Forests by the Chesapeake Bay are growing two to four times faster than expected these days, researchers have found - a signal that rising carbon dioxide in our atmosphere might be triggering noticeable changes in ecosystems in the Mid-Atlantic.
And though scientists warn it's no panacea, the accelerated growth in stands of hardwoods monitored for the past 22 years is an indication that forests might dampen or delay the impact of climate change at least for a while, by soaking up some of the greenhouse gases that most scientists believe are warming the planet.
"We clearly see an increase in growth in these forests lately," said Geoffrey Parker, a senior scientist at the Smithsonian Environmental Research Center in Edgewater. The 58-year-old forest ecologist said he's been methodically measuring the girth of trees since he began working at the facility in 1987.
By measuring the circumference of a tree's trunk, scientists are able to estimate its "biomass," the combined weight of its wood and leaves. On average, they say, the woodlands they're tracking are bulking up by an extra 2 tons per acre annually. That's as if a new tree 2 feet in diameter sprang up every year.
Parker said he and his colleagues, Sean McMahon and Dawn Miller, aren't sure exactly what's driving the growth surge or when it began. But they note that carbon dioxide levels in the air at the Smithsonian's research center have increased 12 percent in the time since Parker began monitoring the trees. That's roughly the same increase tracked in the air above Mauna Loa Observatory in Hawaii - the site of the longest continuous measurement of carbon dioxide in the Earth's lower atmosphere.
The trio of scientists published their findings this week in the Proceedings of the National Academy of Sciences.
Other studies have shown that plants will grow faster when raised in air enriched with more carbon dioxide. Parker's Smithsonian colleague Bert Drake has demonstrated a similar effect with a long-term study of marsh grass growth at the Edgewater center.
But this forest study is different because it has found the same effect in nature without artificially manipulating CO2 levels, noted William H. Schlesinger, president of the Cary Institute of Ecosystem Studies at Millbrook, N.Y.
"There's been a lot of interest among ecologists and climate-change scientists as to whether forests are growing faster as a result of rising carbon dioxide in the atmosphere," said Schlesinger, who reviewed the Smithsonian study for publication. "Carbon dioxide being the raw material for photosynthesis, it kind of makes sense it might stimulate the rate of growth."
A warming atmosphereThough naturally occurring, CO2 also is a man-made product of burning coal, oil and other fossil fuels. It's one of a group of chemical compounds, including methane, that are dubbed "greenhouse gases" for their ability to trap the sun's rays as they reflect off the Earth's surface and warm the atmosphere.
Since their study began, Parker said, the mean temperature at the Edgewater facility has risen by three-tenths of a degree Celsius, with most of the overall increase driven by higher minimum temperatures. The growing season also has lengthened by nearly eight full days, with frost ending earlier in the year and starting later in the fall.
"We don't have evidence to point to any of those as causative agents," Parker said, "but it is suggestive."
The Smithsonian scientists have been tracking growth in 55 plots of trees on the research center property and elsewhere in Anne Arundel and Prince George's counties. The wood plots vary in size, with some covering up to 2 acres. Relying on documented information about the land use of the tracts, the scientists say the age of the trees ranges from 5 to 250 years old.
Because the tree stands or groups they studied included trees of varying ages, they've concluded that the growth spurts they're measuring now are relatively recent. If not, Parker said, then the trees they're studying are a lot younger than the researchers believe them to be. They're pretty sure of the ages of most woodland plots, he said, because there are records of when they were last logged or when they were abandoned as farmland, allowing trees to grow up.
Though they can't be sure why the trees are growing faster, Parker said researchers have ruled out other obvious explanations.
Trees tend to grow faster when fertilized, and the Chesapeake Bay is suffering from a surfeit of plant nutrients in the form of phosphorus and nitrogen from sewage, farm and lawn fertilizer and air pollution. But Parker said the trees the Smithsonian researchers have studied have been on land that hasn't been farmed in recent years, and hasn't been subject to nutrient-enriched surface runoff or ground water. Nitrogen fallout from power plants and motor vehicle emissions, though higher than the national average, also has declined since 1983, he said.
Climate computer models have suggested temperate hardwood forests such as those around the Smithsonian center should experience greater growth as carbon dioxide concentrations increase in the atmosphere and the climate warms moderately. That's good news of a sort, because forests serve as a "sink" for carbon, absorbing it from the air and locking it up in their roots and branches.
"If trees are growing faster," said Schlesinger, "then they take CO2 out of the atmosphere at a greater rate and that slows the rise of carbon dioxide in the atmosphere - and potentially slows the onslaught of global warming."
A short-term benefitBut the beneficial effect of higher carbon dioxide might be limited. Parker said Duke University researchers, for instance, found that a stand of loblolly pines exposed to air enriched with carbon dioxide grew much faster for a while, but then stagnated from a lack of nutrients.
A 2008 University of Maryland study also suggested that the makeup of the state's forests might change as climate warms, with hardwoods in western and northern Maryland giving way to more heat-tolerant southern pines and oaks. Warmer summers and lack of precipitation might increase the risk of forest fires as well, it notes.
Matthias Ruth, director of the University of Maryland's Center for Integrative Environmental Research, said the Smithsonian scientists' findings are "perfectly consistent" with what computer climate models project. But he cautioned that the beneficial effects of forests soaking up carbon dioxide likely won't last long.
"As trees grow faster and we change climate, diseases will become an issue," he predicted. The findings now suggest there's a brief "window" before harmful changes wrought by rising greenhouse gases overwhelm the beneficial effects.
Schlesinger also pointed to the possibility that tree-damaging pests and diseases might take a toll as the climate warms.
None of those has occurred in the western shore forests to date, but Parker said he figures the growth spurt there might hit some sort of ceiling eventually, probably from insufficient nutrients or precipitation. At that point, the forests would lose some of their ability to soak up more carbon dioxide, and, if they die or burn, would release the climate-altering gas back into the atmosphere.
The Smithsonian ecologist said he hopes their study will inspire others who have been monitoring trees or forests for long periods to check their data for similar growth spurts. Meanwhile, Parker said he and his colleagues aim to try to get a better handle on just what might be triggering the growth in this area and when it began. They plan to study tree rings and trunk cores to see if they can spot the years when growth was elevated.
"It's tantalizing enough that I would go after those factors with an experimental approach," he said. "But right now what we have is a clear effect in search of a cause. … I just find it curious that this is happening."
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