But a close look at your favourite trees, be they woodland oaks, field-side ashes or plantation spruce, might reveal dead branches, discoloured leaves or curiously sparse foliage. For all is far from well with Britain's trees.
The most comprehensive comparative study of tree health, published by the United Nations Economic Commission for Europe and the EC, has found that Britain's trees are the least healthy in Europe. Even the Czech and Slovak republics and Germany - source of some of the most shocking pictures of forest death - fare better overall than Britain. Another inquiry, for the Department of the Environment by a team of experts led by Professor Mike Unsworth of Nottingham University, has found that the situation is deteriorating.
But what is the scale of the problem? Using the amount of leafiness in the crowns of trees as an indicator of health, experts consider trees damaged if more than a quarter of their foliage is missing. According to this criterion, nearly 60 per cent of the trees surveyed in Britain - oak, beech, Scots pine, sitka and Norway spruce - are classified as damaged. For the former Czechoslovakia the figure is 41 per cent, for Germany 25 per cent. Some of our nearest neighbours have, it seems, much healthier trees. In France, for instance, only 7 per cent of trees surveyed are in the damaged category.
In Britain, this assessment is done by the Forestry Authority, at a cost of more than pounds 100,000 a year. In 1991, the most recent year for which results are available, the authority examined nearly 9,000 trees in woodlands and plantations in England, Wales and Scotland.
To assess tree health there is no point in looking at old trees. Increasing infirmity is a natural part of the ageing process in trees as in humans. Before whole trees die, individual branches - often large ones - usually die first, frequently giving the tree a 'stag-headed' appearance. Such dead wood is, in itself, an extremely valuable habitat for a wide range of insects and fungi. When Shakespeare wrote:
When lofty trees I see barren of leaves,
Which erst from heat did canopy the herd
he was probably referring to the natural ageing of lofty field-growing oaks or elms.
Unhealthy young or even middle-aged trees are a different matter. Dr John Gibbs, principal pathologist with the Forestry Authority, points to trees such as 40- to 70-year-old oaks, which are commonly in poor condition. One of their obvious symptoms is sparse foliage, especially in the canopy, and twigs with no leaves.
Dead branches can sometimes be spotted in broad-leaved trees such as birch, and in conifers like pine and spruce; reddening of the foliage in what should be deep green Norway spruce; and unusually small leaves on beech. Yews - stately churchyard trees and a component of many lowland woods - are frequently unhealthy too. In a study commissioned by Greenpeace, Dr Andrew Tickle found that over two-thirds of churchyard and woodland yews were moderately or severely defoliated. Many suffer a discolouration of their needles.
Not all this damage is unnatural. Storms can snap off branches. And caterpillars of some moths can be so abundant that they devour the leaves of an oak tree in weeks, if not days, providing a feast for woodland birds such as warblers and flycatchers. Such trees, nevertheless, almost always recover.
Pollution has been blamed for much of the damage done to trees of many species across Britain. The Department of the Environment's recent report acknowledges that pollutants are major contributors to tree damage in some parts of the country. Britain's industrial chimneys, power stations and - increasingly - car exhausts produce over three million tons of sulphur dioxide and 2.75 million tons of nitrogen oxides every year. Much of this falls back to the ground as dry, acidic dust, or dissolves in water to produce acid rain and mist. It contaminates trees directly, and indirectly via the soils from which their roots gain sustenance. Ammonia gas - from fertilisers and livestock manure - is another air pollutant which can affect tree health.
One effect of increased levels of pollutants is to make trees more susceptible to other forms of damage, such as drought, insect attack or late frost. Ozone, for instance, can make young trees more sensitive to frost and acid rain can actually invigorate insect pests such as the green spruce aphid, which can quickly devour the leaves of conifers. Professor Unsworth's inquiry suggests that while the concentrations of pollutants in most of rural Britain may not be enough to reduce tree growth, they can cause subtle changes in the structure, physiology and biochemical processes of trees.
On top of pollution, trees have had to cope with a number of hot, dry summers over the past decade. Dr Gibbs says that ash trees in parts of rural Cambridgeshire have suffered damage to their branches and leaves - a condition known as dieback - as a result of dry summers. 'Healthy trees in the county are not exposed to different pollution levels or weather patterns,' he says. 'But the healthy trees are on sandy, quick-draining soils whereas those suffering dieback are usually growing on heavy clay soils.' The ashes on quick-draining soils develop a deep root system so they survive better in summer droughts. Heavy clays retain more water - so the trees do not root as deeply, but in a drought they suffer. More frequent summer droughts are one predicted consequence of global warming.
With trees on only 7 per cent of its land area, Britain is one of the least wooded countries in Europe; the Countryside Commission wants to double tree cover by the end of the 21st century. This is a laudable goal - but unless we are able to guarantee a healthier life for many of our existing trees, we could be nurturing any new ones towards a very precarious future.-
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