The tree was a big one, with a cross-section of nearly a metre at what would have been breast height, had it still been standing. It had already been sliced into big drum-shaped sections ready for splitting, and I relished the prospect of getting down to work. But, stopping for a moment, I asked what had brought the tree down. "It seems to have been in good health," said my host. "There's no obvious reason such as disease or high winds, but more and more like this are falling every year."
It was an alarming statement. This, after all, was a stand in the great mixed mesophytic forest of Appalachia - the oldest and largest relatively unbroken block of deciduous woodland in North America. The mesophytic (it means "mixed plants") is anomalous for a forest outside the tropics because its elevation means it has escaped the Ice Ages, and over hundreds of millions of years has evolved a canopy made up of many, rather than a few, dominant tree species. The only other forest like it in the world is in south central China. Accelerated death rates in what should be healthy trees could indicate something seriously wrong.
The pioneering research on the forest was done by E Lucy Braun, who studied the mixed mesophytic all her life and, as a 27-year-old professor, first named it in 1916. In 1934 Braun began an exhaustive survey of the woods, studying around three dozen canopy species growing in typical mixed mesophytic areas: the American beech and tulip poplar; red, black and white oaks; four species of basswood; magnolias and three kinds of maple; hickory, sweet buckeye, sweet gum and the yellow locust tree. The American chestnut was widespread in her days but is now gone, as is the red mulberry.
In recent years much research has concentrated on the effects of pollution such as acid rain and ozone haze on tree health. Some industry groups and the US Forestry Service have consistently denied that the forests are in trouble. And Jim Renfro, an air resource specialist for the National Park Service at the Great Smoky Mountains National Park, concedes that "we don't have evidence that air pollution is [directly] causing a dieback of hardwoods". But the figures he cites on the extent of the human assault on the environment are startling.
The Great Smokies are named for the clouds of water vapour that sometimes shroud the tops of this, one of the highest parts of the Appalachian chain. Despite the name, long-range visibility used to be excellent. But pollution has reduced it sharply over the past 50 years, says Renfro. Average visibility is down from 90 miles (146 km) to only 25 miles (40km), and less in the summer. Over 70 per cent of the haze is from sulphates, which are mostly produced by the electric utilities and factories of the booming Ohio and Tennessee valleys. In the moist mountain air, sulphates turn into sulphuric acid, and the resulting rain and water vapour is five to 10 times more acidic than usual (with an annual average pH of 4.4 to 5 as against a natural background rate of 5.6). In spring, concentrations can be more than 100 times higher again, bathing the growing tips of trees in liquid as acidic as vinegar (pH2).
The effects of sulphates are well understood. They permeate the mountain soils, where they alter the basic ecology by accelerating nutrient uptake. In the Great Smokies "the availability of nutrients [to the trees] is [now] controlled more by acid deposition than natural processes like litterfall and decomposition," says Renfro. Boosted by the sulphates, trees such as the red spruce take up not only beneficial calcium and magnesium, but also toxic aluminium.
Other factors are also at work. For example, the ability of mountain streams to neutralise acid levels, another indicator of ecosystem health, has dropped by a factor of five. During heavy rainfall, acidity can increase suddenly to levels that fish and other aquatic organisms cannot tolerate. Heavy deposition of nitrates (byproducts of industrial combustion) means that soils are nitrogen-saturated, which Renfro says is "unusual, even for old growth forests". Ozone, made by a reaction of man-made nitrogen oxide and mostly natural hydrocarbons, damages the foliage of at least 30 tree species, with another 60 well-documented but not proven. Concentrations are higher and more persistent in the mountains than in nearby cities like Knoxville and Atlanta.
Professor Orie Loucks, Eminent Scholar of Ecosystem Studies at Miami University in Ohio, says research is needed to determine exact rates of death and decline on a species-by-species basis in various locales, but insists preliminary studies point to a grave situation.
The normal "background mortality rate" in the era before the long-range transport of air pollution was between 0.5 and 0.7 per cent annually. Loucks estimates that, even in the least affected areas of the mixed mesophytic, the mortality rate is now four to eight times higher. Anything over 2 or 3 per cent death a year is a disaster, rendering the forest moribund, he warns. Also, the production of mast - the seed stock of the forest - is 50 to 80 per cent down, and "no seed means no regeneration".
Loucks hypothesises three main reasons. First, he thinks the assumption that the region's rich soils contain enough calcium to neutralise the acid is wrong. Second, when foliage is damaged by ozone, a tree is less able to regenerate its roots. As these suffer, the tree is less able to take up nutrients it needs from the soil, and as a result is less able to generate new foliage: a vicious feedback loop. Third, many trees, unlike crops, do not respond happily to increased levels of nitrogen (three times greater today than before the Second World War). The balance of carbon and nitrogen in their tissues is upset, and affecting the production of secondary metabolites, which they use to resist disease and insects. Loucks estimates that trees in the eastern US are receiving around three times their tolerance levels for nitrogen.
But Renfro sees some hope: much of the damage may be reversible. Reducing sulphate and nitrogen oxide emissions would improve air quality quickly, with tangible benefits for recreation and public health. Also, computer models suggest soils and streams would recover their acid balance within a matter of decades. Lowering pollutant levels must be a priority, he says.
But regulations to contain emissions can only be part of the picture, says Cielo Sand Myczack, of The Dogwood Alliance, a local environmental group. Among the biggest threats to the forest, she says, are more than 100 highly-mechanised large-scale chip mills sited in the region within the past 10 years, which can reach quickly into remote areas and chip 300,000-700,000 tonnes of hardwood each per year for export to make fine- grade paper. The Dogwood Alliance are urging a region-wide study of their impact and a moratorium on new permits for chipping operations.
Can the forest survive? The balance of economic and political power, and the drive for "growth" and profitability on the current industrial model will mean that environmental campaigners will have a hard fight. Whether they will succeed is as unknowable as the reason for the death of that mighty black oak in its primenReuse content