Adrian Thomas introduces some of the visitors and residents

If you planted the seeds of Quercus robur, what would grow? Would it be a.

trees, b. flowers, c. vegetables, or d. grain? This was a genuine pound;1 million question on Who Wants To Be a Millionaire? By answering a, the contestant went away a very happy man, for Quercus robur is none other than that most familiar of trees, the Oak. Actually, let me qualify that - Quercus robur is the Pedunculate Oak, one of our two native species of oak tree. Its UK distribution is predominantly eastern and southern, while in the wetter west and north it is often replaced by our second species, the Sessile Oak.

The two can readily be told apart if you know what to look for but, to the untrained eye, they are essentially very similar trees. Both have the typical oak leaf with the wavy lobed edges, and both can grow into magnificent 500-year old specimens if given the chance. Together with their abundance and wide geographical spread, it is no wonder the oak is so familiar. Mature trees can reach up to 45 metres tall and can spread almost as wide. Reputedly, the largest oak in Britain is the Major Oak in Sherwood Forest, whose trunk at head height is some 13 metres round and whose wood is estimated to weigh 23 tons, the equivalent of four bull African elephants. The lower limbs of many oaks are often close to ground height, making for tempting climbing, and indeed it was in the Major Oak that Robin Hood and his Merry Men were said to have hatched many of their plans. It’s just a shame that it’s not old enough for that to be possible!

Nevertheless, in days gone by outlaws probably did find refuge in oaks, and they would not have been alone up there. They would have shared their lofty hideouts with the wealth of wildlife for which oaks are celebrated, arguably the richest and most diverse ecosystem in the British landscape.

The most basic ecological function of oaks is as a structure in and on which creatures can live. Indeed, for some of its inhabitants, the oak might as well be made of inanimate bricks and mortar. To them it is a high-rise hotel with a choice of rooms to suit all tastes, from the wind and sun-scorched crown to the damp, dark north-face of the trunk.

Enjoy your stay

And what a massive hotel it is, with acres of bark to occupy and miles of branch corridors. Many bird species nest here, or take shelter, or use its loftiness to escape from predators. Woodpeckers excavate their own rooms, and bats, Stock Doves and Jackdaws take advantage of the hollow core of older trees. Indeed, Barn Owls, that have taken so willingly these days to manmade cavities, would once have made their natural homes inside an oak tree.

Many primitive plants such as mosses and liverworts, plus more than 300 lichen species, also take full advantage of all those penthouse opportunities. They are called epiphytes, plants and their kin that merely grab an opportunistic foothold rather than attacking the oak in any way. In particular, the Atlantic oakwoods of western Britain, mild and moist in the influence of the Gulf Stream, are often festooned with dangling beard lichens, their trunks furry with mosses. There is even a fern, the Common Polypody, that grows on oak branches very successfully, and a miniature world of creepy-crawlies wander through these extensive “micro-forests”.

One very common plant that uses oaks to haul itself up towards the light is the Ivy. It is not a true parasite in that it does not penetrate the bark to plunder nourishment from the tree, but it does compete with the tree for water and nutrients. The winding stems and the dense leaves of the Ivy offer yet more microhabitats for yet more wildlife.

But while many living things merely derive lodging from the oak, many other residents are definitely there for the restaurant as well as the beds. For them, the oak tree serves up a rich and varied menu. Admittedly, meal of the day for much of the summer is “leaf”. While a tree is only occasionally defoliated to the point of being threadbare, look at almost any oak leaves and the telltale signs of nibbling guests are clear to see.

Grub’s up

Close to the ground, deer browse foliage within their reach, but it is much smaller mouths that do the most damage higher up. Moth caterpillars in particular can be abundant. More than 100 species of British moth have been found feeding on oak trees, and a few species reach almost plague proportions. The Winter Moth and the Mottled Umber are two of these, the caterpillars of the latter using a curious form of locomotion whereby they arch their abdomens up to their head-ends, and then stretch back out, earning them the nickname of “loopers”.

