The occurrence of cavities in fruit trees: effects of tree age and management on biodiversity in traditional European orchards

The formation of tree-cavities is an important ecological factor since many animals in woody habitats are cavity users. Recent research focuses on tree-cavity formation and the associated cavity networks in forest ecosystems. However, although traditional European orchards are important habitats for secondary cavity users, ecological research on the factors associated with the occurrence of cavities in fruit trees is widely missing. In particular, fruit tree pruning management may affect decay-cavity formation due to the pruning wounds allowing heart rot and decay to enter the tree. Here, we present a cross-sectional study investigating 608 fruit trees in 30 study plots of three European fruit-growing regions to identify factors associated with decay-cavity occurrence in fruit trees. Presence of decay-cavities was positively related to trunk diameter. Moreover, fruit trees of low vitality and with woodpecker-cavities featured more often decay-cavities than trees of high vitality or without woodpecker-cavities. Apple trees featured higher numbers of cavities at younger age than other fruit trees. Occurrence of decay-cavities was also related to the past removal of large main branches. Accordingly, traditional orchards are cavity-rich habitats if they comprise high proportion of old fruit trees, in particular apple trees, and if pruning management produces large pruning wounds. Thus, differential tree pruning and fruit-growing traditions across Europe result in different cavity densities in traditional orchards. Preservation of existing and potential cavity trees and selective removal of large branches from apple trees are recommended as conservation measures establishing high cavity densities and increasing the associated biodiversity in the agricultural landscape.