Forests by the Chesapeake Bay are growing faster these days, researchers have found, in a growth spurt that seems linked to rising carbon dioxide in the air and warming climate.
In a study published this week in the Proceedings of the National Academy of Sciences, a trio of scientists at the Smithsonian Environmental Research Center in Edgewater report that stands of hardwood trees they've been monitoring over the past 22 years along the bay's western shore have been growing two to four times faster than expected.
It's a signal that rising carbon dioxide in our atmosphere may be triggering noticeable changes in ecosystems in the mid-Atlantic. And though scientists warn it's no panacea, it's an indication that forests may dampen or delay the impacts 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 Smithsonian senior scientist. The 58-year-old forest ecologist said he's been methodically measuring the girth of trees since he began working at the Edgewater 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 two tons per acre annually. That's as if a new tree two 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.
Though 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 range in size, with some covering up to two acres. Relying on documented information about the land use of the tracts, they say the age of the trees ranges from five to 250 years old.
Because the tree stand 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 they 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.
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's 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."
Though they can't be sure why the trees are growing faster, Parker said researchers have ruled out other obvious explanations than changing climate