Oak (Quercus spp.) is a relatively minor component of the Caledonian Forest, but is important in providing a habitat for many species of insects, lichen and birds.

Global distribution

Sessile or durmast oak and pedunculate or common oak have a similar distribution, occurring throughout most of Europe, from southern Scandinavia to the Mediterranean, and from Ireland and western Scotland to the Ural Mountains in Russia. The range of sessile oak does not extend quite as far east in Russia as that of pedunculate oak. About 450 species of oak have been described worldwide, of which 25 are native to Europe, but only sessile and pedunculate oak have such a broad distribution, and they are the sole species native to the north and west of the continent.

In the past, large areas of Europe were covered by temperate deciduous forests in which these two oaks predominated, but only a small proportion of those forests remain today, as the majority of the land has been converted to agriculture.

In the past, large areas of Europe were covered by temperate deciduous forests in which these two oaks predominated, but only a small proportion of those forests remain today, as the majority of the land has been converted to agriculture.

Distribution in Scotland

Both species of oak are distributed throughout much of Scotland at lower elevations, usually less than 300 metres, although pedunculate oak is more common in the south and east of the country and sessile oak predominates in the north and west. In some areas, such as the south side of the Moray Firth and in the Borders, both species are common, and the historical planting of oaks by people over the centuries has blurred the differences in their ranges.

On the west coast, the sessile oaks of the Atlantic oakwoods are particularly important, as they form bryophyte-rich temperate rainforests under the influence of the mild, wet oceanic climate. These woodlands are of international significance because of their unique assemblage of species, especially mosses, lichens and ferns, but are all greatly reduced in extent.

In the mountainous interior of the Highlands, where the native pinewoods of the Caledonian Forest predominate, oaks are less common, as the harsher climate and poorer soils limit their growth. However, in glens such as Moriston and Strathfarrar, areas of sessile oakwood do occur on the drier south-facing slopes at lower elevations. In Glen Affric a few sessile oaks can be found at the eastern end of the glen near Badger Falls. However, the presence of the flora associated with upland oakwoods suggests that, in the past, oak was more widespread on the south-facing slopes there.


> Content contributors

Oaks are members of the beech family, Fagaceae, and are long-lived trees which grow relatively slowly, at least in their initial years. Both pedunculate and sessile oak are large trees, reaching up to 40 metres in height. However, because of its more north-westerly distribution and ability to grow on higher ground, sessile oak is more usually only up to 30 metres tall in Scotland. Trees of both species regularly live to be 500 years old, and individuals of 1,000 years old are known, although in some cases that age has been achieved by coppicing them over long periods of time.

The bark of both species is grey and fissured, often covered with various lichens, and old trees can reach an exceptional girth – some individuals have a circumference of 12 metres. The large spreading branches produce substantial domed crowns, sometimes resulting in the trees being wider than they are tall. The branches of the sessile oak tend to be straighter than those of the pedunculate oak, which can become twisted or even gnarled with age.

Oaks are deciduous trees, and in winter their buds are distinctive by being clustered at the end of the twigs. The buds are composed of rusty-brown overlapping scales which protect the young undeveloped leaves inside from frost and cold. In spring the new leaves appear later than those of other trees such as birch, and in the Highlands it is late April or early May when the buds burst. The leaves are up to 12 cm. long, and are characteristically lobed, with pedunculate oak having three to five pairs of lobes on each side of its leaves, while those of sessile oak have five to eight lobes per side.

One of the features which differentiates the two species is the leaf stalk or petiole. On the pedunculate oak this is very short, typically less than 0.5 cm. long, whereas on the sessile oak the petiole is 1-2 cm. in length. Further distinguishing features between the two oaks include the presence of two small lobes or auricles at the base of the leaf on pedunculate oak, in contrast to the wedge-shaped base of a sessile oak leaf. The latter also has fine hairs on the underside of the leaf, especially along the midrib, but these are absent in pedunculate oak. Sessile and pedunculate oak hybridise naturally with each other, producing trees of intermediate characteristics, and this can make it difficult to accurately identify a tree to the species level.

Because of the large numbers of insects and other invertebrates which feed upon oaks, many of the leaves can be tattered and have numerous holes in them by late July. Oaks then produce a new flush of leaves, especially on young trees, and this phenomenon is called lammas growth, because it occurs around the time of Lammas, the Celtic festival of first fruits, on 1st August. In autumn, the leaves turn various shades of yellow and brown, as chlorophyll is withdrawn from them and carotenoid pigments become visible instead. In the Highlands, the leaves are shed at the end of October, but in milder areas a few leaves will stay on the trees until December.


