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Forests are a crucial resource for carbon mitigation, currently offsetting around 20% of North American carbon emissions. As temperatures continue to rise, scientists are rushing to understand how climate change is affecting forests and their carbon sequestering abilities. A new study, in the Proceedings of the National Academy of Sciences, provides some valuable insight into how warmer winters might hinder the ability of trees to store carbon—despite warmer summers encouraging their growth.

The study used six plots of forest located in the Hubbard Brook Experimental Forest in New Hampshire over a period of 10 years to study the effects of changing soil temperatures and reduced snowpack. The researchers left two of the six plots untouched (reference plots), while warming the soil of two plots in the growing season by 5°C, and warming the soil of the remaining two plots in the growing season by 5°C while also removing snow periodically through the winter to induce up to four freeze/thaw cycles. The soil was warmed with buried heating cables.

Over the 10 years, researchers measured tree growth, snow levels, and soil temperature for each plot. The plots consisted mostly of red maple trees, and the growth measurements were only taken from these trees.

All plots that experienced warmer soil during the growing season showed more tree growth than those without the warming cables. However, the uptake of carbon by trees with reduced snowpack in the winter and those that were only warmed in the growing season showed a marked difference in their carbon sequestering abilities.

When the researchers evaluated the tree stem biomass carbon—a measure of how much carbon the tree has taken from the surrounding environment—they found that the trees with heated soil during the growing season increased carbon uptake by 63% compared to the reference plots. Meanwhile, the trees with reduced snowpack in the winter only stored 31% more carbon than the reference plots.

This indicates that the reduced snowpack—and thus, increased freeze/thaw cycles—in the winter impaired the carbon sequestering abilities of the trees by a significant amount. The plots that maintained a more consistent snowpack experienced a protective effect, while the increased freeze/thaw cycles appear to have caused enough damage to tree roots in the other plots to impair their carbon uptake.

Many current models on the effects of climate change have relied on the fact that trees experiencing warmer growing seasons are increasing their carbon uptake due to increased soil nutrition. While this study provides supporting evidence of this idea, it also shows the impact of warmer winters and increased freeze/thaw cycles. The study authors indicated that future models should incorporate this effect to get a fuller understanding of the impact of climate change on forest carbon sequestration.

They write, "Our findings advance our understanding of forest ecosystem carbon cycle response to climate change by showing that not incorporating the effects of a shrinking winter snowpack into Earth system models are likely to lead to overestimating rates of carbon sequestration of northern temperate forests in response to climate change by at least 50%."

The researchers note their limited statistical power due to the relatively small area of forest and the limitations of studying effects on only one type of tree. To better understand the future effects of a warmer planet and increasing freeze/thaw cycles in temperate forests, future research should study the effects on other trees and the overall ecosystems.

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