Duke Forest Carbon Experiments
The Duke Forest near Durham, NC, is home to a range of free-air carbon enrichment (FACE) experiments that are helping to answer questions about how forests adapt to rising levels of atmospheric carbon dioxide, and the effects of those adaptations on how and where trees store, or sequester, carbon.
In the mid-1920s, Duke University started buying small farms and woodlands as a buffer for its new campus and for possible expansion. In 1931, a total of 4,696 acres were placed under the stewardship of Dr. Clarence Korstian, dean of the Duke School of Forestry, as the Duke Forest. Over the years, using income from forest products, the university purchased more acres; today the Duke Forest covers over 7,000 acres in six main tracts located in Alamance, Durham, and Orange Counties.
The early objectives for the Duke Forest were to demonstrate practical and economical techniques for managing timber, develop an experimental forest for research in the sciences associated with growing timber, and provide an outdoor laboratory for forestry students. Since the 1990s, research on the Duke Forest has broadened beyond forestry to encompass a variety of disciplines in the natural and environmental sciences. Today, the Duke Forest is nationally recognized as a premier facility for environmental science research, with projects sponsored by the Forest Service, U.S. Department of Energy, National Science Foundation, and National Aeronautics and Space Administration.
Fertilization and Carbon Sequestration
Duke Forest’s free-air CO2 enrichment (FACE) site is internationally known for experiments on carbon sequestration and effects of climate change. The first FACE ring (the prototype) was installed, along with a control, in 1994 in a loblolly plantation area of the Duke Forest. Since 1994, six additional rings have been installed. From above, a FACE plot looks like a circle drawn onto an expanse of loblolly pine trees; the circle is ringed with tubes that release CO2 into the forest canopy through computer-controlled valves, the output automatically adjusted to account for the ambient movement of air. The CO2 levels are set at about one and a half times today’s levels to approximate the levels predicted for the year 2050.
SRS team leader and research physiologist Kurt Johnsen and fellow scientists Chris Maier and John Butnor with the SRS Southern Institute of Forest Ecosystem Biology Team began experiments on carbon cycling and fertilization on the first prototype plot in the 1990s. The studies they conduct with Duke researchers Ram Oren, Heather McCarthy, and others have become the longest running forest experiment in the world using FACE technology.
In 1998, to begin evaluating whether future forest growth and carbon sequestration is dependent on site fertilization, the researchers inserted a barrier across both the prototype plot and its control, fertilizing half of each twice every year with ammonium nitrate. Results from these studies have provided important information about what will happen to pine plantations under elevated CO2—and how they could be managed under those conditions for both timber and carbon sequestration benefits.
“Because this is the longest running elevated CO2 experiment on forest trees, we’ve been able to really look at the interaction between soil nutrition and CO2 in terms of growth and processes in loblolly pine,” says Johnsen. “Our findings have clear implications for the use of pine plantations for short-term carbon sequestration as well as broader implications about the role of forests as carbon sinks.”
On a single plot, researchers are able to show what happens under elevated CO2 with and without enrichment of the nutrient-poor soil that’s the norm across most of the South. As you might expect, growth in the unfertilized halves starts to slow and carbon storage drop under the elevated CO2 conditions predicted for the future. Interestingly, it is belowground where carbon starts to be lost. Trees fare much better in the fertilized halves of the plots.
In 2005, recognizing the importance of the nutrient studies on the prototype plot, researchers with studies on the remaining Duke Forest FACE facilities decided to partition their plots and fertilize half of each. Along with research on direct responses to elevated CO2, scientists using the FACE plots continue to gather data about tree physiology that is, in turn, incorporated into models to predict forest responses to both environmental and management changes.
Narrowing It Down
In 2006, SRS and Duke collaborators published findings that provide a more precise understanding of what happens to the ability of trees to sequester carbon under elevated CO2. They found that trees can only increase wood growth from elevated CO2 if there is enough leaf area to support that growth. Leaf area, in turn, is limited by soil nutrition; without adequate soil nutrition, trees respond to elevated CO2 by transferring carbon belowground then recycling it back to the atmosphere through respiration.
“With sufficient soil nutrition, forests increase their ability to tie up, or sequester, carbon in woody biomass under increasing atmospheric CO2 concentrations,” says Johnsen. “With lower soil nutrition, forests still sequester carbon, but cannot take full advantage of increasing CO2 levels. Due to land use history, many forests are deficient in soil nutrition, but forest management—including fertilization— can greatly increase growth rate and wood growth responses to elevated atmospheric CO2.”
The researchers further tested their hypotheses using data from FACE sites in Wisconsin, Colorado, and Italy. In the 2006 articles, the scientists identify critical areas for further study, but the overall consistency they found across these diverse forests bodes well for developing accurate models to predict the ability of the world’s forests to sequester carbon.
Along Comes an Ice Storm
On December 4, 2002, the Research Triangle area of North Carolina was hit with an ice storm that pulled power lines to the ground and left thousands from Raleigh to Durham in the dark. In the Duke Forest, planted pines took a beating. While young trees were flexible enough to bend without breaking and most older trees (over 30 years) were strong enough to take the weight of the ice, middle-aged pines snapped off at the top, their broken limbs littering the forest floor. The Duke Forest resource manager estimated it would take over $55,000 to clean up the mess.
On the FACE site, some 30 percent of the trees in the experimental plots were damaged. “At first we were quite dismayed by this ice storm because we thought it had wrecked our experiment,” says Johnsen. “To adjust our data, we needed to quantify how much destruction had taken place, so we pulled everything back to the lab and started measuring.”
What they found was a surprising pattern in the destruction. “The trees under high CO2 had consistently less damage from the ice storm than those in ambient conditions,” says Johnsen. “There were fewer damaged trees, and less damage per tree. We still don’t know exactly why, though I suspect it’s because they have bigger branches. We won’t be able to tell until we harvest the trees and measure them.”
The results may provide an important clue about what will happen to loblolly pine under global warming. “Freezing weather limits the current range of loblolly pine, since the tree is susceptible to damage from cold and ice,” says Johnsen. “We’re seeing that elevated CO2 increases the production of seed and cones in loblolly pine. If it also decreases damage from ice storms, loblolly pine could start migrating north under climate change conditions.”