Temperate broadleaf forests, such as the stands of red oak common in New England, absorb more carbon than expected along their edges, but are more susceptible to heat stress.
Over centuries, as humans have cleared fields for farms, built roads and highways, and expanded cities, we’ve been cutting down trees. Since 1850, we’ve reduced global forest cover by one-third.
We’ve also changed the way forests look.
Much of the world’s woodlands now exist in choppy fragments, with 20 percent of the remaining forest within 100 meters of an edge, like a road, backyard, cornfield, or parking lot.
Scientists have studied fragmented forests for decades, mostly to gauge their effects on wildlife and biodiversity. But recently, two Boston University scientists, Andrew Reinmann, a postdoctoral research associate, and Lucy Hutyra, an associate professor of earth and environment, have turned their attention to another issue: the effects of forest fragments on carbon storage and climate change.
[caption id="attachment_4074” align="aligncenter” width="680”] Lucy Hutyra and Andrew Reinmann. Photo by Cydney Scott[/caption]
The research offers some good news and bad news about forest fragmentation. It suggests that while these forests may be more valuable carbon sinks than previously thought, they are also more sensitive to climate change.
says Reinmann, lead author of the paper.
The annual atmospheric concentration of carbon dioxide, a potent greenhouse gas and agent of global warming, has increased by more than 40 percent since the start of the Industrial Revolution and continues to rise. Forests play a critical role as a carbon sink, absorbing about 25 percent of the CO2 emissions we humans put into the sky.
Most of our understanding of forest carbon dynamics comes from studying intact rural forests like Hubbard Brook in New Hampshire’s White Mountains and Harvard Forest in Petersham, Massachusetts, not from studying forest fragments.
To find out, Reinmann and Hutyra gathered data from 21 fragmented forest plots around Boston, measuring about 500 trees. In eight of those plots, they went a step further, taking sample cores from trees above 10 centimeters in diameter, a total of 420 cores from 210 trees.
[caption id="attachment_4075” align="alignright” width="300”] Photo by Cydney Scott[/caption]
They used the cores, and other data, to calculate how fast the trees grew. A tree’s size and growth rate indicate how much carbon it can absorb and also how much stress it’s experiencing.
Reinmann and Hutyra found that forest fragments grow faster along the edges than intact forests, absorbing more carbon than expected.
says Reinmann, who notes that the effect extends in about 20 meters from the forest edge.
Temperate Broadleaf Forests
Curiously, the finding may hold only for temperate broadleaf forests common in New England, the Appalachians, Canada, and Europe. Amazon rainforest has the opposite effect when fragmented, with lower biomass and less carbon storage along the edges.
Though this seems like a win for patchy New England forests, deforestation is still bad for carbon sequestration overall.
Offsetting this somewhat good news is the paper’s other finding.
These forest edges, more exposed to wind and sun, grow more slowly when stressed by heat.
says Reinmann, who found that the “magic number” for local trees is about 27° C (80.6° F), which corresponds to the average high temperature in July, our hottest month.
And the really bad news: if regional temperatures continue to increase at a steady pace, the current carbon benefit offered by forest edges may decline significantly.
Reinmann and Hutyra are currently expanding the work to study rural forests and are so far finding even larger effects there. They are also hoping to use high-resolution imaging and more precise chemical analyses to look closer at core samples to see how growth and photosynthesis change over days, seasons, heat waves, and other environmental stressors.
More data may lead to better models, says Hutyra.
The work was funded by the National Oceanic and Atmospheric Administration, the National Aeronautics and Space Administration, and the National Science Foundation.