In Temperate Forests, Edges Hold More Carbon than the Middles

(Inside Science) -- Almost everyone loves a vast, dense forest -- including scientists. But in the temperate zone encompassing North America, Europe and much of Asia, farms, roads and housing developments have removed much of the forest, creating a patchwork quilt where there was once an unbroken green blanket.

In a paper published today in the Proceedings of the National Academy of Sciences, and presented last week at the American Geophysical Union meeting in San Francisco, a team of scientists report that such fragmented forests may actually fight harder against climate change. The paper authors found that New England oak forests grew nearly twice as fast around their edges than in their interiors.

"The growth and the size of trees near the edge of forest compared to what is just inside is pretty phenomenal," said Nick Haddad, an ecologist at North Carolina State University in Raleigh who was not involved in the research. "Through that lens, there can be a green lining around the big cloud of forest fragmentation."

But the study authors also found that this surprising benefit will likely decline as the climate warms, and emphasized that further fragmenting forests will not slow climate change.

About half of a tree's mass is the element carbon, and growing trees soak up carbon from the atmosphere via photosynthesis. This carbon can return to the atmosphere, however, if forests are burned or cut and left to rot, or when trees die naturally. Still, scientists have estimated that forests globally take up about a billion tons of carbon a year more than is lost to fire and deforestation.

While that is not nearly enough to offset the roughly 10 billion tons of carbon that human activities emit annually, scientists have suggested that by protecting existing forests and allowing forests to regrow where they have been cut down, countries can slow the pace of climate change as they transition away from fossil fuels.

But estimates of forests' carbon-absorbing potential are based on a relatively small number of studies, many done in large, intact forests such as the Amazon rainforest. In studies of forests that have been penetrated by farms or roads, scientists have found that biodiversity and other measures of forest health usually decline, because forest edges are vulnerable to damage from wind, heat, invasive species and other factors. Recently, scientists found that tropical forest edges also contain substantially less carbon than the interiors, suggesting that the total amount of carbon held in tropical forests -- which provide a portion of the world's overall carbon sink -- might be smaller than previously estimated.

Curious if a similar effect occurs in temperate forests, Boston University ecologist Lucy Hutyra and postdoctoral researcher Andrew Reinmann launched a study in oak-dominated forests in Massachusetts. The scientists used trees’ annual growth rings to measure their growth rates in study plots that extended from the forest edge to 30 meters into the forest. They found that trees at the forest edge actually grew new wood 89 percent faster. Though the study did not determine a cause for the growth enhancement, Hutyra and Reinmann suspect that added sun exposure at the forest edge plays a major role.

Unlike in the tropics, nearly all original temperate forests have been cut down, and many are now growing back, making temperate forests an outsized contributor to the total global carbon sink. They are also more fragmented: Hutyra and Reinmann found that nearly 18 percent of forests in southern New England -- comprising the states of Massachusetts, Connecticut and Rhode Island -- grow 20 meters or less from an edge, indicating a level of fragmentation higher than almost any other forest on Earth. If the edge effect from their study held for all temperate forests, the global temperate forest carbon sink estimate, which accounts for around 60 percent of the total net forest carbon sink, would increase substantially.

However, Haddad noted that because Hutyra’s and Reinmann’s study plots were mostly next to suburban yards, trees on these forests' edges were likely protected from disturbances, and possibly even aided by fertilizer applications to nearby lawns and gardens. This may have given them an edge, so to speak, over trees growing next to farms or roads. The authors "were looking, in some ways, at the most idealized landscapes," Haddad said. "I suspect that the patterns will not be as strong in those harsher and more dynamic environments."

Haddad added that carbon uptake in other temperate forest types, such as pine forests in the southern U.S., might respond differently to fragmentation.

By comparing growth rates with past climate data, Hutyra and Reinmann also found that the growth boost on the forest edge lessened substantially during hot years, suggesting that temperate forest edges are more vulnerable than interiors to heat stress. So as the planet warms, fragmented forests may sop up less carbon.

That is one reason that despite her team's results, Hutyra recommends against trying to fight climate change by punching more holes in the forest. The other is that carbon lost from the removed trees would outweigh carbon taken up at newly created edges.

"We're not advocating for fragmenting our forest," Hutyra said, noting that before Europeans arrived, southern New England's forests absorbed 31 percent more carbon than they do today. "If we just had forest and pasture … we'd be better off."