Hybrid American Chestnut Trees Are Carbon Storage Champions

Douglass Jacobs, an associate professor of forestry and natural resources, found that American chestnuts grow much faster and larger than other hardwood species, allowing them to sequester more of the heat-trapping greenhouse gas than other trees do over the same time period.

"Maintaining or increasing forest cover has been identified as an important way to slow climate change," said Jacobs, whose paper appears in the June issue of the journal "Forest Ecology and Management."

"The American chestnut is an incredibly fast-growing tree," Jacobs said. "Generally the faster a tree grows, the more carbon it is able to sequester. And when these trees are harvested and processed, the carbon can be stored in the hardwood products for decades, maybe longer."

Purdue scientist Douglass Jacobs examines a two-year-old chestnut tree at an experimental farm near the university. (Photo courtesy Purdue)

Once a dominant forest species throughout much of the eastern United States, the American chestnut grew from Maine to Mississippi, treasured for its annual yield of nuts, rot-resistant wood and towering size.

But at the beginning of the 20th century, an introduced Asian fungus caused widespread blight that killed chestnut trees across their range. About 50 years ago, the species was nearly gone.

New efforts to hybridize remaining American chestnut trees with blight-resistant Chinese chestnuts have resulted in a species that is about 94 percent American chestnut with the protection found in the Chinese species.

Jacobs said those hybrid new trees could be ready to plant in the next decade, either in existing forests or in former agricultural fields that are being returned to forested land.

"We're really quite close to having a blight-resistant hybrid that can be reintroduced into eastern forests," Jacobs said. "But because American chestnut has been absent from our forests for so long now, we really don't know much about the species at all."

Jacobs studied four sites in southwestern Wisconsin that were unaffected by the blight because they are so far from the tree's natural range.

He compared the American chestnut with black walnut and northern red oak trees at several different ages, and also cross-referenced his results to other studies using quaking aspen, red pine and white pine in the same region.

In each case, the American chestnut grew faster than the other species, growing as three times more aboveground biomass than the other species at the same point of development. American chestnuts can grow to 120 feet tall. One tree in North Carolina had a trunk diameter of 17'6".

American chestnut trees also sequester more carbon than the others. "Each tree has about the same percentage of its biomass made up of carbon, but the fact that the American chestnut grows faster and larger means it stores more carbon in a shorter amount of time," Jacobs said.

Since American chestnut trees are most often used for high-quality hardwood products such as furniture, they hold the carbon longer than wood used for paper or other low-grade materials.

Jacobs said trees absorb about one-sixth of the carbon dioxide emitted globally each year. Increasing the amount that can be absorbed annually could make a considerable difference in slowing climate change, he said.

"This is not the only answer," Jacobs said. "We need to rely less on fossil fuels and develop alternate forms of energy, but increasing the number of American chestnuts, which store more carbon, can help slow the release of carbon into the atmosphere."

Jacobs said that since this study looked at aboveground carbon sequestration, future studies would seek to understand more about how forests that contain American chestnuts store carbon below the ground.

The roots of American chestnuts are vulnerable to new exotic pests. Blight-resistant hybrids have already proven susceptible to Phytophthora cinnamomi, or root rot, which preys upon tree roots in wet, southern soils.

"This threatens to be almost as bad as the fungal blight," Jacobs said. "In the future, we may need to select for this resistance in new hybrids. Luckily, the Asian chestnut shows some resistance to this fungus as well, although the breeding process would take a long time."

One of the greatest obstacles to reintroduction of the American chestnut is the host of laws and regulations that now govern the lands in the chestnut's original range, Jacobs said.

On many public lands where the chestnut used to thrive, such as the Great Smoky Mountains National Park, human interference is discouraged and often illegal. But Jacobs said some interference and harvesting will be necessary to reintroduce the chestnut, calling for a unified and proactive approach and exceptions to certain laws that govern public lands.

The Stry Foundation, Electric Power Research Institute, and Purdue University's Hardwood Tree Improvement and Regeneration Center funded the research.

Copyright Environment News Service (ENS) 2009. All rights reserved.