Perhaps the most familiar of oak caterpillars is that of the Green Oak Roller Moth. The adult lays its eggs in June, but the caterpillars don’t hatch until the following May. Then they quickly roll themselves up in an oak leaf, like a child playing in a rug, and eat, in private, within.

However, shake the branch of an oak tree in June and, sensing trouble, the caterpillars leap out into the unknown on a silken rope. There they dangle, waiting for danger to pass before they clamber back up their lifeline to resume eating.

As well as the oak’s leaves, there are alternative dishes available. Some creatures, such as aphids, tap into the sap coursing through the finer veins of the leaves. But the bark and wood, although substantial, are difficult to access for all but the most determined of mouthparts. One creature that manages it is the Oak Bark Beetle. Only 2mm long, the female bores a maternal gallery into the living layer of trunk. Here she lays her eggs, and when they hatch, the larvae slowly gnaw side-tunnels perpendicular to their mother’s original excavation. On felled or dead trees, these networks are often revealed, signs of a tree under secret attack.

Perhaps even more insidious as invaders of the heart of the tree are the mycelia - the fungal roots - of various species of Bracket fungi. Some, such as the Oak Polypore, are rare enough in themselves to have their own Biodiversity Action Plan. One food source you do not instantly think of with oak trees is their flowers. In fact, it is hard to think what an oak flower looks like. Being wind pollinated, it has no need for showy petals or insect-attracting nectar and the copious “blossom” is green and inconspicuous. Nevertheless, the straggly pollen-laden male catkins are a favourite food of the Common Dormouse.

Far more familiar is the oak’s fruit, which we all know as acorns. A single mature oak tree can, in a good year, produce 50,000. The imported Grey Squirrel is perhaps the biggest beneficiary of this bounty these days, but the most notable native acorn gatherer is the Jay. Come October and November, this beautiful peach-coloured relative of the Crow can be seen flapping slowly across much of the British countryside, seeking out the best oak trees. With more food on offer than it can eat, it also takes the opportunity to prepare for leaner times. It collects several acorns at a time in its gullet, and flies off to bury them one by one elsewhere, with a single Jay hoarding up to 5,000 acorns in one autumn. Throughout the winter, it will return to its stash, apparently using visual clues to go straight to each acorn. No one knows how many of its acorns it recovers, but inevitably many are missed, and this is thought to be one of the main ways by which oaks expand their range.

Acorns, leaves and flowers are obvious ready-made food sources. Some creatures, however, use a quite radical strategy to extract nourishment.

Incredibly, they force the oak to prepare them a special meal, on request.

This is the curious world of gall-inducing insects.

More than 40 different species employ this technique, ranging from weevils to aphids and, in particular, tiny wasps. Their method is cunning: it is thought that they secrete a chemical that mirrors the growth hormone of the oak itself, tricking the tree into increased cell production which results in large warty growths. These are the galls, and they are usually formed around the grub or larva of the insect, protecting and feeding it. Oak galls come in many different shapes, according to which parasite is involved. Some of the most obvious are spangle galls, which can freckle the underside of oak leaves with dark spots. Also very familiar are marble galls, which look like a hard brown spherical nut attached to twigs; the little pinhole in one side shows where the newly-hatched wasp made its way into the outside world. There are also button galls, kidney galls, oyster galls and artichoke galls.

But it is another of the really familiar twig-adorning galls, the “oak apple”, that adds further fascination to the gall wasp saga. The cynipid wasp larva that lives within this greeny-brown squidgy gall hatches into a male or female winged wasp that heads out in search of a mate. The fertilised female then flies, not in search of oak twigs but to the ground underneath the tree. There she burrows down to the oak’s fine roots beneath the surface and lays her eggs. A gall forms, this time on the root, but from all these subterranean larvae hatch wingless females. They climb the tree to lay their eggs without any need of finding a mate, one of nature’s examples of parthenogenesis. It is the larvae that hatch from these eggs that then feed within the developing “oak apples”.

All the many organisms that feed directly on the oak are themselves almost always just another link in an intricate series of food chains. Small insects are fed on by larger ones, which are taken by birds or small mammals, which are themselves taken by birds of prey or larger mammals. The Green Oak-roller Moth caterpillars, for example, are vital in the breeding cycle of woodland birds such as Great Tits and Blue Tits, which time their huge single brood to coincide with the bumper crop of wriggling baby-food.

