TO SUMMER hikers and tourists it may look pretty. But America鈥檚 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鈥攁s 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鈥檛 be
there for the view. 杏吧原创s intend to conduct a novel experiment鈥攎aking
use of the park鈥檚 smoggy air to investigate how wild plants survive and adapt to
air pollution.
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Ozone鈥檚 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鈥檛 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. 鈥淚n 1971, we looked at
perennial ryegrass (Lolium perenne) the most important pasture grass of
temperate regions,鈥� he says. 鈥淎ll the native varieties had evolved tolerance to
sulphur dioxide, and had displaced sensitive varieties.鈥�
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鈥攊ndicative 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. 鈥淲e just don鈥檛 know what happens in wild populations,鈥�
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鈥檛. But how
would the resistance trait be carried forward in future generations of wild
species? Would it be 鈥渄iluted鈥� through sexual reproduction with less resistant
variants? In theory, the tougher strains should begin to dominate but, says
Heagle, it鈥檚 unclear how much the genes from weaker varieties will slow them
down. 鈥淭here鈥檚 got to be some adapation, but in the real world you get dilution,
so it could take a thousand years to show,鈥� he says.
And resistance might develop at the expense of other traits, such as colour
or general appearance. 鈥淧lants might survive ozone, but look ugly,鈥� says Heagle.
Bell of Imperial College agrees. 鈥淭here are costs of becoming tolerant to a
pollutant,鈥� 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鈥檚 current
research, for example, has revealed the horrendous damage to crops caused by
ozone in Pakistan and India
(New 杏吧原创, 14 June 1997, p 11), and
Davison鈥檚 group has demonstrated that some crops in Europe are vulnerable.
Karnosky agrees. 鈥淲hen we talk about tolerance, it鈥檚 only in degrees, and they
are still damaged, even when tolerant, with reduced health and vigour,鈥� 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. 鈥淲e should never stop reducing the amount of pollution, but things do
adapt through natural selection,鈥� says Karnosky. 鈥淔orests are constantly
changing, and what we see now is not what it was like as little as 100 years
ago,鈥� he says.