> Content contributors

Oaks are monoecious, meaning that each individual tree has both male and female flowers on it. The flowers appear about 7-14 days after the leaves burst, and the male flowers consist of drooping, tassel-like catkins, up to 4 cm. long. The female flowers are tiny and occur at the leaf axils, where the leaf stalks join the twigs. Each catkin releases several million pollen grains which are distributed by the wind and, when pollination is successful, adhere to the receptive stigma on the female flowers. Fertilised flowers develop into acorns, which ripen in the autumn and are up to 3 cm. long. The acorns are seated in hard, warty cups called cupules, and in pedunculate oak these are borne on stalks called peduncles, which are up to 8 cm. long. By contrast, the acorns of durmast oak are either stalkless or have very short stalks less than 0.3 cm. long, and it is this sessile feature which gives this species its common name.

An oak will produce its first good seed crop when it is 40-50 years old, and acorn production varies, with trees sometimes producing very few in a given year. Large numbers of acorns are produced intermittently in what are known as mast years, and these occur every three to five years, when a mature tree can produce up to 50,000 acorns. The acorns are shed before the leaves fall, and they begin to germinate almost immediately. The leaf litter which accumulates on top of them provides protection from frost and hides them from seed-eating animals and birds, but very few of the acorns produced by a single oak are successful in growing on to become a mature tree.


> Content contributors

In the UK, oak provides a habitat for more organisms, and especially insects, than any other tree. Because of its large size and longevity, it plays a unique role in forest ecosystems and many species have adapted to live with it.

Underground, the roots of oak trees have mycorrhizal associations with various species of fungi. In these mutualistic or symbiotic relationships, both partners benefit from their interactions, with the fungi receiving sugars and carbohydrates which the tree photosynthesises using the sun’s energy, while the tree receives nutrients and minerals from the fungi, which it is unable to access directly in the soil itself. Examples of mycorrhizal fungi associated with oak include several species of milk cap, for example the yellowdrop milkcap (Lactarius chrysorrheus) and the oakbug milkcap (Lactarius quietus), as well as various species of Russula, such as the purple brittlegill (Russula atropurpurea). Over a dozen species of bolete form mycorrhizal associations with oak, including the orange oak bolete (Leccinum quercinum) and the very rare Strobilomyces floccopus. There are also a number of tooth fungi which occur with oak, including the velvet tooth fungus (Hydnellum spongiosipes) and the fused tooth fungus (Phellodon confluens).

Not all fungi are beneficial though and oak mildew (Microsphaera alphitoides) is a white powdery fungus that appears on oak leaves in late summer. Another fungus with the unusual common name of nut disco (Hymenoscyphus fructigenus) grows on acorns, especially in wet weather. A number of bracket fungi are associated with oak, including the oak mazegill (Daedalea quercina), which takes its name from the wavy pattern of elongated gills on its underside and grows on dead branches and stumps. This fungus is the only home for a small rove beetle (Gyrophaena strictula), a fact which illustrates the complexity and uniqueness of habitat that an oak tree provides.

A remarkable diversity of lichens, totalling over 300 species, have been recorded growing on oaks. These range from common species such as Hypogymnia physodes and oakmoss lichen (Evernia prunastri) to various ‘beard’ lichens in the genus Usnea, and tree lungwort (Lobaria pulmonaria). This latter species has disappeared from much of its former range, due to atmospheric pollution, but still occurs in more pristine areas such as Glen Affric.

Bryophytes are the group of non-flowering plants which include mosses and liverworts, and 65 species grow on the trunks and branches of oaks. They are particularly abundant in the temperate rainforest ecosystems of the Atlantic oakwoods, where the trees are festooned with mosses. Common species include broom fork-moss (Dicranum scoparium) and slender mouse-tail moss (Isothecium myosuroides), while rarer species from the Atlantic oakwoods include western featherwort (Plagiochila atlantica). Frequently to be seen growing amongst the moss on the branches of oaks is the common polypody fern (Polypodium vulgare).

In the Highlands, oak grows in association with specific groups of plants in a vegetation community called upland oakwoods. This typically includes birch trees, usually downy birch (Betula pubescens) but sometimes silver birch (Betula pendula), and common plants such as wood sorrel (Oxalis acetosella), dog violet (Viola riviniana), tormentil (Potentilla erecta) and bracken (Pteridium aquilinum). Other flowering plants typically occurring in these oakwoods include wood anemone (Anemone nemorosa) and bluebells (Hyacinthoides non-scripta).

