Holey Waters

Natural History, Sept, 2000 by Peter J. Marchand

On a dry hillside, a rain-filled tree hole may be the only available nursery for gnats, midges, punkies, mosquitoes, marsh beetles, and a host of other small organisms.

The man looked skeptical as I explained how I had wandered onto his property in the woods of western Virginia--how I had been searching for tree holes that captured and retained rainwater, and that my interest lay in the aquatic organisms inhabiting these pools in places where there was no other water around. Expecting to be invited off the premises, I was not prepared for his response. "I heard one of my dogs lapping water out of a tree when we was up back of my cabin last week," he drawled. "If you want, I'll take you to it." Half an hour later, I was staring into a pool of tea-colored water caught between the twin trunks of a huge white oak. Mosquito larvae danced just below the surface, and as I probed the excelsior-like mesh of recalcitrant leaf veins and decomposing organic material at the bottom, I quickly turned up other organisms. It was perfect! Though I would find many similar water holes up and down the Appalachian Mountains during the next two weeks, none were more exemplary than what this woodsman had led me to.

In a landscape devoid of ponds, water-filled tree holes are sometimes hidden reservoirs of biodiversity, providing a habitat for upwards of 140 species, including protozoa, flagellate algae, swarms of bacteria, and numerous invertebrates whose larval stages are aquatic, as well as occasional mosses and vascular plants. Included among the invertebrates are moth flies, wood gnats, midges, punkies, mosquitoes, marsh beetles, and beelike or wasplike syrphid flies. A dozen or so insects in these families are seldom, if ever, found elsewhere.

While not uncommon in the deciduous forests of the eastern United States and western Europe, tree holes capable of supporting simple aquatic communities are found only under rather specific circumstances. Most occur where closely grouped trees grow together at their bases, eventually grafting and forming at their junctures a catchment basin (known as a pan) lined with seamless, waterproof bark tissue. In parts of the eastern United States, numerous hardwood stumps remained after the land was cleared during the nineteenth century; when the stumps resprouted, the clumping of new trees led to the formation of many of today's pans. Some large species--beech, for example--may also form pans in their buttressed roots and occasionally in the crotches of their branches. (These trees are an important source of water holes in Europe.) Tree holes can develop when bark is injured, allowing fungi and bacteria to enter the tree and create a cavity through decay. In this case, the tree often responds by producing callus tissue, which walls off the cavity and eventually enables it to hold water. Breakage caused by wind or ice storms often leaves branch stubs vulnerable to the formation of such decay cavities. Other possibilities exist as well; tree holes have been found in almost all broad-leaved genera and even in a few spruces and firs.

Not all tree holes are created equal, however. I have investigated many seemingly ideal cavities that failed to provide suitable habitat for aquatic insects for lack of two essential conditions. The first is an adequate supply of rainwater that is channeled down the tree trunk into the cavity or pan. During a rain, this so-called stem flow not only provides the water necessary to maintain the aquatic community (direct interception of rainfall or canopy drip is less likely to result in standing water in a tree hole) but also delivers nutrients washed off leaves and bark. And during storms, the stem flow may flush potentially toxic accumulations of metabolites, such as ammonium and hydrogen sulfide, from the pool. The second requirement is a supply of organic matter sufficient to provide an energy base for the community. Food chains in this habitat generally involve the serial processing--in hand-me-down fashion--of successively smaller organic particles by a number of organisms that feed on dead plant matter, from leaf-shredding marsh beetles all the way down to protozoans. (Many browsers, such as mosquito larvae, also act as predators of protozoa and bacteria.) Without a source of material from the outside, the marsh beetles--keystone species that regulate food supply to lower-level consumers--have little to work with.

In deciduous forests, autumn leaf fall provides almost all the energy necessary to sustain the tree-hole community. The best tree holes, therefore, will be positioned so that they intercept both water and leaves. But the right combination of stem flow and leaf fall is critical. Too much organic matter often has a detrimental effect on the community, reducing both species diversity and population densities, probably because it reduces the amount of standing water.

Despite their sometimes tenuous nature, water-filled tree holes offer insects at least one major advantage over lakes and ponds: greatly reduced predation of their larvae. Once the insect leaves the tree hole as an adult, however, the score may be evened, for success isn't counted until the life cycle has been completed. Both insects and habitat are widely dispersed, so the greatest challenges for organisms that breed only in tree holes are to find mates and to locate new tree holes where they can deposit their eggs. The pans or cavities must have sufficient resources to carry another generation of larvae into adulthood. How the insects find the holes in the first place remains a mystery, but one important criterion in site selection is probably the amount of standing water they hold.