 onsider these few facts:
- Forests occupy about 31 percent of the Earth's land area.
- Forests make up over 90 percent of the Earth's biomass.
- Forests account for two-thirds of the carbon that is
"fixed" or withdrawn from the atmosphere. Forests thus play a
major role in how much carbon is free in the atmosphere, which in turn
affects the magnitude of the "greenhouse effect."
- Forests regulate not only the flow of water, but local,
regional and global climate.
 Now add another fact: We know next
to nothing about what effect increased ultraviolet-B radiation will have
on forests as the stratospheric ozone shield continues to disintegrate over
the next century. Also, since global processes do not operate in isolation,
how will the UV-B effect on forests affect their ability to cope with anticipated
global warming?
Most research on the effect of increased
UV-B on plants has been done on annual plants, such as crop plants, says
tree physiologist John Bassman. But trees are much different in their relationship
to increased UV-B.
The most obvious difference is their
longevity and the resulting increased exposure to UV-B radiation. With conifers,
a single needle can stay on the tree and be exposed to UV radiation for
up to 20 years. Another difference is trees' annual dormancy and their overall
exposure to greater environmental extremes. Also, their large size results
in considerable physiological complexity, such as the transport of water
from its roots to leaves far above the ground.
Finally, whereas an annual plant
might be able to adapt to climatic change, a tree is slow to adapt because
it is so slow to respond genetically.
Although public perception of increased
UV-B radiation has been diverted lately by global climate change, the problem
has not gone away. In fact, even if ozone-depleting emissions were halted
immediately, the detrimental gases already in the stratosphere break down
slowly. Scientists estimate that their effect on the ozone layer could continue
for another 100 years.
So what effect, ask Bassman and others,
will the resulting enhanced UV-B exposure have not only on individual trees
but on forest ecosystems?
Studies on agricultural species have
shown that about 60 percent are at least moderately sensitive to high levels
of UV-B radiation. Among other effects is a lower rate of photosynthesis.
One of Bassman and his colleagues'
primary interests is what effect UV-B might have on RUBISCO, or "ribulose
1,5-bisphosphate carboxylase oxygenase." Ultraviolet-B radiation affects
many important proteins, including DNA and RNA. RUBISCO is not only the
most abundant protein on Earth, it is the primary enzyme responsible for
capturing carbon dioxide from the atmosphere.
Based on work that's been done on
crop and herbaceous plants, Bassman and others believe that increased carbon
dioxide and global warming will offer a buffer against UV-B damage to a
certain extent. Increased carbon dioxide can enhance plant growth.
"But other things associated
with that make the problem less than straightforward," says Bassman.
From the broadest possible perspective, he continues, carbon dioxide is
going to have a positive effect at least on physiology. But combine that
with the negative effect of UV-B radiation on photosynthesis and the result
is far from certain.
One thing Bassman worries about is
whether the increased UV-B radiation will change carbon allocations within
trees. They may have to put more of their photosynthetic products into protective
mechanisms at the expense of growth.
There could be more severe direct
effects, also, says Bassman. But considering the role of trees in regulating
atmospheric carbon, even small effects could in turn have large effects
on climate change. One earlier series of studies on loblolly pine showed
that enhanced radiation caused a 20 percent decrease in biomass. Another
study on sweet gum, however, resulted in no reduction in biomass, even though
it did affect the rate of leaf elongation. As with other plants, the effect
of increased UV-B seems to vary from species to species.
Along with Gerald Edwards and Ron
Robberecht, Bassman has begun a project to gather more information on the
effect of enhanced UV-B radiation on trees. But doing so is not a simple
matter. In fact, one reason so little is known is the difficulty in exposing
trees to measurable amounts of UV-B radiation.
Ambient UV-B exposure varies constantly.
Clouds, the angle of the sun, and the density of the surrounding canopy
all affect how much radiation a tree is receiving.
Only three or four studies across
the country are attempting to mimic the natural environment outside the
greenhouse. Bassman has rigged up a system that allows him to measure the
UV-B output of the sun. It tracks the output second by second, then supplies
multiples of that amount of UV-B to the trees, simulating natural exposure
to enhanced levels of radiation. So if a cloud goes over the sun, the lamp
levels correspondingly go down. As the cloud passes, the light level goes
back up. The trees are subjected to the amount of extra UV-B caused by a
25 percent reduction in stratospheric ozone and a 50 percent reduction.
Bassman and his colleagues are examining
the effect on four species: poplar, red oak, ponderosa pine, and Douglas
fir. They will consider UV-B's effect on a number of processes: growth and
biomass distribution, carbon uptake, carbon allocation and its partitioning
into various chemical fractions, and leaf development, anatomy, morphology
and aging.  | 
wsu.edu.