TO SUMMER hikers and tourists it may look pretty. But America's Great Smoky Mountains National Park, shrouded in a hazy cloak of ozone and sulphur dioxide, may be a veritable hell for its rich spectrum of plant life-as well as for visitors who suffer from asthma.
Ironically, it is the notoriously high pollution levels in this swathe of land straddling Tennessee and North Carolina that create the beautiful pastel-coloured sunsets that draw in the sightseers.
This month there will be a few extra visitors to the park, but they won't be there for the view. Scientists intend to conduct a novel experiment-making use of the park's smoggy air to investigate how wild plants survive and adapt to air pollution.
Ozone's ability to damage plants is hardly news. When airborne concentrations of the gas exceed around 40 parts per billion, some plants wilt and die. Others become peppered with unsightly spots and blotches. But so far, the focus has been on agricultural crops. There is a lot of evidence that crop plants can be bred for resistance to high levels of ozone, but much less is known about whether wild plants can also develop resistance.
Last month, ozone concentrations in the park, boosted by winds that swirl from distant cities such as Chicago, Detroit and even New York, reached an all-time high of 128 parts per billion. This is more than three times the level that is considered to be dangerous for agricultural plants. So how will wild plants be affected?
The researchers hope that the Great Smoky Mountains Park project, funded by the National Geographic Society, could give them the answer. So in Britain, Alan Davison, professor of agricultural and environmental science at the University of Newcastle, has lots of questions.
Davison wants to know to what extent the noxious pollution levels are killing off certain species. He also hopes to find out how quickly individual species adapt to the harsh conditions. The answer may have important implications for the way we try to preserve rare flora in the future.
Later this month, Davison will fly out to the park to join Howard Neufeld of the Appalachian State University in Boone, North Carolina, and Art Chappelka of Auburn University in Alabama. They are going to set up two gardens in the park, each at a different altitude. Neufeld has selected five species known to be sensitive to ozone for the gardens, including tall milkweed (Ascelpias exaltata) and cutleaf coneflower (Rudbekia laciniata).
Neufeld suspects that altitude influences the extent of damage, because ozone generally lingers for longer at higher altitudes. By analysing the genetic make-up of hardy and weak specimens of a species at both altitudes, Davison aims to identify important genes and eventually cross-breed strains to produce hardy varieties.
During the study, which will initially run for two years with long-term follow-up, he hopes to see whether vulnerable strains will simply die or naturally cross-breed with the hardier varieties and develop resistant traits. By forcing individual species to evolve tolerance or die, it should then be possible to work out the long-term effects of high ozone levels.
The results of earlier research projects have hinted at the outcome. In the mid-1990s, Davison and his colleagues at Newcastle showed that the common plantain (Plantago major), an abundant weed that colonises paths and waste ground throughout northern temperate regions, develops tolerance to ozone.
However, back in the early days of the Industrial Revolution, researchers found that wallflowers couldn't survive in the sulphur-choked air of Leeds, a Yorkshire town that spearheaded the Industrial Revolution. Around the same time, Scots pine was wiped out in all the industrial cities in northern England, and rhododendrons struggled to grow in urban Manchester.
Ironically, says Davison, some plants benefited from the smog. Sulphur pollution eradicated a rose disease, black spot, and tar spot, which affects sycamore trees. The two diseases only returned when the air became cleaner. Hardy natives
In the 1970s and 1980s, pioneering work by Nigel Bell of Imperial College, London, showed that some grasses could handle pollution. "In 1971, we looked at perennial ryegrass (Lolium perenne) the most important pasture grass of temperate regions," he says. "All the native varieties had evolved tolerance to sulphur dioxide, and had displaced sensitive varieties."
Following the publicity surrounding acid rain damage to European forests at the time, clean air legislation helped to cleanse the air of sulphur. The focus then switched to ozone. During sunny weather, oxides of nitrogen combine with hydrocarbons emitted from vehicles and power stations to form ozone.
In 1992, as levels of the gas continued to remain worryingly high, Dave Karnosky of Michigan Technological University in Houghton identified enclaves of aspen in the US which had become resistant to ozone. In more recent, and still unpublished experiments, Karnosky isolated and cloned aspens that were either resistant or sensitive to ozone. Next, he planted mixtures of the two at sites of high, intermediate and low ozone pollution, to see how they would cope. His preliminary results, presented this May in Houghton at a conference on air pollution and forest damage, show that at the highest ozone level, the sensitive clones were 40 per cent more likely to die. Resistant clones at this level had trunks twice the diameter of their sensitive counterparts-indicative of their better health.
None of the previous studies reveals how quickly individual species of wild plants become completely resistant, and what impact this might have on rival species and biodiversity. "We just don't know what happens in wild populations," says Allen Heagle, professor of plant pathology at North Carolina State University. This is what the Great Smoky Mountains Park study may reveal.
The list of unanswered questions is long. Heagle has shown that if you expose individual clover plants to ozone, some will survive and some won't. But how would the resistance trait be carried forward in future generations of wild species? Would it be "diluted" through sexual reproduction with less resistant variants? In theory, the tougher strains should begin to dominate but, says Heagle, it's unclear how much the genes from weaker varieties will slow them down. "There's got to be some adapation, but in the real world you get dilution, so it could take a thousand years to show," he says.
And resistance might develop at the expense of other traits, such as colour or general appearance. "Plants might survive ozone, but look ugly," says Heagle. Bell of Imperial College agrees. "There are costs of becoming tolerant to a pollutant," he says. Bell notes that if the pollution disappears, the sensitive strains might re-emerge to outcompete the resistant strains, which may have to waste energy to defeat the pollution.
The Smoky Mountains research should bring some valuable new clues to what is going on. But however hardy wild plants turn out to be, the researchers stress the importance of continuing to reduce pollution levels. Bell's current research, for example, has revealed the horrendous damage to crops caused by ozone in Pakistan and India (New Scientist, 14 June 1997, p 11), and Davison's group has demonstrated that some crops in Europe are vulnerable. Karnosky agrees. "When we talk about tolerance, it's only in degrees, and they are still damaged, even when tolerant, with reduced health and vigour," he says.
Effects on human health, such as aggravation of asthma and other bronchial conditions, underline the need to curtail air pollution. But emergence of plants that cope with our pollution is a reminder that nature has a history of adapting to stress. "We should never stop reducing the amount of pollution, but things do adapt through natural selection," says Karnosky. "Forests are constantly changing, and what we see now is not what it was like as little as 100 years ago," he says.
Author: Andy Coghlan
New Scientist issue: 1st July 2000
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