Field Guide to Northern Tree-related Microhabitats: Descriptions and size limits for their inventory in boreal and hemiboreal forests of Europe and North America
A tree-related microhabitat (abbreviated as TreM) is a morphological feature present on a tree, which is used by sometimes highly specialised species during at least one part of their life cycle. These features may serve as shelters, breeding spots, or crucial hibernation or feeding places for thousands of species. Trees bearing at least one TreM are called habitat trees (Fig. 1). Various biotic and abiotic events can create TreMs: for example, a falling tree can injure the tree bark, snow can break off a tree top, fire can create fire scars, and a woodpecker can excavate a breeding cavity in the trunk. For some TreMs, such as vertebrate nests and witches’ brooms, the tree is merely a physical support. Only morphological features that are known to have a direct link with one or more associated species are classified as TreMs (Larrieu et al. 2018). Each TreM provides very specific conditions to the inhabiting species, depending on its characteristics, such as size, shape, position in the tree, degree of decomposition of the surrounding wood, condition of the bearing tree (living or dead), exposure to sunlight, microclimate, and moisture content. The diversity of TreMs in a forest stand directly influences the diversity of species because different TreMs provide optimal conditions for various species to thrive. However, TreMs are ephemeral, and when one deteriorates or ceases to exist, the species that are reliant on it must locate and colonise a new TreM. Thus, the more often a specific TreM occurs in a stand, the easier it becomes for the associated species to establish in a new TreM when the previous one is no longer viable. To reinforce biodiversity in a stand and thus improve its resilience, we need to know which TreMs are present, and to preserve and favour them through adapted management practices.
