Ecology of large cavities in trees: a review

Abstract Large tree-cavities (LTC) are conspicuous structures with specific ecological functions in large trees. We summarize the distribution, dynamics, ecological roles and conservation issues of LTC based on nearly 370 sources where these could be distinguished (based on entrances ca. 9 cm wide or larger). Large live broadleaved trees provide most LTC; live conifers and snags have importance in certain ecosystems. The resulting landscape distributions differ from those of smaller cavities by (i) an order of magnitude lower densities in forests (> 10 LTC ha –1 qualifies as a conservation hotspot) and higher sensitivity to management; (ii) relatively frequent occurrence in semi-open natural, rural or urban landscapes, if certain trees are retained until senescence. The slow wood decay processes, crucial for LTC development, introduce mechanical challenges that are best met when tree remains alive. Fast-growing broadleaved trees can form LTC at an earlier age, but slower-growing species produce persistent cavities. Relatively few vertebrate species are obligate LTC users, but they are more often threatened and often have symbolic meaning for people. Especially, those species less capable of defending cavities against predators tend to use (and compete for) only a subset of available cavities. For invertebrates, LTC form micro-ecosystems that host specific assemblages. Maintaining LTC as functional parts of human-dominated ecosystems requires unprecedented planning horizons and distinct approaches to tree retention and recruitment depending on the landscape type and species. In terms of landscape planning, both ecological and socio-economic pre-conditions of LTC development and their ecosystem functioning beyond habitat provision are insufficiently studied.