Tree cavities in forests - The broad distribution pattern of a keystone structure for biodiversity

We explored the main factors affecting the global distribution of tree cavities - a habitat component of mostly biotic origin that is crucial for many animal species. We considered the influence of eight environmental variables (ranging from the single-tree to the biogeographic-region scale) on cavity density in a meta-analysis of 103 published studies. The global median density of cavities was 16 ha(-1), with densities highest in Australasia and lowest in the Palaearctic region. Two major factors influencing density were identified: cavity density was positively related to the amount of precipitation, and was higher in natural than in managed forests. These effects suggest that the distribution of tree cavities largely reflects the incidence of fungal heart-rot in trees, and that forest management, by affecting wood decay processes, can have a broad-scale impact on tree microhabitat availability. Although air temperature, forest composition and wood hardness had suggestive univariate effects, neither these variables nor biogeographic region explained any additional variation in multifactor models. In regions where woodpeckers are present there was an upper limit to the density of woodpecker-excavated cavities (approximately 10-20 cavities ha(-1)) that was considerably lower than the highest total cavity densities encountered (up to 140 ha(-1)). This indicates that primary cavity-nesters are particularly important keystone species in cavity-poor forests where wood decay processes are suppressed either climatically or by forest management. (C) 2011 Elsevier B.V. All rights reserved.