By Carolyn Krause
The future may also bring higher temperatures and drought if global climate changes as predicted. So, it seems practical to raise fast-growing trees that not only provide fuel by capturing carbon from the atmosphere (helping to deter climate change) but also flourish under dry conditions.
It's not easy to tell a male willow from a female willow because each tree does not express its gender identity until it is 6 to 20 years old. However, Jerry Tuskan and Greg Roberts, researchers in ORNL's Environmental Sciences Division, in collaboration with Swedish scientists through the International Energy Agency, have found a potentially useful method for early gender identification in willows.
|Gerald Tuskan uses genetic engineering
techniques to determine the gender of a
hybrid willow tree in an early stage of
|Interior of a plantation of Swedish willow trees
at the State University of New York in Syracuse.
"We have identified a DNA marker for gender determination in hybrid willow trees," Tuskan says. "The marker is present in all female hybrid willow and absent in all males."
The marker will ultimately be used to isolate and characterize the DNA
sequence responsible for gender selection. Use of this DNA sequence should
greatly increase researchers' ability to identify highly productive
drought-resistant trees for biofuels.
One of the longest studies of a forest tree species in an atmosphere enriched in carbon dioxide (CO2) suggests that in this environment trees absorb airborne carbon more efficiently and that some of the absorbed carbon ends up being stored in soil around tree roots. The ORNL study examined the response of trees to additional atmospheric CO2) from increased energy production using fossil fuels. Elevated CO2) levels may cause global warming and possibly disruptive climate change.
This study addresses a variety of questions. How do forest trees respond to rising concentrations of atmospheric CO2) from increased fossil fuel combustion, forest burning, and other sources? Do they grow faster? Do their trunks and branches become larger than normal? Are their leaves larger or smaller? Do they grow more efficiently even when deprived of nutrients and water? Will their response to elevated levels of atmospheric CO2) be to absorb and store more carbon from the atmosphere, slowing global warming?
To help address these questions, ORNL has been conducting a series of long-term studies of a forest tree species in a CO2)-enriched atmosphere. The results of the study, which is sponsored by DOE's Global Change Research Program, were first reported in a letter to the May 28, 1992, issue of Nature by researchers in ORNL's Environmental Sciences Division. They are Richard J. Norby, Carla A. Gunderson, Stan D. Wullschleger, Elizabeth G. O'Neill, and Mary K. McCracken. The letter, entitled "Productivity and compensatory responses of yellow-poplar trees in elevated CO2)," was the third most cited 1992 scientific paper in ecology and environmental sciences, according to the January 1995 issue of Science Watch.
The increased fine-root production, Norby says, suggests that some of the additional carbon in the atmosphere may be eventually absorbed and stored in the soil rather than in trees. Fine roots, he adds, die and decay rapidly, but their residue is an important source of the carbon in soil. The possibility of increased carbon storage in soil is an important focus of DOE's new Terrestrial Carbon Processes Program, which aims at identifying and quantifying natural mechanisms of the terrestrial ecosystems that may affect trends in atmospheric CO2) concentration. Program scientists will develop the scientific understanding needed to model, predict, and quantify the role of terrestrial ecosystems in regulating the balance of global carbon.
"Our results suggested strongly that, because less leaf area is needed to sustain the same growth rate and because fine-root production increased, the yellow poplar trees grow more efficiently in a CO2)-enriched atmosphere," Norby says. "The increased productivity showed up as additional fine roots rather than in wood.
"The increased efficiency implies that these trees may be better able to withstand environmental stresses, such as drought, and survive even when nutrients are limited. Thus, they may last longer as a natural resource for removing carbon from the atmosphere and storing it."
ORNL scientists examine the response of yellow poplar and white oak trees grown in chambers whose atmosphere is enriched in carbon dioxide. They found increased productivity that showed up as additional fine roots rather than in wood.
The interaction of elevated CO2) with the stresses associated with predicted increases in air temperature is the current focus of the ORNL research group.
