Scientists working in the area do know one thing: It’s not going to be as simple as “more trees, more carbon sequestered.” And there are other, maybe more pressing questions: How will the forests we rely on change in response to climate conditions? Can forest management play a part in adapting forest ecosystems to climate change?

Team leader and research physiologist Kurt Johnsen and fellow scientists with the SRS Southern Institute of Forest Ecosystems Biology Team have spent decades developing innovative approaches to answering questions like these on study sites across the South, experimenting with methods that range from sophisticated electronics and genetics to getting down in the dirt and digging up roots.

FACE It

“We know for sure that atmospheric carbon dioxide is increasing,” says Johnsen on a recent trip to the freeair CO₂ enrichment (FACE) site in the Duke Forest near Durham, NC, where he and SRS research biological scientist Chris Maier chart the physiological effects of increased CO₂ on loblolly pine trees. “That’s intriguing for those of us interested in forests, because CO₂ is the basis for photosynthesis. The first question we want to address is how these increased levels affect how trees operate.”

“Then there’s the related question about carbon sequestration and the opportunity to slow down the rate that CO₂ is increasing by managing forests,” continues Johnsen. “To be able to answer those questions, we need to be able to quantify how much carbon is sequestered in different parts of the tree, and what happens when atmospheric CO₂ increases.” That’s where FACE comes in.

From above, the FACE site looks like a circle of silver tubes poking up above a sea of loblolly pines. The tubes release CO₂ over the tops of the trees through computer-controlled valves, the output automatically adjusted to account for the ambient movement of air. Down on the ground, you can see that many of the tree stems are cuffed and wired; tangles of cables crisscross the forest floor among litter baskets and soil probes. The cables connect up to big red tool boxes that carry ACES, a patented system designed by SRS plant physiologist John Butnor to measure the carbon coming off tree trunks and soil. There’s a constant sound of rushing air on the FACE site; Johnsen and Maier have to raise their voices to be heard.

Pointing to a tree cuff, Johnsen gives a simplified version of what goes on out here. “This cuff is measuring how much CO₂ or carbon is coming off the tree stem. What we’re doing here is breaking down the carbon cycle in trees into smaller parts to get a mechanistic understanding of how carbon is cycled through the forest. We’re also looking at how fertilization affects growth and carbon sequestration under heightened CO₂.”

This brings up the potential importance of loblolly pine in carbon sequestration. Globally, it’s the species most widely used for plantation forestry. Because loblolly pine plantations are so extensive and grow so rapidly, they have great potential for sequestering atmospheric carbon. “Loblolly pine plantations are relatively simple ecosystems, and we already know a lot about this species,” says Johnsen. “This means we’re able to quantify the carbon dynamics for these stands relatively easily, which helps us develop the tools and protocols to measure other types of stands.”

Johnsen stresses that results from one site don’t easily translate to another. “This is why we study how the different components of the carbon cycle respond to the environment,” he says. “The varying responses are then combined to create models to estimate what will happen on other sites. This also provides us the tools to develop forest management to optimize the amount of carbon that’s being sequestered in products such as timber—or stored below the ground in root systems.”

Belowground, Carbon Banks

The soil at the FACE site in the Duke Forest is very rocky, so it’s been almost impossible to measure tree roots using nondestructive probes and ground penetrating radar. Most methods used to analyze carbon storage in taproots—the main roots that usually grow straight down—require digging them up, but researchers are not ready to cut down the pines on the FACE site. Fortunately, a project in the Coastal Plain of South Carolina with forest industry partner MeadWestvaco has provided a timely opportunity to study the role tree taproots play in carbon sequestration.

Named for a nearby town, the Cross Carbon study was set up 4 years ago to look at whether adding the organic matter left after harvesting on a pine plantation could raise soil carbon levels and increase productivity on the wet, sandy soils of the Coastal Plain. When the study started, the trees on the plantation site were at the end of their rotation, so SRS researchers were able to take measurements before and just after harvest. Since it’s a pulpwood site, harvest consists of chipping on site for transport to a mill. Typically, the residue—branches and limbs—are pushed aside into large piles. The study looks at whether adding different levels of this organic material back into the soil will cause trees to grow faster and bigger.

It’s also been a good opportunity to look at what happens to carbon in root systems.

“Better growth directly relates to carbon sequestration, and not just because of the carbon that’s sequestered in tree stems,” says Maier. “The faster you grow and harvest trees on this site, the more taproots you leave in the soil. We’ve come to see the taproots—which can take 10 to 20 years to decompose—as pockets of residual carbon that can provide the added benefit of short-term carbon sequestration to intensively managed pine plantation sites.”

To track what happens to carbon as the roots decompose, Maier and Johnsen went in and mapped all the taproots after the stand was cut, marking them with nails so they can be easily relocated using a metal detector. They’ll keep sampling the roots over the next decade.

