03. Species and ecology of oak habitat

The oak is Sweden’s most common deciduous broadleaf tree, growing naturally as far north as the Dala River. Many species of large, old trees provide space for a range of micro-habitats, but the oak is the tree that provides habitat for the most species. On really old oaks, one can find beetles, moths and butterflies, lichens and fungi, and there are several species that are reliant on the dead wood. Text: Nicklas Jansson

New generations of oaks

Just like many of the species it hosts, the oak requires both light and warmth, as well as a relatively open location, in order to regenerate. Oak regeneration benefits from different types of disturbances such as flooding, fire and grazing. It may sound like a paradox to say that grazing helps oak regeneration, as the animals often eat oak seedlings, but in pastures, regeneration can succeed in larger groups of thorny bushes. Oaks often grow around wetlands and larger rivers that regularly flood. In these habitats, the oak, with its ability to survive flooding, has a competitive advantage over other trees. Oaks do not regenerate in woodlands with dense canopies.

Oaks and species diversity

The oak, which in Sweden actually consists of two species, the common oak Quercus robur and the sessile oak Quercus petraea, is the most common of the thirteen deciduous broadleaf trees found in Sweden. Despite being so common, oak makes up just one per cent of the country’s timber volume. Oaks grow naturally in mixed stands with other deciduous trees which, if they are old and hollow, can have a similar fauna. In Sweden, the oak provides habitats for more species than any other tree.

The oak is an important host for almost 900 species, and close to 900 other species can also use the oak as a habitat. Beetles, moths and butterflies, lichens and fungi are particularly well represented among the species that live in oaks. To be suitable for many of these species, the oak must have reached a relatively high age – often 200-300 years – and have developed special structures and habitats. These can include cavities, sap runs, coarse bark, dead parts of trunks and branches, fruiting bodies of various fungi, bird nests and bats, as well as well-developed mycorrhiza, a sort of “fungus root” where various fungus species supply the tree with water and minerals, and in return receive organic com- pounds from the plant. Also, there can be cavities with wood mould, a mixture of decaying organic material that accumulates in the bottom of the cavities. Wood mould can consist of wood fragments, fungi, remains of leaves and seeds, dead animals, faeces from birds and mammals, as well as droppings from insect larvae in various stages of decay and decomposition.

When it comes to saproxylic beetles, oaks have both the largest number of species, more than 500, and the highest proportion of monophages (29 per cent). “Monophagous beetles” means they only feed on one kind of dead wood and from one tree species and are therefore usually the most endangered.

For the species that do not live exclusively on oak, other important deciduous trees include alder, ash, beech and lime. The beech, Fagus sylvatica, is the tree that shares the most species with the oak.

There are several reasons for the high number of species that live in oaks. Most often cited is that the genus Quercus is old, rich in species, and spread across much of the world, in many different biogeographical settings. This has provided the opportunity for many different species to develop. Another reason is that oaks often grow large and old, making space for many different microhabitats. And for many species, it is beneficial to be able to live in the same place for a long time.

Lichens

Another species-rich group that does well on oaks is lichens. A large number of lichen species, of which many are red-listed, live mainly on the bark – but also on the dead wood. Generally, semi-open woodlands with a long, continuous supply of old trees and high humidity are rich in lichen species. Consequently, woodlands with plenty of old oaks close to wetlands and various watercourses are particularly rich in species. And on the Swedish west coast, there are species-rich oak habitat with so called oceanic lichens, that is, lichens that benefit from the proximity to the sea. Different lichens colonise oaks at different ages. Chaenotheca phaeocephala and Calicium adspersum colonise oaks that are slightly younger than 100 years, while Lecanographa amylacea and Schismatomma decolorans wait until the oaks are some 200 years old. The colonisation rate is low, and is highly influenced by the distance to other trees where the lichen grows, indicating that many lichen species have limited ability to disperse. When sites become overgrown, the colonisation speed of lichens is negatively affected. Once a lichen has colonised an oak, it remains there for many centuries. Because of the long lag between the reduction in the number of old trees and the disappearance of the species from the entire landscape, there is an extinction debt.

Lecanographa amylacea, or ”Gammelekslav”, grow on old oak trees with its main distribution in Östergötland, Kalmar and Blekinge counties. Photo by Jens Johannesson.

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.

Seeping wood mold with droppings from leather beetles and fragments of variable chafer. Photo by Annika Lydänge.

Noble chafer, Gnorimus nobilis. Photo by Jonas Hedin.

03. Species and ecology of oak habitat