Many organisms have evolved with oak to produce the abnormal growths that are known as galls. Over 40 species, including midges, mites and wasps, are responsible for stimulating the oak, by means which are not fully understood, to produce unusual growth forms on its leaves or twigs, within which the larva of the insect lives and feeds. Some galls are well-known and easily seen, such as the marble gall – a brown spherical ball on the twigs which is induced by the gall wasp (Andricus kollari). Some of the most beautiful galls are the spangle galls which are found on the underside of the oak’s leaves. Four different tiny wasps in the genus Neuroterus are the causative agents for the growth of these galls, up to a hundred of which can be found on a single leaf. The spangle galls are induced by the asexual generation of the wasps, while the subsequent sexual generation of the wasps cause different galls to form on the leaves and catkins. Some galls, such as the oak apple, which is caused by another wasp (Biorhiza pallida), support complex communities of insects that consist of parasitespredators and inquilines (insects which exhibit cuckoo-like behaviour).

Large numbers of moth larvae feed on oak leaves, and the caterpillars are usually well-camouflaged in shades of green, to reduce the risk of predation by birds. Common species include the oak hook-tip moth (Drepana binaria), the winter moth (Operophtera brunata) and micro-moths such as Phyllonorycter quercifoliella, whose larvae produce mines in the leaves. In other species, the adults are superbly camouflaged as well, including the evocatively-named merveille du jour (Dichona aprilina), which, with its green and black patterning, blends in perfectly on lichen-covered trunks. Many species of butterflies are found in oakwoods, and in Scotland these include the speckled wood (Pararge aegeria) and the pearl-bordered fritillary (Boloria euphrosyne). The purple hairstreak (Quercusia quercus) is the only butterfly whose larvae feed exclusively on oak, but it is scarce in Scotland.

Numerous beetles are associated with oak trees, including the oak bark beetle (Scolytus intricatus), whose larvae create a distinctive pattern of galleries in the tree’s wood. Several nut weevils, including Curculio glandium and Curculio venosus, lay their eggs in acorns during early summer, and the larvae feed inside the acorns, pupating after they have fallen to the ground. A rare carabid beetle (Calosoma inquisitor) which feeds on moth larvae on oaks, has recently been found for just the second time in Scotland.

Given the abundance of insects which live on oaks, it is unsurprising that spiders are common on the trees as well. Some species spin webs amongst the leaves, while others, such as the crab spiders, rely on their camouflage to ambush their prey. Common species on oaks in Scotland include the orb web spider (Metellina segmentata) and the garden spider (Araneus diadematus).

Insects are also the food source of birds, and many species take advantage of the large numbers of invertebrates on an oak tree. Common species in oakwoods in Scotland include the wood warbler (Phylloscopus sibilatrix), chaffinch (Fringilla coelebs) and tree creeper (Certhia familiaris). Other birds feed on acorns, especially the wood pigeon (Columba palumbus) and the jay (Garrulus glandarius). The latter is particularly important as a dispersal agent for acorns, as it will transport them up to a kilometre away from the tree and bury them in the ground. The jay doesn’t recover all the acorns it stores in this way, so some survive to germinate and grow into new trees.

Many of Britain’s common mammals are found in oakwoods, ranging from the wood mouse (Apodemus sylvaticus) and bank vole (Clethrionomys glareolus) to the badger (Meles meles) and pipistrelle bat (Pipistrellus pipistrelllus). The red squirrel (Sciurus vulgaris) feeds on acorns, and like the jay, aids the tree’s reproduction through its habit of caching nuts for the winter. However, it has been displaced from many oakwoods by the non-native grey squirrel (Sciurus carolinensis) which has been introduced from North America, and which is better able to feed on acorns. In the past, before they were extirpated from the UK, wild boar (Sus scrofa) would have relished the autumnal bounty of fallen acorns as well. Roe deer (Capreolus capreolus) and red deer (Cervus elaphus) both occur in oakwoods in Scotland, and their browsing of seedlings is the main factor preventing the regeneration of oak (and other trees) in ancient native woodlands such as those in Glen Affric.

Although it is not a major component of the Caledonian Forest, oak is nonetheless very important because of the habitat it provides for so many other species. It is likely to have been more abundant in the past, before it was removed by selective exploitation. Its regeneration now will add to the richness and diversity of the restored forest.


> Content contributors