The first opportunists to come along and invade this new habitat are fungi and bacteria and they quickly start digesting wood until the superficial wound grows larger and deeper. Inspect a few trees and likely as not, you will discover an open tree hole that descends down a few inches inside the tree. You may not appreciate having an unsightly hole in an ornamental or otherwise valued tree, but there are plenty of living creatures for which a tree hole is just the right place to live. Ecologists have found that tree hole communities have distinct populations made up of a unique assemblage of plants and animals, many of which are also found on the forest floor. The best tree holes from a naturalist's point of view are those resulting from a limb being torn away, because this often leaves a slight cavity in the trunk.
Rain water is trapped in the hole in summer, snow in winter.
Since evaporation is unlikely in such a protected, shaded cavity, its microclimate is perpetually moist, even soaked, and often maintains a constant temperature throughout a given season. I've investigated those into which I have thrust almost my entire arm. When I point out a northern tree hole, my companions usually show reluctance to probe the cavity with a finger, even though nothing dangerous lives inside. In the tropics, though, I always have second thoughts myself, for highly venomous centipedes and other creatures ready to bite or sting might easily be residents of the shadowy recess. If you take a small container and dip out some water and sediment, or suck out bottom material with a large plastic basting pipette, then place your collection in a shallow white dish, be prepared for a surprise.