The results of the ORNL paper are sometimes incorrectly cited as evidence that trees will not grow bigger and faster in a CO2)-enriched atmosphere unless they are provided with additional nutrients, including fertilizing pollutants such as sulfates and nitrates from acid rain.
"That this interpretation is wrong is shown by the large response of white oak trees to elevated carbon dioxide levels at the same experimental site at ORNL," Norby says. "Both species increased their growth efficiency despite a deficiency of nutrients at the site. When our results were corrected for short-term experimental influences on leaf area, we found that both white oak and yellow poplar trees showed about a 35% increase in productivity in elevated carbon dioxide. These results suggest a potential increase in forest capture of carbon even when nutrients are limited."
The scientists reported the results of their 5-year study of 28 mature trees on the Oak Ridge Reservation in a March 1995 issue of the journal Nature. Authors of the article "Interactive effects of ambient ozone and climate measured on growth of mature forest trees" are S. B. (Sandy) McLaughlin, an ecologist in ORNL's Environmental Sciences Division, and Darryl J. Downing, a statistician in the Computing and Mathematical Sciences Division . Together they used statistical techniques to separate the effects of ozone from other factors. An article on the work also appeared in the March 21, 1995, issue of the New York Times.
Ground-level ozone forms when hydrocarbons and nitrogen oxides in the air react in the presence of sunlight. Sources of these chemicals include emissions from vegetation, fossil-fuel power plants, and highway vehicles.
In the 1980s, declines in growth of loblolly pine trees in the Southeast were observed. This decrease aroused concern because loblolly pine is an important component of southern pine forests. Logging of southern pine forests, which cover an estimated 60 million acres, contributes $4.5 billion to the regional economy annually.
The growth of the trunk of each tree in the experiment was monitored over 5 years using a dendrometer. This instrument consists of a metal band positioned around the stem that precisely measures fluctuations in the stem's rate of expansion.
"When ozone levels in the air remained about 40 parts per billion, as happens almost every day in the eastern United States, tree growth was reduced," McLaughlin says. "The decline was more pronounced under drying conditions. When ozone levels were especially high, we found that the stems actually contracted as water became very limited. Ozone seems to multiply the adverse effect on trees of lack of moisture."
Evidence from other studies, McLaughlin notes, suggests that tree exposure to ozone can lead to increased water losses through foliage and lower uptake of water (because of reduced root growth). As a result, ozone may impair a tree's ability to efficiently use water available for growth.
Thanks to a wide range of temperature, rainfall, and other weather conditions, the scientists were able to measure differences in growth reduction for periods of both high and low moisture. In 1988, the driest year of the study, high ambient ozone levels reduced tree growth by an average of 13%. In the wettest years, 1989 and 1992, little growth reduction occurred. The average growth reduction attributed to ozone for 5 years was 5%.
"Predicted results of future climate change from some computer models," McLaughlin says, "include periods of increasing temperatures and drought. Our results suggest that trees exposed to ozone under these climatic conditions would experience more significant growth reductions."
Studies at ORNL by Rich Norby and others have shown that elevated levels of atmospheric carbon dioxide may increase productivity of yellow poplar and white oak trees, at least in terms of increased root growth. But if the levels of this greenhouse gas are high enough to cause significant warming of the climate and drought, the McLaughlin-Downing study of loblolly pines suggests that overall growth rates of some trees may be reduced, not increased, because regional ozone levels are also projected to increase. Of course, ORNL scientists note, these studies must be repeated for other species of trees, under various conditions such as reduced availability of water and nutrients, to better understand the detailed effects on trees of elevated levels of carbon dioxide and ozone.
To answer this question, scientists in ORNL's Environmental Sciences Division are conducting a precipitation experiment in a forest on DOE's Oak Ridge Reservation.
Paul Hanson checks the plastic troughs used in ORNL's "throughfall displacement
experiment" at the Walker
Branch Watershed on DOE's Oak Ridge Reservation.