Clonal Advantages

With 21 plots on approximately 15 acres, the Cross Carbon study is one of the largest of its kind, but what makes it even more important is the fact that all the trees planted on the site are clones. Because clones are genetically identical, researchers can be sure that the results they get from adding different levels of organic material are related to the treatments rather than to genetic differences in the trees themselves.

And there’s more. There are actually two different loblolly pine tree clones grown on the Cross site, and the difference between them—one is tall and narrow, and carries about half the foliage of the other—may help scientists answer questions about how to adapt forests quickly to changing climatic conditions or establish forests on degraded sites.

“We suspected that the trees with just half the leaf area would grow just as fast as the others, which has been the case so far,” says Johnsen. “These trees will be less affected by lack of nutrients, easier to grow in problematic conditions. Knowing more about how individual clones grow and respond to certain conditions will help land managers select trees that will grow better in specific situations.”

Johnsen wants to take it even further, towards developing ideotypes—multitrait characterizations of trees—which managers can use to create specific products such as timber, bioenergy stocks, or carbon sequestration benefits—or to restore forests to sites with specific conditions.

“In this case, the tree with the narrow crown and less leaf area is an ideotype that we think may be useful for sites that have less nutrition,” says Johnsen. “It might also be useful for short-term biofuels plantations because you could plant the trees close together and cut them in shorter rotations.” But it may be the other clone that holds the key to processing wood into biofuels, an alternative that might help reduce the use of carbonemitting fossil fuels in the not-sodistant future.

Breaking down lignin, the binding agent that holds cells in wood together, is what makes processing wood into the slurry for both pulp and biofuels expensive both financially and ecologically, since the process often involves using toxic chemicals like dioxin. The second clone planted at the Cross Carbon site has a natural mutation in the gene that controls the last step of lignin production. “We think the lignin produced in this tree will be more easily broken down, using less energy to convert it to biofuels,” says Johnsen.

Adaptation is Key

“We’re a society that relies on forests, and not only for products,” says Johnsen. “If forests start changing rapidly, we need to be prepared to do something to keep ecosystems intact— plant different species, for instance. There’s a need to develop management systems that can respond fairly rapidly. This is where clonal forestry comes in, making it possible to quickly get a forest planted on a site, even though it may not be the forest you ultimately want.”

Coming up with ways to select tree species and varieties that will adapt under rapidly changing conditions led Johnsen back to a long-term experiment in Mississippi and a new collaboration with SRS genetics researchers.

The Harrison Experimental Forest, just north of Gulfport, MS, is also home to the SRS Southern Institute of Forest Genetics (SIFG), where project leader Dana Nelson, who is also project leader of the larger SRS Forest Genetics and Ecosystems Biology unit, directs studies that range from sequencing the loblolly pine genome to measuring the response of clonal varieties to different environmental conditions. Experimental plots were established in 1960 on the Harrison to test responses over time of three pine species— longleaf, loblolly, and slash—to four different levels of fertilization put in the second year of the project. The most interesting result is that the onetime fertilization seems to have made all the difference on the gulf coast site, with effects persisting for over 45 years. Katrina turned up another interesting result.

“Along the gulf coast, longleaf pine was largely replaced by loblolly because it was thought to grow faster, and it does in short rotations,” says Johnsen. “At the Harrison, we’ve been able to show that after about 25 years, longleaf catches up with loblolly—and Katrina showed us that longleaf is a lot less vulnerable to hurricane damage. So we know that for that area, longleaf pine will keep carbon out of the atmosphere a lot longer than loblolly pine, especially if hurricanes become more frequent. This is important information for land managers deciding how to replant hurricanedamaged areas.”

Johnsen and Maier have teamed up with SIFG researchers to install a new version of the pine studies at Harrison. Before harvesting most of the old study plots, researchers will go in and dig up taproots for analysis, following their decomposition over time. Then they’ll replant the plots, partially with seeds collected from the site and partially with genetically improved stock. They want to test whether the improvements in site quality from that one-time fertilization 47 years ago persist into the next generation, and whether trees can be designed to take even better advantage of a relatively simple treatment.

It’s about growing trees better for meeting the goals of landowners. “I don’t think there will be many cases where people will manage forests just for carbon sequestration,” says Johnsen. “But it could definitely provide a benefit—along with ecosystem restoration, recreation, wildlife habitat, and income from timber—to landowners looking for incentives to keep their land in forests.”

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For more information:
Kurt Johnsen at 919.549.4012 or kjohnsen@fs.fed.us
Chris Maier at 919.549.4072 or cmaier@fs.fed.us

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Collaborators include: Duke University, Boston University, the Forest Service Rocky Mountain Research Station, and Colorado State University