Mini connections

The inter-connectedness of this mini-universe can be quite astonishing.

There are, for example, beetles that only live on fungi that only live on oaks. There are parasitic wasps whose grubs only feed on the grubs of oak gall wasps. However, the ecosystems of the oak are at their richest when we look at the habitat-scale and even landscape-scale ecosystems of the oakwood. Following the end of the last Ice Age 10,000 years ago, when forests effectively marched back across Europe, it was the oak that would become the dominant species across great swathes of Britain.

It would be easy to suppose that the wildwood, left to its own devices, would consist of nothing but oak trees, that they are the final stage in habitat succession in the wildwood, the “climax vegetation”. However, this is not the case - oaks are actually very poor at regenerating under their own shade, and are eventually ousted by trees such as Ash, Beech and Lime.

But what oaks have going in their favour is that they are rapid colonisers.

As giant trees fell over in the ancient wildwood, oaks were one of the quickest trees to fill the void. It meant that there were probably more oaks in the UK than any other tree for thousands of years.

The richness of the oakwood ecology has as much to do with the way the trees modulate their environment as with the tree itself. En masse, oaks create a dark and sheltered world. Beneath the spreading canopy a rich understorey grows up, typically of Hazel and Hawthorns, Holly and Bramble, and on the woodland floor is a range of ancient woodland flowers, many adapted to flowering quickly in spring in advance of the canopy of leaves closing out the light. Species associated with ancient oakwoods include Dog’s Mercury, Sanicle, Primroses and Bluebells in the south, and Bilberry, Wavy Hair-grass and Bracken in the north. Given the abundance of life in the branches above, there can actually be more weight of living things in the deep layers of leaf litter at the oak’s base. Springtails, false scorpions, woodlice, ants, worms, centipedes and many more - this is the world of detritivores and decomposers that recycle the copious amounts of dead matter that the oak discards.

It is here, in the soil itself, that one of the most bizarre aspects of the oak’s ecology weaves its magic. For a tree so large, it gains much of its strength from a symbiotic relationship with one of the finest living structures in the woodland - mycorrhizal fungi. The countless hair-like threads of these fungi wind around the tree’s roots, penetrating living cells and tapping into the carbohydrates produced by the oak. In return, the tree derives minerals from the fungi which it would find hard to extract from the earth. Only when the fruiting bodies of the fungi emerge, toadstools such as the Purple Brittlegill and Velvet Tooth Fungus, do we get any inkling of the life-enhancing interactions going on below.

Even when the oak casts its leaves late in the autumn and shuts down for the winter, and most of the living things associated with it shut down too, one cannot but help sense the vitality that will burst forth once more next spring, the spring after that, and very possibly for several hundred more.

This is the tree with more insects, more lichens, more birds associated with it than any other... the Mighty Oak indeed.

Adrian Thomas is a wildlife journalist and press officer for the Royal Society for the Protection of Birds

Meet the residents

1 - Aphids secrete honeydew onto crown leaves in spring and summer, and attract ants and butterflies that feed on the honeydew as well as predators such as lacewings and ladybirds.

2 - Tawny Owls often roost in an oak, while Grey Squirrels build their dreys close to a fork in the branches.

3 - Caterpillars of the Green Oak Tortrix feed on the thousands of leaves in May.

4 - Great Spotted Woodpeckers excavate nesting holes; Pied Flycatchers and Blue Tits (shown here), and Great Tits, often use smaller natural holes.

5 - The myriad of corridors of branches, leaves and twigs is home to weevils, beetles and spiders.

6 - Bramble and Holly are among the plants that can grow in the understorey.

7 - Wood Warblers and Chiffchaffs nest at ground level among the vegetation.

8 - The ground directly under the oak can be very shady - shade-loving plants, such as Bracken, grow here, and flowers that bloom before the leaves open in May, such as the Wood Sorrel.

