Pruning promotes the formation of an insufficient number of cavities for hollow-dependent birds in Iberian Holm-oak dehesas

Abstract

Holm oak Quercus ilex dehesas cover large areas in the Mediterranean region, and constitute a paradigm of well preserved low-intensity agro-ecosystem supporting a high number of bird species. The canopy of holm oak trees is periodically pruned, but the role of pruning in the process of tree-hollow formation remains unstudied. Here I investigate the occurrence of tree-hollows in relation to tree characteristics (vitality, trunk diameter and fork height), infection by drilling Coleoptera and pruning intensity, and assess likely origin, use and suitability of hollows for non-excavator birds in dehesas of Spain. 26.3% of all sampled oaks had at least one hollow, and the number of pruning cuts predicted the occurrence of hollows. 97.0% of hollows were situated on pruning cuts suggesting that pruning may have favored decay cavity formation in Iberian dehesas. 43.6% of hollows were estimated suitable to hold the nest of a small-sized cavity-nesting bird, but only 11.9% might have potentially held the nest of a medium-sized cavity-nesting bird, rendering estimated 3.9 and 1.1 potential cavities/ha, respectively. The low densities of suitable hollows would point toward a lack of nest-sites which might limit cavity-nesting birds in holm oak dehesas. Future conservation actions targeting in protection of old and dead holm oak trees instead of selective removal, together with land-scale measurements of enhancement of riverine forests stands may enrich and ensure the persistence of cavity-using species in Iberian dehesas.