THE TREE OF LIFE
From little acorns mighty oaks do grow – and so mighty is the oak that it supports more wildlife than any other tree in the UK
From the top of its spreading crown to the ends of its roots, which can extend as far below the ground as its branches reach into the sky, the oak tree provides shelter and food for hundreds of different organisms. In fact, this native tree supports a greater variety of wildlife than any other species in our islands. It certainly lives up to its reputation as being a ‘mighty oak’.
Like a crowded high-rise block, the oak is inhabited at every level. Birds and squirrels build nests in the crown. Several British bat species may also roost in old Woodpecker holes or under loose bark, as well as feeding on the rich supply of insects in the tree canopy.
And there are certainly plenty of insects: the oak provides habitat for more than 280 species of insect, such as wasps, moths, beetles and weevils, all of which devour the leaves. Flower and leaf buds are the exclusive food plants of the caterpillars of the spectacular Purple Hairstreak butterfly, for example.
Ivy, mistletoe, lichens, mosses, algae and fungi invade the branches and bark. Birds, insects, and mammals feed on the acorns. Even the roots of the young oak are sought out by such insects as weevils and, as the oak lets in quite a lot of light through its leaves, flowering plants grow underneath it, such as bluebells and primroses. Even its shed leaves are vital. They are soft and break down with ease in autumn to form a rich leaf mould beneath the tree, supporting invertebrates such as the Stag Beetle, and numerous fungi, including the Oakbug Milkcap.
You may think the oak must be quickly overpowered by this invasion of wildlife, but once a sapling becomes established the oak can live for up to 800 years (making it the longest lived of our native deciduous trees). It can take up to 100 years for an oak to mature - acorns are not produced until the tree is at least 40 years old, and peak acorn production usually occurs around 80-120 years. After this point, the oak slows its previously rapid growth, and incredibly, oaks even shorten with age in order to extend their lifespan.
There are over 450 different species of oak in the world, but only two are native to Britain and Ireland: the Common or English Oak (also known as pedunculate) and the Durmast Oak (or sessile).
It isn’t always easy to tell the difference between the two. When it is growing in the open, the English Oak is gnarled and tends to have lower, more horizontal and wider-spreading branches so that the main trunk is hidden beneath a mass of boughs and leaves. This is the tree most likely to be climbed by children; there must be few of us who haven’t scaled a mighty oak at some point in our lives, to sit on their sturdy branches
The Durmast Oak has a straighter, less gnarled trunk, with branches growing from higher up.
The two often grow in the same woods, along with other trees. When in woods, the two can be difficult to differentiate – and to make matters more confusing, one species is frequently fertilised by the other and the result is a hybrid with characteristics of both species. If you wish to be certain which tree you’re dealing with, get closer up…
The leaves of the English Oak are pale green and virtually hairless, with two obvious ‘ear-lobes’ (auricles) at the base. They have deep, rounded indentations all round, and grow in bunches. In autumn, acorns grow on long stalks called peduncles (hence its alternate name).
Alternatively, the leaves of the sessile oak are dark green, have no auricles and the indentations are not so deep. Just like their straighter trunks, their leaves are also ‘straighter’. Leaves grow on long stalks and have a few hairs on the midrib of the underside. Unlike the English Oak, the Durmast Oak acorns sit on
English oak woodlands are the most common, and are usually found on heavy clay soils throughout lowland Britain. For a wood comprising all Durmast Oaks, find shallow acid soil, such as there is in the Lake District.
Trees are flowering plants, but many of their flowers are not spectacular, large or colourful, and the oaks’ are no exception. Inconspicuous female catkins (flowers) are pollinated by the wind-carried pollen grains from male catkins (so large petals needed to attract insects for pollination are unnecessary). The oak’s acorn crop varies from year to year – in a bumper year each tree can produce as many as 50,000 acorns. But few of the hundreds of thousands that fall every year grow into full-sized trees.
Acorns start to form in early summer (the warmer the summer, the larger the acorns) and they are a wonderful, bright green. At the very start of autumn they change a deep caramel, then during a few weeks of early autumn they fall to form a dense carpet.
They do not stay long on the ground, as they are seized by hordes of birds and animals either to be eaten or stored away for the winter. Even large mammals such as badgers and deer take advantage of the falling acorns. Jays and squirrels in particular bury them (sometimes quite a distance from the wood) and then forget about them. This is one of the ways the oak is spread across the countryside, and acorns need to germinate and root quickly to prevent drying out.
When looking at the tiny acorn, nestling in its neat cup, it is hard to imagine that one day it will grow into such a vital tree for so much of Britain’s wildlife. No wonder it is such an icon for this country.