It’s one of the signature pastimes for kids from the Northeast: building a snow fort. When fresh snow blankets the backyard, hollowing out a snowbank or building an igloo creates a cozy shelter from the elements. In fact, it can get pretty toasty in those forts once its builders are huddled inside. That’s because snow is an excellent insulator—the fort’s walls trap body heat.

But snow isn’t just good for keeping kids warm—it keeps trees warm too. In northeastern forests, snowpack insulates the soil and root systems that keep trees alive. “You can think of it as like a down comforter that sits on top of the soil and helps to keep it nice and warm through the winter,” says Alix Contosta, a research scientist who studies winter climate change. Snow contains air pockets that trap heat accumulated in the soil during the growing season. Insulating snowpack—defined as a layer of snow that’s at least six inches deep—keeps the soil temperature above freezing so that roots remain healthy and protected. 

Now studies show that climate change is warming winter faster than any other season in the Northeast. Contosta’s research found that the region has lost three weeks of snow cover over the last century. In addition, erratic winter weather patterns like freeze-thaw events have become common. Rapid shifts from freezing to above-freezing temperatures stress the soil ecosystem and degrade tree roots when the ground is uninsulated.

So what does this mean for tree health? Andrew Reinmann, an ecologist who studies plant ecophysiology, led an experiment to investigate how reduced snowpack affects the growth of sugar maples, a dominant species in northeastern forests. He and his team spent five years removing snowpack from a section of New Hampshire forest and comparing its growth to another untouched section. 

They found that maples without an insulating snowpack grew 40 percent less. There are two distinct mechanisms likely at play here, according to Reinmann. The first is the cold itself. Uninsulated ground is vulnerable to deep freezing, and as the soil temperature plummets, the cold kills microbes and damages tree root tissue. What’s even more harmful are the physical changes that happen in the soil when deep freezes are followed by unseasonably warm temperatures. When this cycle repeats throughout winter, potholes can form underground, just like the ones on the roads each spring. Tree roots get pinched and pulled apart as groundwater freezes and thaws, leaving them abraded. 

When roots get damaged, they lose some of their capacity to absorb water and nutrients during the growing season. This results in stunted tree growth, like the growth decline observed by Reinmann and his colleagues. A 40 percent reduction in growth is pretty huge, says Reinmann. But their study only looked at one tree species. Further research will need to assess the species-level differences in how reduced snowpack relates to stunted tree growth. 

Just as climate plays a role in determining tree growth, tree growth plays a role in regulating the climate. When trees photosynthesize, they pull carbon dioxide from the atmosphere and use it to make sugars, which then become wood, branches, leaves, and roots as the tree grows. This process, known as carbon sequestration, works to balance man-made carbon emissions and inhibit climate change. As trees age and grow larger, they contain more carbon, but they also absorb carbon at a higher rate. But if trees aren’t growing to their full capacity, they aren’t storing as much carbon, which could worsen winter warming. 

Based on his research, Reinmann predicts that insulating northeastern snowpack could decline by up to 95 percent by 2099. In theory, this would significantly diminish the carbon storage capacity of the region’s forests. But the relationship between reduced snowpack and carbon sequestration becomes more complicated when considering the effect of climate change on the growing season. While winter is the Northeast’s fastest warming season, spring and summer are also getting hotter—and longer. There’s still a debate among experts as to whether an amplified growing season could offset the damage from winter snowpack loss, explains Reinmann. 

Experts agree that both forests and their stewards will need to adapt to withstand a drastic shift in winter climate conditions. Forest managers can’t control the weather, but they may be able to develop ways to preserve the snow that reaches the ground. Contosta hopes to fund future projects to develop snowpack preservation strategies, like insulating fallen snow with sawdust to help it survive unseasonable temperatures—a technique known as snow farming. 

While warmer days may discourage northeastern residents from mourning the loss of winter, these changes are a harbinger of mounting damage to our first line of defense against climate change: forests. But these projections aren’t inevitable. “We don’t have to land in the worst-case scenario,” says Contosta. Action can be taken to reduce its harmful effects before they snowball out of control.