Carbon is the chemical foundation of all living things. Animals consume carbon indirectly – either by eating other animals or by eating plants. However, plants absorb carbon (in the form of carbon dioxide, or CO2), directly from the air.
“The global atmospheric concentration of CO2 has been reported to have risen by 35 percent over pre-industrial levels and is expected to continue to rise through the end of the century,” says U.S. Forest Service Southern Research Station (SRS) scientist Thomas Eberhardt, now a project leader at the Forest Products Laboratory. “The amount of CO2 available to plants can change the chemical makeup of their tissues.”
Several studies have examined how increased CO2 affects tree leaves and wood, but Eberhardt and his colleagues studied the effects on sweetgum bark. Their study was recently published in the journal Trees.
A tree’s bark is its skin and its circulatory system. Although the outer bark is not living, it seals in moisture and protects the living parts of the tree from insect attacks, fire, and other threats. The inner bark is living, and transports nutrients. Changes in the chemical composition of either the inner or the outer bark could have a range of ecological effects. “Changes could affect how well living trees can resist insects, diseases, and fires,” says Eberhardt. “They could also affect how long it takes litter or dead trees to decompose, and this could alter the cycle of nutrients in the soil.”
Eberhardt studied sweetgum on a plantation established in 1988 in eastern Tennessee as part of the Oak Ridge National Laboratory Free-Air CO2 Enrichment experiment. Since 1998, towering rings of white plastic tubes pumped extra carbon dioxide into two of the five study sites. In 2009, after 12 years of treatment, 10 trees were harvested.
Eberhardt and his colleagues separated the inner and outer bark for analysis. The scientists measured the amount of trace metals; extractives, which are organic compounds that dissolve in water or other liquids; main structural components such as lignin; and ash content, which describes how much of the bark’s dry weight is from inorganic compounds.
Sweetgum trees grown in elevated CO2 had a number of changes in the chemistry of their inner and outer barks. The ash content in their inner bark was 33 percent higher than ash content of trees grown under ambient CO2 concentrations. “The biological and ecological consequences of such a dramatic increase are not known,” says Eberhardt. “However, increases in ash content tend to make it harder to use wood for producing bioenergy, lumber, paper, and other items.” The inner bark also had more extractives and higher amounts of trace metals, especially manganese. Researchers also saw changes in the outer bark, which had lower levels of nitrogen.
“The chemical composition of trees’ bark influences forest ecology as well as how people can use and harvest them,” says Eberhardt. “The ever increasing amount of CO2 in the atmosphere affects trees in ways we’re just now beginning to study.”
For more information, email Thomas Eberhardt at firstname.lastname@example.org.