Replacing Grass With Trees May Release Carbon

By Cat Lazaroff

WASHINGTON, DC, August 8, 2002 (ENS) - Previous estimates of the amount of carbon stored by trees and shrubs may have been too high, suggests a new study released today. The research could force climate experts to recalculate the benefits of growing trees as a way to offset human caused emissions of carbon dioxide, a greenhouse gas linked to global warming.

The study by researchers from four universities explored whether the trees and shrubs now encroaching on former U.S. grasslands are helping to mop up some of the carbon dioxide emitted by vehicles, power plants and other sources. The team concluded that in many locations, the trees may be absorbing less carbon than what is emitted by soil once covered with grasses.

Replacing native grasslands with tree farms, like these hybrid cottonwoods being grown for the pulp and paper market in Washington state, may actually increase the release of carbon to the atmosphere. (Photo by Warren Gretz, courtesy National Renewable Energy Laboratory)

The researchers said their results may force a revision of previous carbon storage estimates, such as those published in the June 22, 2001 issue of the journal "Science."

That "Science" report highlighted "woody encroachment" as one of two important factors for its estimate that the atmospheric carbon "sink" - amounts of former carbon dioxide gas stored away in various natural repositories - had been "relatively stable" in the United States for more than a decade. The invasion of trees and shrubs formed between 18 percent and 34 percent of the total estimated North American carbon sink in the "Science" study - the largest non-forest sink.

But the current study found that in wet locations, the extra carbon saved in the wood of encroaching trees and shrubs was more than offset by the carbon lost from the underlying soil. In dry locations, the situation was reversed.

Policy makers have hoped that growing more trees would help slow or stop the addition of carbon dioxide to the atmosphere, a process most scientists blame for ongoing global warming. Growing plants incorporate carbon from the atmosphere into their tissues, where it remains until the plants die and decay.

In the case of the wood in trees, carbon may remain sequestered for centuries. In the case of grasses, carbon from the plant matter will return to the atmosphere in only a matter of years.

A coring rig deployed at the Central Plains Experimental Range in northeastern Colorado helped the researchers measure the carbon stored in soil. (Photo courtesy Robert Jackson, Duke University)

Carbon stored in soil "can remain in the soil for centuries," explained Jackson. "Furthermore, the global soil carbon pool is about twice as large as the plant pool."

The rich black soils beneath many grasslands provide a long term carbon repository, Jackson explained. But much of the grasslands that once carpeted the southwestern U.S. are changing as a result of fire suppression and cattle grazing.

Many of those former grassland environments have been invaded by drought tolerant woody tree and shrub species. Jackson's research group - including Jay Banner of the University of Texas at Austin, Jackson's graduate student Esteban Jobbagy, William Pockman of the University of New Mexico, and Diana Wall of Colorado State University - focused on the soil underlying what had been southern and western grasslands.

"This kind of analysis for grassland soil carbon was something new," said Jackson.

In research supported by the National Science Foundation and Andrew W. Mellon Foundation, Jackson's group first examined global records comparing amounts of carbon in grasslands, shrublands and woodlands in various climates and environments worldwide.

That data search found that "as you move to increasingly wet environments, grasslands have a lot more soil carbon than shrublands and woodlands do," Jackson said. "That was somewhat of a surprise. The analysis suggested that sites with the potential to store the most plant carbon also had the potential to lose the most soil organic carbon."

A soil coring rig at Sevilleta, central New Mexico. (Photo courtesy Robert Jackson, Duke University)

The average precipitation levels at those plots spanned "the whole range of grasslands in North America," Jackson said, from the edge of the western deserts to the edge of the eastern forests.

The team probed soils more deeply than previous studies have with the aid of a drilling rig than can penetrate up to 10 meters beneath the surface. By analyzing different carbon isotopes, they were able to determine the original sources of the soil carbon, because carbon taken in by woody vegetation is different from that processed by grasses.

"We found a clear negative relationship," the authors wrote in "Nature," between the amount of precipitation and changes in soil organic carbon "when grasslands were invaded by woody vegetation." Drier sites consistently gained soil organic carbon, while wetter sites lost it.

Losses of "organic" soil carbon at the wetter sites were "substantial enough" to offset the increased "plant biomass" carbon stored in the growing wood, they reported.

Why this precipitation tied carbon loss is occurring is still unclear, Jackson noted.

"We don't know the exact mechanism yet, but we have some suggestions," he explained. One is that "grasslands send a lot of their carbon below ground, so that carbon goes immediately into the soil."

"In addition to changes in the amount of carbon entering the soil, the quality of the [plant] tissue also changes," he added. "Woody tissue is typically more difficult to decompose than herbaceous tissue."

Much of the United States' historic grasslands has been replaced by shrubs, trees, and human developments. This is the Konza Prairie Biological Station, typical of the native tallgrass prairie of eastern Kansas. (Photo by Alan Knapp, courtesy Kansas State University)

"It had been proposed that the woody species might even increase soil carbon compared to the grasslands," Jackson explained. "People really didn't think that grasslands would store more carbon in the soil than woodlands."

In a "News and Views" article accompanying the study published in today's issue of "Nature," Christine Goodale and Eric Davidson of the Woods Hole Research Center in Massachusetts note that the new study will make predicting the effects of carbon sinks on climate change much more complicated.

"Woodlands, savannas, shrublands and grasslands cover about 40 percent of the Earth's surface, and so their potential role as carbon sinks - or sources - is a key factor in the global carbon budget," they wrote. "Measuring the effects of woody encroachment at particular sites is one challenge; extrapolating the results to regional or larger scales is quite another. Particular sites are certainly large sinks for carbon, but the global extent of grassland replacement by shrubland is highly uncertain."

The authors said forming better estimates of the sink potential of various regions will require "comprehensive assessments of the extent to which shrubs are replacing grassland, along with field measurements and model simulations of the size and variability of plant and soil carbon stocks across a range of climate conditions."