Today's experiments help inform tomorrow's climate models.
When the Free-Air CO2 Enrichment, or FACE, site was planted with 6,000 sweetgum tree seedlings in 1988, Colleen Iversen was nine years old.
Today Iversen, a University of Tennessee graduate student and Department of Energy Global Change Education Program Fellow, is conducting research on the Oak Ridge National Laboratory site where the trees have been fed a diet of carbon dioxide at levels approaching those anticipated in the coming decades. Through FACE and other long-term experiments, Oak Ridge National Laboratory has become a leader in demonstrating the potential effects of global climate change on terrestrial ecosystems. By establishing experiments that manipulate environmental conditions associated with atmospheric and climate change, ORNL researchers are uncovering clues about how trees, plants and the soils that hold them could be affected.
"ORNL is a world leader in research on terrestrial climate change response," says Paul Hanson, an ORNL ecologist who has studied the effects of both drought and increased rainfall on eastern forests. "Terrestrial response to climate change is our bread and butter and is recognized as such."
From fields to forests, a variety of experiments at ORNL has enabled researchers to peek into a future characterized by higher temperatures, changes in levels of rainfall and higher concentrations of carbon dioxide in the atmosphere. Scientists can reasonably predict future levels of carbon dioxide and potential warming trends, and have conducted experiments to reveal how plants will respond. There remains, however, much uncertainty about how these changes will affect entire ecosystems, from the production of trees and plants to carbon storage to the availability of other nutrients in the soil. ORNL experiments are contributing to the fundamental understanding of how the components of ecosystems—from the wood and vegetation above the ground to the roots and microbes beneath—function as a whole.
"We started with very-small-scale studies in the growth chamber, and that transitioned to open-topped chambers in the field and ultimately to the FACE experiment," says Stan Wullschleger, who leads ORNL's plant molecular ecology group. "The group's research has evolved from small-scale, short-term studies to current, large-scale, long-term experiments designed to understand the physiology and the ecology of ecosystems."
Single factor experiments
FACE is one of the longest-running experiments at ORNL. For the past 10 years sweetgum trees on a 4.2-acre site have been exposed to elevated levels of carbon dioxide in an effort to determine what effect this greenhouse gas would have on a real-life ecosystem. ORNL is one of several FACE sites. Others include a loblolly pine stand located at Duke University; a desert plot in Nevada; aspen and birch stands in Wisconsin and a soybean field in Illinois.
In 2004, Oak Ridge National Laboratory published findings in the Proceedings of the National Academy of Sciences showing that increased carbon dioxide levels in the air cause trees to produce more fine roots and leaves. Uncertainty about the effect of CO2 on terrestrial plant life was an important limitation of climate change models being generated to predict the potential impact of a warming earth, says Rich Norby, an ORNL Corporate Fellow who oversees the FACE project.
"CO2 is the primary driver of the green-house effect," Norby says. "We always keep in mind that our experiment does not replicate the future. You can make big mistakes by trying to pretend the research is more than it is. The objective is an understanding that can be used to inform ecosystem models. We want the models to be informed by real-world observations that are realistic."
Now researchers including Iversen are focused on discovering what that means for the world of soil, water, roots and microbes underground. "We do not know a lot about the interchange between plants and soil," information that could provide answers to questions about the ultimate effect of carbon dioxide on the growth and decomposition of trees and on the effectiveness of soil in sequestering carbon, Iversen says. "Researchers refer to underground processes as the 'black box.'"
Another key piece of a changing climate is precipitation. While scientists do not yet know with certainty how levels of rainfall will be affected by a warmer world, Hanson has examined two possible scenarios for forests of the eastern United States and Tennessee in particular. In a study begun in 1993 and completed early last year, Hanson and his colleagues altered rainfall over plots of trees located on the acreage surrounding ORNL, providing some plots with a 33% increase in precipitation and others with a 33% decrease.
The researchers are now in the process of publishing the study's final conclusions. Hanson said the experiment showed that because of their deep, established root systems, large trees are highly resilient and drought tolerant. However seedlings are more likely to die in drought conditions.
"Future rainfall changes do have a potential impact, but the impact is not going to be one that we observe as a catastrophic change that produces a die-off of the existing eastern forests. In the absence of a major disturbance event, it would take about 100 years or so for changes in the forest to be observed," Hanson says. "Drought does, however, have the potential to change long-term composition of biodiversity in the forest."
Information from experiments like FACE and the precipitation projects help inform models that combine the results for a more comprehensive outlook. "We are dependent upon modeling results to project what the future might look like and therefore how society might prepare for climate change," Hanson says. "One of the things we've done is use modeling to combine results from single-factor manipulations into combined analyses of multi-factor effects. We have combined experimental lessons learned from precipitation change, ozone increase, nitrogen input, warming and elevated CO2 studies and run them through models to reveal a collective response. Those results reveal important interactions.
"If we just simulated a CO2 study, the data would predict a beneficial future. If we simulated only warming, we would find more negative impacts. But if you look at these factors together, they can mitigate or counter one another."
No Occam's razor
A newer experiment sitting adjacent to FACE's sweetgum stands takes a real-life look at changes that occur from multiple factors associated with climate change including carbon dioxide, temperature and precipitation. Because such conditions are not yet possible to reproduce in multi-acre-sized plots like FACE, ORNL researchers have set up a series of chambers known as the Old-field Community Climate and Atmosphere Manipulation experiment, or OCCAM—a tongue-in-cheek play on the "Occam's razor" principle that the explanation of any phenomenon should make as few assumptions as possible.
At OCCAM, 12 four-meter open-top chambers house a variety of plants found in Tennessee old fields including wildflowers, grasses and, more recently, a sprinkling of tree seedlings. The treated chambers receive four combinations of high or low carbon dioxide and higher or lower temperature—the higher temperature treatment is 3.5°C above ambient. Each chamber is divided in two, with one side receiving less moisture than the other.
Using sensors and a regimen of regular measurements, a collaboration of ORNL and University of Tennessee scientists is observing how these multiple factors change the phenology of the plants—such as production, flowering and leafing times; the chemistry and resulting impact on plants' decomposition; effects on systems beneath the soil such as nitrogen fixation and changes in the amount of particular species present.
"We have watched two of the plant species change significantly over the course of the experiment, with one becoming dominant and one declining for a while and then coming back," says Aimee Classen, a soil ecologist at ORNL and UT whose primary interest is in aboveground and belowground interactions. "We have also seen impacts on the nitrogen budget in our experiment. Researchers do not fully understand the phenomena, but it is very important. If plant carbon increases as a result of elevated levels of CO2, in order to maintain that plant growth you must have nitrogen. The question is, where is the extra nitrogen going to come from?"
Experiments such as OCCAM reveal both how little is known about future ecosystem response to climate change and the importance of future scenarios including multiple variables. "OCCAM is not straightforward, which is an appealing aspect of the project," Classen says. "What is particularly exciting about multifactor experiments is that they are difficult to interpret. We could simplify our system to produce seemingly clear answers. In the long run, however, that would tell us very little about a large and complex problem."—Larisa Brass
Web site provided by Oak Ridge National Laboratory's Communications and External Relations