Forests are an invaluable resource in the fight against climate change, currently offsetting approximately 20 % of North American carbon emissions. Scientists are working hard to understand how climate change is affecting them. carbon sequestration capacity. The new ten-year study, published in Proceedings of the National Academy of Sciences, provides important insight into how warmer winters and reduced snow cover may be preventing trees from storing carbon, even as warmer summers promote their growth.

The research was conducted over a decade at the Hubbard Brook Experimental Forest in New Hampshire. The researchers used six forest plots to examine the effects of changing soil temperatures and reduced snow cover. Two plots served as reference plots, remaining untouched. In two others, the soil was warmed by 5°C during the growing season using buried heating cables. The remaining two plots also had their soil warmed by 5°C during the growing season, but in addition, snow was regularly removed in the winter to induce up to four freeze-thaw cycles in the soil. Measurements focused on the growth of mostly red maples, snow cover, and soil temperature for each plot.

The results showed that all plots with warmer soil during the growing season experienced greater tree growth compared to the reference plots. However, the difference was significant in their ability to sequester carbon. Trees with warmer soil during the growing season increased carbon uptake by 63 % compared to the reference plots. In contrast, trees with reduced snow cover in winter (and warming during the growing season) stored only 31 % more carbon than the reference plots. This means that reduced snow cover and the associated increased freeze-thaw cycles in winter have significantly impaired the ability of trees to store carbon.Plots that maintained more consistent snow cover experienced a protective effect, while increased freeze-thaw cycles appeared to cause enough damage to tree roots in other plots to disrupt their carbon uptake.

These findings have important implications for climate models. Many current models assume that trees increase carbon uptake during warmer growing seasons due to increased soil nutrition. While this study supports this idea, It also highlights the significant impact of warmer winters and increased freeze-thaw cycles.The study authors suggested that future models should include this effect to gain a fuller understanding of the impact of climate change on carbon sequestration in forests. Specifically, they state that not including the effects of decreasing winter snow cover in models would likely lead to overestimation of carbon sequestration rates in northern temperate forests by at least 50 %.

The researchers noted limitations in their statistical power due to the relatively small forest area and studying the impacts on only one tree type, red maple. To better understand the future impacts of a warmer planet and increasing freeze/thaw cycles in temperate forests, future research should focus on other tree species and overall ecosystems. Spring