There are worlds within worlds, but here is one not many people consider, even though they repeatedly walk by one example after the next. Everyone reading this enjoys a few trees nearby, whether in a village yard or an entire forest. Whatever the tree, evergreen or deciduous, it is vulnerable and can be wounded. If wind tears off a branch, or woodpeckers and insects work at the outer layers of the trunk, living wood is destroyed and dead wood is excavated. 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. 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. As tree holes grow older, they enlarge until a "good" one is big enough to put your entire fist in. I've investigated those into which I have thrust almost my entire arm. But as long as there is even the least depression, a tree hole is a technically miniature pond, complete with a detritus-filled bottom and often simple green plants (algae and mosses) growing at the water's surface around the perimeter—the "shoreline." 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.
How does one learn about the hidden life of a tree hole? 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. A magnifying glass, especially a powerful one, reveals a miniature zoo of small creatures moving through the decaying wood fibers and accumulated sediment. The most obvious creatures may be mosquito larvae, called wrigglers, but not the common sort from real ponds or water-filled depressions lying out in the open. This species of mosquito breeds exclusively in tree holes and is known, logically if not imaginatively, as the tree hole mosquito.
As one might expect, tree hole communities differ according to the part of the world where they are found. In tropical Panama, there is a brilliantly colored tree frog that suspends its eggs from the sides of a hole in a tree trunk to make sure emerging tadpoles drop into the water. There too, is a species of poison arrow frog whose tadpoles are transported to a water-filled tree hole by the male, while the female is off doing other things.
Much, much more exists in a protected little tree-hole-lake. Each one contains an enormous community of small creatures adapted to life in the dark water, busily feeding upon decaying wood, reproducing, capturing and eating one another. It is a mystery how some of these specialized inhabitants arrive in an isolated tree hole, with it so separated from others. Some crawl over the bark seeking just such a haven. Others fly on exploratory missions, finally ending up in a place just right for their needs. Still others hitchhike on some other creature, a nuthatch, perhaps, as it probes for food in the hole. In fact, birds of many kinds are conveyors of life the world over, and this in part is how isolated islands and recent lava flows are colonized. No matter how small, a tree hole attracts certain birds, and they invariably will have tiny eggs, spores, seeds, and active animals riding on their feet and feathers, ready to be left off (or to jump off) upon arrival at a favorable spot.
As the wooden wall of a tree hole softens with decay, it is penetrated increasingly beneath the water's surface by roundworms, rotifers, and protozoans, and by mites, beetles, and ants that prefer the wet pulpy mass above the water level. A tree hole is an active incubator, a culture vessel in which a wide variety of life flourishes, a whole little world with its own complex micro-food web of interactions between organisms. And like a pond or any other ecosystem, large or small, it changes with time, its populations going through biological succession, or one stage following another, with some species adapted to a new young hole, others to a mature hole, still others to one that is either drying out or merging with the nearby soil. A tree hole is simply a reflection of the larger world beyond.
If you really want to be an explorer of tree holes and their inhabitants, and samples examined with a powerful magnifier aren't seen clearly enough, use a microscope if one is available—but be prepared for an astonishing sight of creatures crawling or zipping through the field of view. The smaller the inhabitants, the more diverse they are, the more of them, and the greater the activity. We hear a lot about research biologists in the tropics examining exotic ecosystems in trees and investigating debris on the floor of rain forests, but right here in northern latitudes are miniature examples of precisely the same kinds of hidden habitats; only the players on stage are different. And tree holes are among the best. To be sure, many of a cavity's inhabitants also live under leaf litter on the ground and inside rotting logs lying flat in a forest, but there they live in somewhat different proportions with respect to one another. With its wetness and thick accumulating humus, a tree hole is essentially a discontinuous extension of the forest floor with its organic remains of decaying leaves and fallen trees.
Take a look at whatever tree holes you are able to find nearby. Poke a finger in one, feel its warmth and richness, and try looking for a few of its inhabitants. This protected and hidden world flourishes year after year, shielded from the sun's heat in the summer, unlikely to freeze solid in the depth of winter. It's a very good place to be when conditions outside are tough. Little wonder it is crowded with small lives seeking a degree of security in a very unsure world.
© 2000 William H. Amos
Comments to the author Comments to the author sent via our contacts page quoting page url plus : ('wamos','')">Bill Amos welcomed.
Bill Amos, a retired biologist and frequent contributor to Micscape, is an active microscopist and author. He lives in northern Vermont's forested hill country colloquially known as the Northeast Kingdom, and takes delight in studying the several ponds on his land.
Editor's note: Other articles by Bill Amos are in the Micscape library (link below). Use the Library search button with the author's surname as keyword to locate them.
Tree images were taken by Ian Walker in north of England deciduous woodlands using a Nikon Coolpix 700.
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