Development rates and persistence of the microhabitats initiated by disease and injuries in live trees: A review

Many tree-dwelling species inhabit microhabitats that develop slowly in diseased or injured trees. Conservation planning would benefit from explicit knowledge of these processes, notably for resolving conflicts with timber production, arboricultural or public safety goals. We reviewed published rates of development of five selected common tree microhabitats and the factors affecting those rates in living trees. Most of the 71 relevant studies found globally focused on tree-cavities and exposed wood, while the development of witches' broom, bark pocket and dendrotelm microhabitats remains poorly described. Three time parameters emerge as central: minimum tree age; rate of formation; and persistence time of the microhabitat. Among the studied microhabitats, large side cavities are formed in the oldest trees, seldom hosting large vertebrates in trees <100 years old. In contrast, the development of exposed wood, bark pockets and dendrotelms can be compatible with some approaches of even-aged production forestry. The evidence indicates that tree growth rate is a key effect modifier of microhabitat development, which supports short-term wound occlusion but can also expose the tree to chronic disease and decay. For example, side cavities usually develop more rapidly in fast-growing trees, but persist longer in slow-growing trees. Environmental exposure regulates microhabitat development by affecting these growth and degradation responses and pathogen performance, but typical ranges of natural variation in microhabitat development rates can also be distinguished. To mitigate conservation conflicts with production forestry and the management of human environments, microhabitat prone trees should be considered across multiple management cycles, notably to sustain large side-cavities and witches' brooms. We outline retention forestry approaches, reduction of salvage logging, and veteran tree management in the countryside and urban environments as the key arenas of action. Given the high variation in tree microhabitat development, a successful strategy would combine fine-scale retention of existing microhabitats and landscape-scale prediction and planning for future microhabitat formation.

Automated summary

The development of tree microhabitats is closely related to tree growth rate and age, and has significant impacts on conservation, forestry management, and public safety. Different types of microhabitats require unique management approaches to retain the natural environment.