The bark of a tree is similar in many ways to our own skin. As well as being essential for the survival of the tree itself, a whole array of other species take advantage of this niche in the forest ecosystem.
A short way beneath the outer surface of a trees trunk and branches, there is a layer called the cambium. Each growing season the cambium adds a new layer of cells to the xylem, which it surrounds. The newer xylem or sapwood, transports minerals up the tree from the roots. The older xylem or heartwood, is at the centre of the tree. It is essentially made up of dead cells, and provides much of the strength of the tree. This is all within the cambium. Bark basically consists of the several layers that are outside the cambium. The outer edge of the cambium produces another layer of cells that create phloem, which transports sugars from the leaves to the rest of the tree. Outside that, most trees have a layer known as the cork cambium, which produces the cork the tough outer layer of the tree. This outer layer is all that we usually see of the bark.
The outer cork protects the tree from the elements - from scorching by the sun or drying by wind. It also helps to ward off fungi and the many insects and mammals that would otherwise take easy advantage of the sugar-rich sap or the wood that it surrounds.
The bark of different trees has evolved to make best use of the environment in which each species occurs. Scots pine (Pinus sylvestris) bark offers protection from fire. In prehistoric times, parts of Scotlands woodlands would have been influenced by fires, which would have occasionally swept through areas of forest. While this may seem devastating, when forests were more extensive, pinewood ecosystems would actually have benefited from such natural disturbance, as it can clear away rank vegetation, leaving a fertile bed on which pine seeds can germinate. Many Scots pines have very characteristic thick protective plates on their bark, and it is thought that in areas more prone to fire the bark may become locally adapted to offer extra protection.
Many trees have chemicals within their bark, which protect against fungal and insect attack. Birch (Betula spp.) bark is high in volatile oils (this is why its also great for lighting fires!), and is so waterproof and resistant to decay that tubes of birch bark can still be found on the forest floor after the wood inside has decayed. The bark of oak (Quercus petraea) is very high in tannins, which are toxic and protect the tree from insects.
Aspen (Populus tremula) bark has some fascinating features. On many trees it is a conspicuous greenish-grey colour and it is actually a result of the tree being one of the few that can photosynthesise through its bark. It also has characteristic diamond-shaped marks on the bark that are in fact tiny breathing holes called lenticels.
Different species of trees have very characteristic textures to their bark, that influence what other species live on it. The deep fissures and crevices on the bark of an old oak or Scots pine are a haven for many species of insects and spiders. These invertebrates attract birds, which feed on them. The crested tit (Parus cristatus) is very much a pinewood bird that includes in its diet insects picked from the bark and twigs in the branches. The treecreeper (Certhia familiaris) is a specialist bark forager, hopping up the trunk, and probing the crevices with its specially adapted, thin, curved beak.
As their name suggests, bark beetles (family Scolytidae) are among the insects that use bark. The larvae burrow beneath the bark of various tree species, with the larvae of each beetle species making distinctive galleries, or passages in the wood. These beetles can break through the barks defences, carrying in fungal spores that the bark would usually repel.
Even after a tree has died, bark can be a haven for all sorts of wildlife. Bats, such as the brown long-eared bat (Plecotus auritus), sometimes roost beneath loose bark, and a multitude of invertebrates also live out their lives in this hidden world.
In the Caledonian Forest, perhaps the most obvious demonstration of the life that bark can support is in the lichen and plant communities on the surfaces of trees. Plants that live upon other trees, without actually causing them any harm, are known as epiphytes mosses are a good example. The texture of bark influences the species that live upon it. In an old pinewood it is not uncommon to see many other plants such as blaeberry (Vaccinium myrtillus) and cowberry (Vaccinium vitis-idaea) growing in the thick crevices of pine bark. Similarly, the fissured bark of oak can support many species of fern such as common polypody (Polypodium vulgare).
The texture of bark, and therefore the lichen communities, can alter during the lifetime of a tree. Young hazel (Corylus avellana) has fairly smooth bark, and so attracts lichens that prefer this texture, particularly the Graphidion lichens. (These script lichens are distinguishable by the tiny squiggles on their surface). As the tree grows older, the bark gets rougher and becomes more suitable for other species, including the leafy, frogskin-like lungwort (Lobaria pulmonaria).
The lichen community can also vary on different parts of the same tree. Aspen bark has rough areas, which support various species of strap-like Ramalina lichens, while the smoother areas are host to completely different species, such as the crust-like Pertusaria spp.
It is not only the texture that determines what can survive on tree bark. The chemistry of bark is also surprisingly influential. Aspen bark is not as acidic as that of some other trees such as pine and birch. This feature means that it can support species of plants and lichen that might not otherwise be present in a pinewood, such as the orange Xanthoria parietina. This illustrates how the diversity of certain species (in this case trees) in turn increases the number of other species present.
While bark does an excellent job of protecting the tree, there are some very determined creatures that are keen to get to the nutritious cambium, or the wood beneath it. Many mammals eat bark, and by looking at the height of the damage, we can find out what mammals are present in an area. While this is a natural process, it does cause problems for individual trees, allowing fungi and other organisms to enter. Voles (Microtus agrestis) often eat the bark at the base of young trees, killing young saplings. Deer also strip bark (as well as damaging it by fraying their antlers on it to shed the velvet coating). The bark of aspen and willow is an important food source for the European beaver (Castor fiber). This is obviously damaging to a tree, but from an ecological perspective it shows how bark can support a wide range of different species. Also, when a tree is killed or harmed by bark damage, valuable dead wood habitat can be created for fungi, insects and many other organisms. Beavers usually coppice trees before eating the bark. By felling a broadleaved tree, they actually encourage it to send up new growth, which eventually provides young bushy habitat for nesting birds, and allows light to reach the forest floor.
While barks main purpose is to protect the tree, it also serves as a good example of how every surface, nook and cranny in woodland can provide food, shelter, or both, for myriad living things, thereby increasing the overall biological diversity in the forest.
Sources and further reading
Brown, R.W., Lawrence, M.J. & Pope, J. (2004) Animals Tracks, Trails and Signs. Hamlyn: London.
Mitchell, A. (1982) Trees of Britain and Northern Europe. Collins: London.
Steven, H.M. & Carlisle, A. 1959. The Native Pinewoods of Scotland. Oliver & Boyd: Edinburgh.
Street, L. & S. (2002) The importance of Aspens for lichen. In: Cosgrove, P & Amphlett, A. (eds.). The Biodiversity and Management of Aspen Woodlands: Proceedings of a one-day conference held in Kingussie, Scotland, on 25th May 2001. The Cairngorms Local Biodiversity Action Plan: Grantown-on Spey.