A new study shows that trees in the Boston region grow faster and store more carbon as biomass the closer they are to developed areas.
Courtney Humphries received the 2017 David Perlman Award from the American Geophysical Union for this article.
The woods at the Mass Audubon Habitat Wildlife Sanctuary in Belmont, Massachusetts—a well-heeled suburb six miles from downtown Boston—are pleasant but commonplace, the kind of place that New Englanders go to walk their dogs or get a taste of nature. But to Andrew Reinmann, a biologist at Boston University, this is also a critical ecosystem to study.
That’s because as urban development spreads, patchy woods like this are increasingly the norm. “More and more of the landscape is becoming fragmented,” Reinmann says, as we swish through a thick mat of fall leaves. Forests are important asset in fighting climate change, absorbing an estimated 30 percent of the carbon dioxide we emit from burning fossil fuels. But those estimates come from big forests, says Reinmann, and we know relatively little about how patchy forests function, and whether they provide the same services that large forests do.
A study published on Monday in PNAS by Reinmann and BU environmental scientist Lucy Hutyra shows that forest fragments in New England behave differently than intact forests in surprising ways: they may pull significantly more carbon dioxide out of the atmosphere than predicted.
Reinmann pauses where the forest ends at a row of backyards separated by a low stone wall, a common sight in New England. The reason that forest fragments are different can be found here: the forest edge.
“You can see how the structure of the trees all along the edge is different,” he says, pointing to a stand of oaks with long horizontal branches reaching over the backyards, soaking up the additional sunlight. Slicing and dicing forests with housing developments, roads, and agricultural fields creates a multitude of forest edges and, as Reinmann and his colleagues are finding, conditions at the edge of a forest are different than deep inside it. These effects add up; currently, 20 percent of the world’s forested land is within 330 feet of an edge.
Edge conditions can actually be a boon to the trees that remain. An earlier study from Hutyra’s lab found that urbanization makes trees in Massachusetts grow faster. Her team analyzed tree rings of red oaks adjacent to areas that had been cleared for development over the past few decades. “In the overwhelming majority of the cases, the tree showed a large increase in growth within a couple of years of that edge being created,” says Reinmann—an average doubling of growth rate.
To understand how this growth boost might affect entire forests, the current study looked at plots near forest edges in Massachusetts, including this one in Belmont. The researchers measured tree diameters and took thin cores from trees to study their growth over time. They found a much higher growth rate in the forest within 10 meters of an edge compared with the forest 20 or 30 meters in. “On, average the forest is growing 90 percent faster near the edge,” says Reinmann. In some cases, individual trees are growing faster, and in other cases, they’re growing more densely.
A spatial analysis of the forests of southern New England (Massachusetts, Rhode Island, and Connecticut) shows that almost 20 percent of the forests lie within 20 meters of a forest edge. Given the growth boost at edges, Reinmann and Hutyra estimate, forests in southern New England take up about 13 percent more carbon dioxide than they’re given credit for, and store about 10 percent more carbon.
“A 10 percent difference is a really important one,” says Jonathan Thompson, senior ecologist at the Harvard Forest. “Everything we know about carbon storage comes from big, intact forests, and then we scale it up.” That information is used to model the effects of carbon sinks on climate. “To the extent that all the forests near an edge operate differently, then we need to change our models.”
A growth boost is the opposite of what’s been reported in research on other kinds of forests, particularly in tropical regions, where forest fragmentation by clear-cutting causes trees at the edge to die off. Reinmann says that New England seems to be in a “sweet spot” where creating edges also makes conditions that are beneficial to trees; a similar effect could be at play in similar temperate forests.
Forest edges have long fascinated ecologists, starting with Aldo Leopold, who coined the term “edge effect” in the 1930s and speculated that forest edges may be important habitats for wildlife. More recently, scientists have focused on the negatives of too many edges: fragmentation that lowers biodiversity, degrades habitat, and impedes the movement of wildlife. But Nick Haddad, a conservation biologist at North Carolina State University, says that this study shows that edge effects are complex. “The study is fascinating in showing how productive trees are near the edge of the forest in New England,” he says. “They’re sponges for taking up carbon from the atmosphere.” It also highlights how human activity can sometimes help the plants living around us. “We’re creating garden-like conditions for forests,” he adds.
It’s a surprising piece of good news that a consequence of urban sprawl actually helps to combat some effects of greenhouse gas emissions. But, Reinmann says, “the really important thing to stress is that it does not mean forest fragmentation is a good thing. The carbon sink here is still substantially lower than it would be if we didn't lose any forest.” In other words, slicing up a forest to store carbon is a very bad idea.
And there’s another catch; Reinmann and Hutyra found that forest edges are more sensitive to heat stress. Forest growth rates decline more at the edge than in the interior in hot years. Using climate projections, they estimate that exposure to high summer temperatures in New England could reduce the growth boost at forest edges by one-third or more by the end of the century, suggesting we’ll get less help from forests as the Earth warms.
The research points to a need to better understand how human-modified landscapes differ from pristine ones. Reinmann says the next step is to study which factors are shaping life at the edge in small forest patches. It may be that light, water, nutrients, and other resources are more abundant there, and what lies on the other side of the edge—whether a backyard, a parking lot, or a field—might also matter. Ultimately, there could be better ways of taking care of small forests like the Habitat Sanctuary, not just for our own enjoyment but for the work they do to stabilize the climate.