SCIENCE: Fact: the woods are not dying

Links between air pollution and tree health are proving hard to pin down, says Malcolm Smith

In the early Eighties, Europe's trees seemed to be dying in large numbers across the continent. The term Waldsterben - dying of the forest - was coined to describe the damage to, and death of, trees, which was widely believed to be the result of pollution from European industry.

Now it appears that we may have been mistaken. The links between air pollution and poor tree health are not as firm as many foresters and conservationists once believed, even in central Europe where the Waldsterben was at its worst.

Contrary to predictions, growth rates of forest trees in central Europe are higher now than at any time recorded, though no one is quite sure why.

According to a review of the evidence carried out by Otto Kandler of the Ludwig-Maximilians University of Munich and John Innes of the Swiss Federal Institute for Forest, Snow and Landscape Research, subsequent surveys have failed to confirm that forests are dying or declining over large areas of Central Europe.

There are places where air pollution, particularly sulphur dioxide from industry and power stations, is damaging forests. The Ore Mountains on the German/Czech Republic border, and the Harz and Fichtel Mountain areas in Germany are three examples. Outside such locations, pollution may be affecting some trees but it is certainly a secondary consideration after the impact of climate change - drought in particular - and diseases.

In Britain, however, field and hedgerow trees are suffering considerably more. There is no year-on-year monitoring of them; just observation of all too frequent dead branches, whole dead trees, and sparse foliage. The evidence is to be found along most roadsides the length and breadth of Britain.

"Fieldside trees may be more prone to drought," says Dr Derek Redfern, in charge of tree health estimates for the Forestry Commission. "They often grow on raised banks and may filter out more air pollutants on to their leaves than trees inside a forest. They could also be affected by pesticide drift if they are close to sprayed crops."

Dr Redfern draws attention to the poor health of ash trees in the Midlands and southern England, most of them on farmland. Beeches, too, he says, are often in bad condition on roadsides, the result of road-salt spray, air pollution and limb cutting for vehicle safety.

More recently, alders - trees of riverbanks - have fallen foul of a disease caused by a phytophthora fungus, which invades its roots and stem. Dead roots, strips of bark exuding a rusty liquid, and yellowing leaves are the result. Forestry Commission scientists believe that the disease has already killed up to 11,000 alders and severely damaged 20,000 more.

Elsewhere in Britain, foresters are finding other diseases and conditions that are killing alders. Some are the result of insect pests that previously have not caused serious damage, a phenomenon that is baffling research foresters.

While alder is of no commercial timber value, the tree tolerates flooding and is a natural stabiliser of riverbanks. It also provides wildlife habitat for anything from insects to otters. So its future is being watched by river engineers and conservationists alike.

Last July the Forestry Commission's Arboricultural Advisory and Information Serviceissued a "Tree Damage Alert" warning foresters to look out for the Asian race of the Gypsy Moth. Already in Germany, the caterpillar of the moth feeds on a wide range of broad-leaved trees, on larches and on pines. It can cause serious and widespread defoliation. If the attacks continue over several years, an afflicted broad-leaved tree goes into terminal decline.

Caterpillars were found last summer at one site in north-east London. They had possibly escaped from an amateur entomologist's collection, but foresters will be looking for any survivors as soon as the weather warms up. If they do spread into Britain, larches and pines will be most at risk. Such conifers cannot regrow their leaves.

There was a time when no one much considered the health of trees. After all, a human lifespan is all too short compared with that of a giant spruce or oak. That is why Sir Thomas Browne could write, in the late 17th century, of England's trees: "Generations pass while some tree stands, and old families last not three oaks."

It is not a commentary thatcould be written today. For, as human life expectancy continues to increase, the chances of many of Britain's trees making it to a ripe old age are increasingly remote. Even without pollution and climate warming, Dr Gibbs is concerned that the health of Britain's trees will remain well below par. "With the current level of world trade in timber and timber products, even with import regulations in place, there is an increasing risk of introducing diseases to which our trees have no resistance," he says.

Gauging tree health is not easy. Foresters do it by estimating the density of foliage in the tree canopy. The fewer the leaves and twigs, the less healthy the tree. It is a subjective measure. In the same vicinity as their sample trees, foresters in continental European countries select a well-foliated tree to act as a healthy standard.

On this basis in 1993 - the year for which the most recent results are available - 53 per cent of the Czech Republic's forest trees were significantly defoliated (a quarter or more of their foliage missing), the worst in Europe. Several other East European countries also had unhealthy trees - Poland with 50 per cent; the Slovak Republic 38 per cent. Western Europe's trees are generally faring better; 15 per cent of Belgium's trees, and 8 per cent of France's, turned out to be significantly defoliated on the same assessment.

The problem with tree health is knowing how unhealthy a tree can become without suffering long-term damage or death. In any population of living organisms, a proportion is bound to be off-colour at any one time. So roughly 10 per cent of forest trees looking sickly may be normal. When the proportion is much higher, foresters begin to worry. Somewhere between 17 per cent and 54 per cent of Britain's forest trees show signs of ill- health, so there is no room for complacency.

Oak trees have fared the worst, though there is recent evidence for some recovery. Much of their defoliation - total in some trees - is the result of insect attacks (mainly moth caterpillars eating the leaves) and fungus diseases. A more worrying condition, known as "oak dieback" or "oak decline" has damaged many trees - and killed some - particularly in the south and east of England.

Drought seems to be a significant problem and may have an impact on many of our trees if climatic warming takes off. Trees may feel the impact of last summer's heatwave and drought for years to come.

Storms also seem to be a factor, although whether Britain has become a much windier country in recent years is open to debate. Air pollution - ozone and ammonia in the lowlands, sulphur dioxide and nitrogen oxides in the uplands - are part of the unhealthy cocktail. Trees stressed because of drought and pollution, or loosened by wind, are more disease-prone, too.

Arboreal unhealthiness has become endemic. If there is an after-life, who would want to be reincarnated as a tree?