Four-Tier Woodland Layers
A wood can be rather like a four-storey house with mosses at the bottom, two storeys of, ferns and shrubs, and an attic made up by the canopy of foliage.
What chiefly distinguishes woodland from other habitats such as grassland or heather moor is that it has a dimension which they lack that of height. This provides a series of habitats for many species of wildlife, ranged one above the other from ground level up to 20 or 30m (65 or 100ft). So a useful way of looking at a wood is to think of it as consisting of various layers of vegetation; and of these, four are usually recognised.
At the lowest level is the ground layer – the very small plants, chiefly mosses, which grow on the floor of the wood. Next comes the field layer (or herb layer) – the wild flowers and the ferns. Above them grows the shrub layer (or under-storey), of which hazel is a typical species. And high over the rest of the wood are the upper boughs and twigs of mature oaks, ashes, beeches and others which form the tree layer (canopy).
You don’t need to look at many woods before you realise that there are great differences between them. Some may have all four layers well developed. Others, and they are more usual, may be lacking in one or more layers. Why are there these differences? Often there is no single answer. In ecology it is always necessary to be on the look-out for multiple causes, especially in so complex a site as a woodland. And because practically every British woodland exists to serve a human purpose, the presence or absence of any of the four layers is often due to man’s interference rather than to natural causes.
Here and there, even in a quite natural woodland, you may find an absence or near absence of one or other layer. This often happens if the tree layer is particularly dense with summer leaves that suppress the layers below by cutting them off from the light they need. So the only type of wood which can have all four well-developed layers is one where plenty of light comes down through the tree layer even at the height of summer. In ashwoods, for instance, the divided, pinnate leaves of the ash allow much more light to filter through them than do the leaves of most other large trees. Pure ashwood, however, is not found in many areas. A more common type of wood is a mixture of ash and oak. In such a wood, provided the oaks are not too many or too close, especially if there are a few clearings and a variety of trees of different ages, you will probably find all four layers better developed than in most other woods.
The upper branches and twigs of a tree are structured to expose the maximum number of leaves to the light. Since many of the young leaves, especially of oaks, are highly palatable and nutritious for insects, the canopy becomes the scene of immense activity in spring and early summer. Phenomenal numbers of caterpillars, mainly of mottled umber, winter and green tortrix moths, begin to eat the leaves as soon as the buds open. And the caterpillars in turn are taken in large numbers by tits, warblers, finches, redstarts, pied flycatchers, starlings and many other birds which are feeding hungry broods of nestlings. In years of caterpillar plagues a wood’s entire tree layer may be stripped of its leaves and the shrub layer may also be devastated. In such years an unusual amount of light gets down to the field layer. However, once the caterpillars have pupated, starved to death or been eaten, a replacement flush of leaves appears on the trees. Another group of insects active in the tree layer in spring produces oak apples, marble galls, artichoke galls and many others.
In winter the highest twigs are searched for tiny forms of animal life by sharp-eyed small birds such as the tits, nuthatch and lesser spotted woodpecker. But the most obvious birds of the tree layer are those large species which come in spring to build their nests there the heron, rook, carrion crow, buzzard, sparrowhawk and several others. Many birds also use the tree tops for perching by day. And for roosting after dark.
This complex zone consists of shrubs, the lower branches of mature trees and saplings which, if they survive, will grow up to take their place in the tree layer. Some shrubs and young trees can tolerate quite a lot of shade; others survive only in clearings or where the big trees stand well apart. For hundreds of years, until the early 20th century, the shrub layer was very important in the life of rural Britain. This dense jungle of hazel, hawthorn, ash, hornbeam, sweet chestnut, willow and many other species was completely cut clown (coppiced) at regular intervals to produce the slicks and poles that were in such demand in the farming countryside.
For two or three years after coppicing the ground was carpeted with wild flowers encouraged by the light suddenly let into the wood. There was also a great increase in butterflies, bees, beetles and other insects that revelled in the fragrance and the sunshine. But in another two years all this changed. New tall stems shot up from the cut stumps and the shrub layer again became impenetrable. The flowers began to die out. But there were more small birds; warblers, thrushes and. In the south of England, nightingales, all used the shrubs for nesting. But growth continued and in about three more years the coppiced shrubs grew into a dense mass of tall poles unsuitable for any of these birds. Then the shrubs were felled once more and the cycle began again.
Today most of the old coppices have been destroyed or are neglected. However, in many woodland nature reserves the old coppice system is being restored for the benefit of wild flowers, insects, birds and other creatures and the cut wood is used forand wood-burning stoves and fires.
While the wild flowers of the field layer are at their best in clearings or in the dappled sunlight under ash trees, those beneath the closed canopy of beech woods are few. But even in these shady places there are peculiar plants which, lacking chlorophyll. Manage to do without the process of photo-synthesis by which nearly all other plants take carbon dioxide from the air. Instead these plants, called saprophytes, take their food from dead organic matter as many fungi do. The bird’s nest orchid is the best known of the beechwood saprophytes. Also tolerant of shade are the ferns which, in some woods, are abundant.
The field layer is the world of the wood mice, bank voles and shrews which dwell among the thick cover. Here, too, live hedgehogs, rabbits, hares and their predators the fox, stoat and (mainly in Wales) the polecat. Among birds which hide their nests in the field layer are woodcock, pheasant and, in clearings, the nightjar. Among typical butterflies of the field layer are the speckled wood. Green-veined white and several of the fritillaries.
This is the domain of the shade-tolerant, earth-carpeting mosses, many of which also cover fallen logs and the cut stumps of trees. But mosses are not in every wood. They need almost all-the-year-round moisture and so are commonest in the west and north of Britain, especially in upland woods where rainfall is high and mists are frequent. In drier lowland woods the ground may only be covered by drifts of dead leaves which shelter whole populations of small creatures. Many fungi also inhabit the ground layer. Their underground threads often grow in close association with the root-tips of trees, to the advantage of both fungus and tree. They send up their fruiting bodies-toadstools – mostly in autumn, bringing attractive shapes and colours to the woodland floor.
While it is useful to think of a woodland as built up in several layers, in nature these layers never actually occur in such distinct compartments as shown in diagrams. Also many invertebrates and birds, and mammals such as squirrels and bats, move freely from one layer to another. Even a few plants – ivy, traveller’s – joy, honeysuckle and polypody fern, for instance belong to all layers. So do tree trunks: different mosses, lichens and invertebrates are found on different sections of the trunk from ground level up to the tree crown.
The vitality of a woodland depends on a powerful flow of life-giving substances circulating through all the layers with every plant and animal playing a part by its life and death. For a healthy woodland, though made up of various layers, is really one successful, ever-changing community. For example, a plant provides sustenance for an insect which. In turn, is food for a bird that then falls victim to a larger predator. Plants take nutritive substances from theto the tree-tops and return it again in falling leaves and twigs. And the food-chains circulate organic material through all the woodland layers.