9 - Fungi grow among the leaf litter or the oak’s roots; the leaf litter is decomposed by fungi and eaten by millions of tiny invertebrates (woodlice, false-scorpions, springtails, earthworms) 10 - The ground layer can be dominated by Bilberry and Wavy-hair Grass in the north, Bracken on deeper acid soils, and flowers such as Dog’s Mercury (shown here) and violets in the south. Badgers and other mammals collect acorns in the autumn for their winter food supply.

11 - The oak trunk provides a climbing frame for Ivy and Honeysuckle, providing yet more habitat and food.

12 - Nuthatches, Great Spotted Woodpeckers and Treecreepers clamber over the trunk looking for insects; bracket fungi exploit weak spots to get a footing on the trunk.

13 - Epiphytes, such as mosses, liverworts, lichens - like the Beard Lichen shown here - and Polypody ferns, can festoon the branches.

THE OAK’S YEAR

January - the ground is covered in leaf litter and few plants are in leaf, apart from Wood-sorrel and some ferns; many mosses produce fruiting bodies; female cynipid wasps which cause “oak apple” galls emerge and climb the tree to lay eggs on dormant leaf buds.

February - first Primroses may flower.

March - Chiffchaffs return from African winter quarters and sing in bare branches.

April - first leaf buds burst open in southern Britain - can be as late as early May up north; Bullfinches plunder the oak buds; Wood Warblers and Pied Flycatchers return from Africa to western oakwoods by late in the month; Wood Anemones are at their best; Grey Squirrels have their first litter; Dormice emerge from hibernation.

May - root growth starts; oak flowers emerge about two weeks after the leaf buds burst, and can then be attacked by currant galls and hop galls: Bluebells are at their peak; dawn chorus is deafening.

June - nesting season is in full swing, with birds gathering the rich crop of insects for their young.

July - Purple Hairstreak butterflies flit around the tree canopy; oaks produce a second set of leaves, called Lammas leaves.

August - few woodland plants can now flower because of the dense shade.

September - summer bird visitors depart for sunnier climes; acorns are now maturing fast.

October - acorns start to fall and Jays, Wood Pigeons, Badgers and other mammals start collecting them; green chlorophyll withdrawn from leaves into the trunk and roots and the leaves turn brown and start to fall by the middle of the month.

November - the last leaves fall by the end of the month.

December - the Winter Moth and Mottled Umber are on the wing; Footman Moth caterpillars feed on lichens on mild days; flocks of tits rove through the woodland.

Identifying an oak

Although only two species of oak are native to Britain, many other species of the 500 or so known around the world have been planted in towns and gardens over the past few hundred years. All have acorns borne in “egg-cups”, known as cupules. The commonest are:

* Holm Oak Quercus ilex - native of southern Europe. Planted especially in seaside areas where it withstands salt spray well. The leaves are a very dark green and are evergreen; when young, they are spiny like Holly leaves, hence the Latin name that means “Holly Oak”.

* Turkey Oak Quercus cerris - native of the Mediterranean region. The lobes on the leaves are often quite pointed, and the acorns sit in a woolly cup.

* Cork Oak Quercus suber - another non-native species, whose products you are nevertheless very likely to have about the house. The ecology of the Cork Oak is very important. A native of the Mediterranean, it is only occasionally grown in the UK. It is grown in vast plantations in places such as Spain and Portugal, where the cork bark is stripped off the tree every 10 years or so and made into wine corks. There are fears that the advent of plastic wine corks will see the Cork Oak forests destroyed and turned into barren agricultural land, and with it will be lost a unique wildlife including highly endangered birds such as the Spanish Imperial Eagle.

Sessile Oak Pedunculate Oak

5-8 lobes on each side of the leaf 3-5 lobes on each side of the leaf Leaf stalk (petiole) 10-20mm Leaf stalk less than 5mm

No lobe at base of leaf 2 small lobes (auricles) at base of leaf Fine hairs on midrib on underside of leaf Hairless leaves

Acorns stalkless, or almost so (sessile) Acorns with stalks (peduncles) up to 30mm long

NB

Watch out for hybrids!