed trees. More open older stands with larger trees tend to produce more cones that are attractive to crossbills, although their use was also consistent with crossbills reducing their risk of predation. Within these patches, crossbills selectively foraged on trees having many small, thin-scaled cones, which require less effort and time to extract the seeds. Crossbills were apparently limited to only a very small subset of the closed Scots pine cones because they have relatively small and ineffective bills for foraging on Scots pine. Given that crossbills foraged on only a small subset of the cones within more open stands, a management strategy that would aid common crossbills is to thin middle-aged stands. This would accelerate the development of older stand structure, which would benefit crossbills by increasing the occurrence of suitably open woodlands, benefit other species that rely on older more open stand structure, and potentially decrease the vulnerability of the forest to fire and drought.