Environmental News Service, Aug. 8, 2002
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.
"Assessments relying on carbon stored from woody plant invasions to balance emissions may therefore be incorrect," wrote the researchers in a paper published in today's issue of the journal Nature.
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.
However, the soil forms yet another carbon sink. The "organic carbon" in soils "can stay there for a long time, longer than in the plant," said Duke University environmental scientist Robert Jackson, lead author of the "Nature" study.
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."
The scientists then set up experiments at six research sites in Texas, New Mexico and Colorado, where grasslands had been invaded by trees and shrubs over the past century. They were able to study the effects of this encroachment because the original grasses had been maintained on one side of a fence running through each test plot, while woody species had invaded the other side of the fence.
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."
The results were a surprise to the researchers, and could prove a setback to supporters of tree plantations and other tree growing projects as carbon sinks.
"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."
Copyright Environment News Service (ENS) 2002. All Rights Reserved.
Study: In global warming fight, trees not always best answer
Scientists have overestimated the potential of trees and shrubs to soak up carbon dioxide from the atmosphere, according to a new study.
The reassessment casts doubt on whether planting trees is always a positive step in the fight against global warming, as President Bush and others have suggested.
In the study, published in Thursday's issue of the journal Nature, Duke University scientists say trees and shrubs growing in areas of abundant rainfall are less effective storehouses for carbon than native grasslands they have steadily replaced across much of the western United States.
Vegetation stores carbon that otherwise might trap heat in the atmosphere, driving up temperatures and leading to climate change. Previous studies have ignored what was going on below ground, said Robert Jackson lead author of the study and an associate professor of biology at Duke University.
In wet locations, replacing grass with shrubs and trees actually can lead to a decrease in the amount of carbon locked up in organic matter mixed in the soil, Jackson said. The amount can be enough to offset any gains achieved above ground.
"The study suggests that we need to look very closely at what's below ground before we add up just what's stored above ground in tree trunks," Jackson said.
Scientists studied six pairs of adjacent western grasslands. In one of each pair, trees and shrubs had cropped up sometime in the last 100 years.
In the drier sites, the invasive growth led to an increase in the amount of carbon locked up in the soil. In wetter areas, however, the opposite was the case, Jackson said. It is not clear what caused the change.
"Grasses are deceptively productive," Jackson said. "You don't see where all the carbon goes so there is a misconception that woody species store more carbon. That's just not always the case."
Previously, studies estimated that U.S. shrublands contain about 440 million tons of carbon. The number may be closer to 280 million tons, Jackson said.
That result suggests shrublands, by absorbing carbon from the atmosphere, do less to balance emissions from the burning of fossil fuels than previously thought, Jackson said.
"It would not surprise me at all if they were absolutely spot-on right," said Steve Pacala, a Princeton University professor ecology, who wasn't involved in the study. However, he said he didn't consider the study definitive, given uncertainties in its measurements of the carbon contained in woody roots.
The study helps dispel the notion that humans can plant their way out of global warming, said Daniel Becker, director of the Sierra Club global warming and energy program.
"We are going to need to tackle the industrial sources of emissions head-on rather than just plant a bunch of trees," Becker said.
As part of his administration's strategy for curtailing carbon dioxide emissions, Bush has proposed tax incentives for farmers who plant trees.