Aerial view of the troughs near ORNL used to ensure thattrees in various plots of a predominantly oak forest receive different amounts of soil moisture from rain and snow.
In this "throughfall displacement experiment" conducted at the Walker Branch Watershed on DOE's Oak Ridge Reservation, trees in various plots of a predominantly oak forest receive different amounts of soil moisture from rain and snow. One-third of the precipitation falling through the trees is intercepted by troughs, preventing the water from reaching the soil. The captured water flows by gravity from the "dry" treatment area to the "wet" treatment area, where it provides additional moisture to the soil. This project is believed to be the world's largest experimental manipulation of an oak forest (which also has maple and dogwood trees).
"We found that the experimental system can produce statistically significant differences in soil water content in years having both extremely dry and extremely wet conditions," says Paul J. Hanson, an ESD project researcher. "The experimental design produces soil water changes without affecting temperatures of the soil and forest floor."
Project researchers had predicted that oak trees would be more resistant to drought than maples for two reasons: oaks have longer roots that can reach deep water supplies, and oaks maintain or accumulate high concentrations of water-soluble compounds in leaves, making them more drought tolerant.
"Large differences in drought tolerance were observed between dogwoods and chestnut oaks even in the first year of the study with above normal rainfall," says Timothy J. Tschaplinski, another project researcher. "The oaks were better able to withstand dry conditions.
After almost two years of the experiment, the researchers found no differences in growth responses for mature oaks and maples. However, in the forest undergrowth the leaves and stems of maple and dogwood saplings were found to grow 45% faster in wet areas than in dry ones.
"Our goal," says Hanson, "is to understand how climate change that may result from increasing emissions of greenhouse gases will affect forests. In this way, we will be able to better assess the impacts of greenhouse gas emissions."
What was the dominant cause of this shift? Climate change, increases in atmospheric carbon dioxide, or a combination?
"Studies of Antarctic ice cores indicate that atmospheric carbon dioxide increased rapidly during this period," Cole says. "Also, evidence for changes in temperature and dryness are recorded in pack rat remains and in reconstruction of the earth's surface features, such as ancient lakes."
The photograph (above) and satellite image (below) show alluvial fan deposits
the Organ Mountains in New Mexico. The view in the photo is looking roughly
to the west. In the geologic record in these deposits, an ORNL geochemist found
stable isotopic evidence for a vegetation shift 7000 to 9000 years ago that
resulted principally from an increase in atmospheric carbon dioxide.
To help determine the dominant cause of the vegetation shift, the researchers measured the ratios of isotopes in the desert's soils. In a letter published in the April 7, 1994, issue of Nature, they report that the isotopic signature in the geologic record pins the vegetation shift primarily on increased atmospheric carbon dioxide levels, not climatic change.
The scientists measured the ratios of carbon-13 to carbon-12 and of oxygen-18 to oxygen-16 in carbonates from alluvial fan deposits in the Chihuahuan Desert. They found that the carbon isotope ratios in the geologic record changed, revealing a shift from grass-dominated to shrub-dominated vegetation some 7000 to 9000 years ago. They cited studies indicating that trees and shrubs have a competitive advantage when carbon dioxide levels rise significantly.
They also observed that the oxygen isotope ratios were relatively constant. "Oxygen isotope ratios depend on temperature and moisture," Cole says. "Because these ratios remained fairly constant in that period, we concluded that atmospheric carbon dioxide change, rather than climate change, was the dominant cause of the vegetation shifts."
The research results suggest that stable isotope ratios in well-preserved unaltered alluvial-fan soils could be useful indicators of global carbon-dioxide change. Thus, isotope ratios in the geologic record could fill gaps in knowledge about recent and ancient levels of atmospheric carbon dioxide and their relationship to vegetation and climate. Such information could be used to improve the ability of global climatic circulation models to predict future climate accurately.
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