The Hidden Life in Tree Holes
Tree holes – formed by the action of fungi, lightning or the excavations of animals – make ideal, securely hidden dwelling places for a host of different animals.
A walk through woodlands anywhere in Britain will reveal trees disfigured by long gaping fissures, deep callus-ringed holes and shallow depressions – all of them capable of harbouring a highly specialised community of animals that varies according to whether the hole is predominantly wet or dry.
Rot holes and pans
When the bark of a tree is damaged, the underlying layer of sap wood, and eventually the inner heartwood, may be attacked by fungi. Various species of beetles can help to spread this fungi as they burrow into the timber. Between them the insects and the fungi cause the damaged area to enlarge and eventually a cavity is formed. This, called a rot hole, is one of the commonest types of tree hole.
The wound that begins the formation of a rot hole may be the result of natural damage by animals, high winds or lightning. It may also be caused by human activities: pollarding, coppicing and theof trees can initiate rotting, as can accidental damage. Some rot holes occur in sheltered positions in trees and remain dry even in heavy rain. Others however, are exposed and rain water fills them, either directly or by running into them down branches. The water may drain away through the rotting wood or it may accumulate, especially when the tree hole is lined with rotted leaves.
Often the openings of rot holes become restricted, even though the cavity is continually enlarging. This is due to the formation of a callus which, in some cases, actually closes the hole. The closure may have an effect on whether or not the hole becomes water-filled, for a restricted opening means that less water will enter – although this may be balanced by a lower rate of evaporation of the tree hole water.
A second variety of tree hole results from the growth pattern of certain trees. They are called pans and differ from rot holes in that they are shallow depressions, lined with unbroken bark, in the tree surface. They may occur when tree crowding leads to the distortion of trunks and branches and in the case of trees like beech, may be found among the buttress roots. In most cases, pans are shallow and. Although nearly always capable of holding water, may lose water rapidly by evaporation.
Both rot holes and pans are extremely common and can be found in a wide range of deciduous trees, including beech, oak, hornbeam and ash. They are less common in conifers but have been recorded in several species, including silver fir.
When tree holes become filled with water they form a specialised aquatic habitat which is colonized by a wide variety of animal life. Numerous protozoans, including the cosmopolitan ‘slipper animalcule’ Paramecium, small crustaceans like Cyclops, and rotifers and springtails are found in almost all accumulations of water, and those in tree holes are no exception. Occasionally larvae of the dronefly Eristalis tenax may be encountered; they are found in many stagnant waters breathing air through a telescopic tail which is held up to the surface. The rotting wood and leaves in these holes provide a rich source of food and can support large numbers of individuals.
Although water-filled tree holes cannot be classed as permanent habitats, they can persist for the entire life of a tree which, in the case of beech for example, may exceed one hundred years. Tree holes in the tropics dry out periodically and hence their fauna must either be capable of withstanding periods of dryness during their life-cycle, or else they must be capable of completing their life-cycles very quickly. In this country, however. Most tree holes do not dry out completely. Even in the summer (they are aided in some cases by water which rises within the tree itself). It has been found that nearly all the fauna can survive as long as the bottom layer of decaying material remains moist.
Wet hole inhabitants
Perhaps the most interesting and important group of animals are those specifically associated with the tree hole habitat in their immature stages and are seldom, if ever, found elsewhere. By becoming adapted for such a specialised habitat these animals have forsaken the variety of aquatic habitats elsewhere and therefore restricted their choice of breeding sites. But in return, they have escaped their natural enemies, for there are no predatory beetles and dragonfly larvae in tree holes. In Britain three of our 33 species of mosquito breed exclusively in tree holes. In some areas all three may be found inhabiting the same tree hole, but often one species pre-dominates. Little is known about the selection of individual holes by mosquitoes, but observations show that our rarest tree hole breeding species, Orthopodomyia pulchripalpis. Selects holes containing water rich in tannin and therefore dark in colour. The more common Aedes geniculates and Anopheles plumbeus are normally found in clearer water.
Female mosquitoes lay their eggs into the water of the tree holes in the summer months. The eggs may either hatch almost immediately or remain dormant until the following spring. The larvae feed on organic material suspended in the water and on bacteria and algae that they scrape from the walls of the cavity and from the surfaces of submerged leaves. The larvae are active swimmers and are often called ‘wrigglers’, which describes their way of moving. They are air breathers and so must spend time at the surface, taking in air through an opening at the end of their bodies. Mosquitoes pass through four larval stages and in spring or summer enter a pupal phase, which, unlike that of many insects, is active. Once again, their common name of ‘tumblers’ describes their movement. Like the larvae they rise to the surface to breathe. But they do not feed. In the summer the pupal skins split and the adult mosquitoes emerge to mate, feed and repeat the life-cycle.
Apart from mosquitoes, a number of fly larvae occur in tree hole water. Among the most numerous are the larvae of the biting midge Dasyhelea dufouri and those of the non-biting midge Metriocnemus martinii. Both pass through four larval stages, like mosquitoes, and feed on debris in the tree hole.
The immature stages of the hover-fly Myiatropa florea are common inhabitants of tree holes, although they are generally found in low numbers. Once again, there is only one generation each year. Unlike the previously mentioned insects, the mature larvae leave the tree hole to pupate in crevices in the bark. The only beetle to breed in tree holes is Prionocyphon serricornis. Whose larvae may live for up to two years in tree holes. In addition to the fully aquatic fauna of tree holes, there is also a group of animals which is attracted to moist situations. They include woodlice such as Oniscus, snails like Helix, and larvae of the cranefly Ctenophora and of the mothfly Pericoma.
Dry hole dwellers
Dry tree holes offer a valuable and important habitat for a range of terrestrial arthropods, as well as for birds and bats. Some arthropods, such as centipedes and spiders, are found in holes throughout the year, while others, including ladybirds, are present only in the winter, using the protected environment of the tree holes as overwintering sites.
The most obvious inhabitants of our dry tree holes are birds, some 20 species of which nest in tree hollows. Tawny owls, little owls and stockdoves are among those occupying the larger holes, while starlings, blue tits, great tits, nuthatches and pied flycatchers reside in the smaller holes. Often these birds use holes which have been made in a previous season by woodpeckers. When other birds take over woodpecker holes, they may line the floor with nesting material – oak leaves and grass in the case of the pied flycatcher and flakes of bark in the nuthatch – or the nest hole may be used totally unlined as in little owls and stockdoves. Nuthatches are noted for the way in which they reduce the size of the entrance of the nest hole with mud in order to keep out larger intruders.
Like all tree holes, the excavations made by woodpeckers may be enlarged by rotting fungi, become filled with rain and so be transformed into aquatic habitats.
Mammals, too, make their homes in dry tree holes. Squirrels nest in holes in the higher branches, and bats may roost there. Wood mice inhabit holes lower down, while stoats and wildcats make use of holes set low in the boles of trees. In fact, most small wild mammals will make use of dry holes at some time in their lives – as refuge from enemies and shelter from rain and snow. Such rodents as field-mice may well gnaw the insides of the holes to enlarge them or to shape them more to their own liking for nesting.