Inspect a few trees and likely as not, you will discover an open tree hole that descends down a few inches inside the tree. Water collecting there creates a fine place for bacteria and fungi to continue their decomposing work, and over the years a tree hole becomes so enlarged its spreading rot may eventually kill the tree after which a whole new habitat is created with a dead log lying on the ground, but that is another story. 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.