Decomposition of dead wood
Most of the insects found on old oaks live off the dead wood or off wood-living fungi. Dead wood exists in many different forms and can vary in size from massive trunks to fine branches. It can be high up in the crown, or down on the ground, in the sun or the shade, or in a dry or a damp location. It can be upright or horizontal, and in various stages of decomposition.
The woodliving beetles that belong to the first succession after a tree or part of a tree has died consume primarily the nutrient-rich tissues under the bark xylem, phloem and cambium. Examples of beetles in this group include several species of bark beetles Scolytinae, jewel beetles Buprestidae and longhorn beetles Cerambycidae. There are 25 species of longhorn beetles that live on oaks – most of them live under the bark on recently died branches and trunks. An exception is the great capricorn beetle Cerambyx cerdo, whose larvae live in living oaks where large tunnels are created in the wood. These tunnels make up important microhabitats for many other beetles. For some fungus species, the tunnels can also function as entrances to the wood in the trunk. Heartwood of oak is very hard, and for most wood-living insect species, fungi must first start to decompose the heartwood before the insects can consume it or gnaw their tunnels in it.
The fungi that first colonise the wood determine much of the subsequent composition of fungi and insects. Different species live on the fungi’s fruiting bodies, the mycelia and the wood that has been modified by fungus activity. The num- ber of species reliant on fungi and their mycelia has probably been underestimated, as very few species eat wood unaffected by fungi.
A large number of fungus species can live simultaneously in the same tree trunk. They can decompose the tree in parallel, or make use of the wood in different stages of decomposition. There are two main types of fungi that decom- pose wood. Some fungi that live in heartwood only break down cellulose and hemicellulose, leaving the lignin as a red-brown, brittle mate- rial called brown rot. Other fungi break down cellulose, hemicellulose and lignin, leaving a soft, whitish, flexible material called white rot.
It is primarily the lignin-rich brown rot which can, in some cases, form thick beds of fragmented wood in the bottom of hollow oaks. These two types of rot provide habitats for different communities of invertebrates.
Fungi, hollow trees and species dependent on them
More than 500 fungus species live together with oaks. Half of these are woodliving, and the other half interact with the roots of the oak via mycorrhi- za. Fungal brackets are fruiting bodies of fungi that decompose wood. Two important and characteris- tic fungal brackets found to cause brown rot on old oaks are beefsteak fungus Fistulina hepatica and chicken-of-the-woods Laetiporus sulphureus. A common fungus that causes white rot in oak wood is the robust bracket Fomitiporia robusta.
Fungi that cause heartrot in oaks colonise the inner part of non-living tissue. As a result, the trunk begins to be hollow after several decades. This process often leaves an intact, functional sapwood under the bark, and the tree can continue live and grow. Because these fungi are not usually deadly for the tree, birds like woodpeckers can excavate their homes in the softer wood and from this, lots of important homes are created for birds like wood pigeons, starlings, goldeneyes and tawny owls, but also for bats.
The decayed heartwood in intact trunks has a relatively stable temperature and moisture regime, which is crucial for many wood-living fungi and insect larvae. A long-lived tree like the oak can offer possibilities for some insect species populations to survive for a century or more. Large, living trees contain a considerable portion of the total amount of dead wood in an old woodland; in reality, dead wood remains longer in living trees than in branches and fallen trunks.
Studies in Sweden have shown that half of the oaks that have reached an age of 200-300 years have cavities in their trunks. A hollow trunk often contains multiple cavities with different types of decay and wood mould. The shape and size of the cavity and the entrances can determine the invertebrate animal that will inhabit it. The hermit beetle Osmoderma eremita and the marbled rose chafer Protaetia marmorata are two larger beetles that live in this habitat. Their larval activities both mix up the wood mould and enrich it with droppings; they also changed the structure, making it granular. The pseudoscorpion Anthrenochernes stellae lives in the same habitat.
When a trunk lies on the ground or when there are dead roots in the ground, beetles like the stag beetle Lucanus cervus, the tanner beetle Prionus coriarius and the rose chafer Cetonia aurata can find habitats because their larvae live off this type of dead wood.
A large portion of the biodiversity in the oak’s habitat is dependent on old trees and dead wood. To conserve biodiversity, we must protect not only the oaks, but also the large variation of habitats associated with them: glades and open swathes in the woodland, woodland edges with flowering bushes, hollow trees, standing and lying dead wood in all successional stages and in various sizes.