Forests in the eastern United States appear to be growing faster than they should be, and increases in temperature and carbon dioxide are the likely culprits.
"We've known for 30 or 40 years that extra CO2 and extra temperature cause trees to grow, most of the climate models predict this," said Geoffrey Parker, of the Smithsonian Environmental Research Center in Maryland. "It's just that there haven't been many field studies that really corroborated it."
Parker's team used a combination of two types of tree data to put together a comprehensive look at how trees along the western edge of the Chesapeake Bay have been growing in recent years. They found that the forest, including both young and old trees, has been adding weight at an exceptionally high rate. In fact, in 90 percent of the measurements taken, the rate of growth of the trees was higher than the expected rate. The results were published in the Proceedings of the National Academy of Sciences.
The researchers narrowed the causes underlying that growth bonanza to three factors. First, local measurements taken over 17 years showed a 12 percent increase in CO2 levels in the area. Temperature measurements from the nearby Baltimore-Washington International Airport over about 100 years indicated a significant increase, as well, and the growing season — based on first and last frosts of the winter — has grown by about seven days.
Parker stressed that there isn't enough information about this effect to truly assess the potential risk or benefit to forests, or to the climate system as a whole. He did say, however, that "if this is a widespread generality that this extra growth is going on, it may well have contributed to slowing the increase in atmospheric CO2." The "metabolism" of the forest seems to have sped up, he said, and it is certainly possible that some negative effects could be associated with such a process.
Jeff Hayward, the climate initiative manager for the non-profit Rainforest Alliance, said there is often a focus on the beneficial effects of forests with regard to climate change, but there are limits to that side of the story.
"One spin on this story might be, 'Wow, yippee, trees might be taking up more extra carbon, and they're growing faster and getting bigger. Isn't this a positive thing?'" Hayward said.
However, both Hayward and Parker pointed out that this trend is unlikely to continue indefinitely. At a certain point, other limiting factors including nutrients and available moisture will limit the rate of the trees' growth.
And even if the increased carbon dioxide could be adding mass to certain forests, there are well-documented negative effects that climate change is having on forests as well. The most striking of these may be the ongoing invasion of pine bark beetles over vast swaths of the Rockies, where millions of trees are being consumed by the beetle infestation. In British Columbia alone, an area bigger than Ireland has already been largely destroyed, and the unprecedented beetle swarms have been linked to warming temperatures.
Another study examining how climate change may be affecting forests looked to the north, where vegetation will most likely expand into the Arctic as the region warms. Also published in PNAS, the study led by University of California, Berkeley, graduate student Abigail Swann found that if deciduous trees like poplar and aspen expand into previously clear parts of the Arctic then multiple feedback loops will kick in and help to accelerate climate change.
"When you consider deciduous trees, a pathway through the greenhouse warming associated with additional water vapor is capable of contributing at least equally as the change in the color of the surface, which was previously considered to be the really important way that trees change climate," Swann said.
The change in color, or decreasing albedo, involved with increased tree cover where snow and ice used to be would still play a major role in absorbing more heat from the sun, but the excess water vapor in the atmosphere as a result of the trees would apparently play an even larger role in driving further warming. Swann said that the two pathways would be additive. Deciduous tress release more water vapor into the air — in a process called evapo-transpiration — than other types of trees that could inhabit the region.
It is difficult to determine on what time scale vegetation could move significantly into the regions studied by Swann and her colleagues; she said it could happen in a few decades or take several hundred years.
The major wildcard in the equation is the carbon that is stored in soil and permafrost, she said. As forests expand northward, it is extremely difficult to guess what will happen to that carbon "sink."
"That's a huge unknown, and so I don't think we can really say what the net balance will be," she told SolveClimate. "I don't think we really know what the soil response is going to be, but if there is significant warming it could easily outweigh any changes in carbon above ground."
Both of these new studies drive home the point that climate change and forests are already affecting each other in profound ways.
"What both of these studies indicate to me is just the realness of climate change, and how disruptive it is," Hayward said. "It's not just some easy thing that you can say, 'well, temperature will go up so that will be positive for these people, and this will be negative for those people.' It's just very complicated, and the effects will be disruptive."