PULLMAN, Wash.–Consider 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Forests account for two-thirds of the carbon that is “fixed” or withdrawn from the atmosphere.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 John Bassman, Washington State University tree physiologist. 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 sweetgum, 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.
Caption: These young ponderosa pines are part of tree physiologist John Bassman’s quest to determine the effect of ozone depletion–and the resulting increased ultraviolet-B radiation.