Large solitary oaks as keystone structures for saproxylic beetles in European agricultural landscapes

Abstract

Abstract Trees outside forests (TOFs) are declining in agricultural landscapes worldwide. Although they provide valuable habitats for saproxylic species, their relative contribution to saproxylic biodiversity with respect to forests is poorly known. Our aim was to investigate the conservation value of oak TOFs for saproxylic beetles in European agricultural landscapes. In our design, plots pairing forest trees and TOFs were sampled in sites nested in two French regions. We studied local habitat conditions (deadwood, tree‐related microhabitats, openness) and saproxylic beetle assemblages. We measured significant environmental contrasts between forest trees and TOFs, and found that the latter hosted higher microhabitat density and diversity. Species assemblages were strongly dissimilar between TOFs and forest trees. Even though α‐diversity and abundance were not significantly different in the two contexts, the between‐tree β‐diversity and the γ‐diversity of both all and rare species were higher outside than inside forests. Furthermore, the potential key drivers of saproxylic beetle diversity were not consistent between forest trees and TOFs. Tree cavity was the only elementary microhabitat whose density significantly affected biodiversity metrics both inside and outside forests. Many more effects on beetle assemblages originating from surrounding conditions were observed in TOFs than in forest trees. We also confirmed that saproxylic beetle diversity is significantly associated to canopy openness. Consistent with our results, environmentally friendly forestry practices should include the retention of sun‐exposed microhabitat‐bearing trees at final harvesting. In addition, environmental policies should favour TOFs as well as oak savanna‐like woodlands which are seriously threatened worldwide.