For forests, an earlier spring: Climate change leads to increased growing season and allows forests to store more carbon dioxide

Over the last two decades, spurred by higher temperatures caused by climate change, Harvard scientists say, forests throughout the Eastern U.S. have experienced earlier springs and later autumns than ever before.

Using a combination of satellite imagery, tower-mounted instruments and on-the-ground observations, research associate Trevor Keenan and Andrew Richardson, associate professor of Organismic and Evolutionary Biology, along with colleagues from 7 different institutions, found that forests throughout the eastern US are showing signs of spring growth earlier than ever, and the growing season in some areas extends further into the fall. That expanded growing season, they say, has enabled forests to store as much as 26 million metric tons more CO₂ than before. The study is described in a June 1 paper published in Nature Climate Change.

"What we find in this paper is an increase in the growing season of forests in the eastern U.S. due to recent climate change," Keenan said. "This has been beneficial for forests in the past, but we do not expect the response to continue unchecked in the future. It must also be kept in mind that this positive effect of warming is but one amid a barrage of detrimental impacts of climate change on the Earth's ecosystems."

Though the fact that forests can store more carbon is a good thing, both Keenan and Richardson warned that continued climate change could lead to more dramatic negative consequences in the future.

"If forests weren't storing additional carbon in this manner, we would be even worse off in terms of atmospheric CO₂ levels, so at the moment, it's a good thing…but this is not going to solve the CO₂ problem," Richardson said. "Yes, 26 million metric tons is a lot of carbon, but it's still small when compared to fossil fuel emissions.

"And climate change isn't just about warmer temperatures," he continued. "It's also about changes in precipitation patterns…so in the future, an earlier spring might not help forests take up more carbon, if they end up running out of water in mid-summer."

To find evidence for the earlier spring, Keenan and Richardson integrated observations from three sources -- satellite imagery, ground observations and instrument towers.

By collecting data across three different scales, Richardson said, Keenan was able to capture both a region-wide picture of the eastern forest -- which stretches along the eastern seaboard from Maine to Georgia, and as far inland as Wisconsin -- as well as a more granular measurement of individual sites.

By using satellite data, Keenan tracked when forests across the region began to turn green in the spring, and when leaves began to turn in the fall. Ground observations made every three to seven days at the Harvard Forest in Petersham and a long-term research site in New Hampshire provided direct information about the state of the buds, leaves and branches.

When combined with data collected from instrument towers, the various data sets allowed Keenan, Richardson and colleagues to paint a richly detailed picture that shows spring starting earlier, and the growing season lasting longer than at any point in the last two decades.

"Basically, we showed that there are three different ways of looking at this, and they all show the same result -- spring is getting earlier," Richardson said. "When you look at the patterns across both space and time, and year-to-year at individual sites, and when you look across different species, the same patterns hold up…that gives us confidence that there's something going on."

Another important finding, Richardson said, is that the research identifies a significant source of error in existing computer models of how forest ecosystems work. It turns out that these models don't properly reflect how spring and autumn temperatures control the start and end of the growing season.

"What that means if you run these models forward 100 years, they won't be accurate," he explained. "This shows an opportunity to improve the models and how they simulate how forests will work under future climate scenarios."

The real power of the findings, however, may be in helping make the effects of climate change more tangible to the general public.

"If you tell people the leaves are going to start coming out in mid-April, and it used to be in early May, that's something people can relate to more easily than describing temperature-change records," Richardson said. "People are just inherently fascinated by the passing of the seasons and weather, and how those two are connected."

"The interesting thing about the findings is that we can watch climate change happen," Keenan said. "Spring is earlier than it used to be, and autumn is later. Everyone can understand that, regardless of their predispositions regarding climate change."