Forests will struggle without winter snow

Trees are among nature’s most effective tools for capturing carbon dioxide. They absorb this heat-trapping gas as they grow, making forests key allies in the fight against climate change.

But what happens when winters get warmer and snow becomes scarce? That’s the question Boston University scientists set out to answer in a long-term forest experiment.

The researchers used underground cables to simulate warmer soil and removed snow to mimic shifting winters. The results suggest that future forest growth – and the carbon storage it brings – might not be as strong as we hope.

Lack of snow slows forest growth

In a recent study, BU researchers showed that warmer summers do boost tree growth. But there’s a catch: when snowpack is reduced in winter, that growth slows significantly. Forests may grow faster in heat, but they struggle without a protective blanket of snow.

“We know from past work that there are several negative effects of lessening snowpack, and we know that temperatures are warming and the snowpack is shrinking,” said study co-author Professor Pamela Templer.

“We wanted to look at the interactions between climate change across the year, and we wanted to be as realistic as possible about the future climate our forests will experience.”

Snow loss harms tree roots

Snow insulates the forest floor. With less snow, the soil freezes more often, damaging roots and disrupting the delicate balance trees rely on.

Instead of a steady cold, the ground experiences freeze-thaw cycles that can be stressful for the plants.

At the same time, warmer summers speed up decomposition in the soil, which can help trees grow faster at least on the surface.

“When we think about climate change, it’s not just warmer temperatures in the summer or warmer temperatures overall,” said Emerson Conrad-Rooney, a PhD student in Templer’s lab and lead author of the paper.

“We need to account for these changes throughout the year that can differ from season to season.”

Snow removal shrinks forest gains

The BU team set up six forest plots, each about the size of a small classroom. Four of them had underground cables that warmed the soil by 5 degrees Celsius (9 degrees Fahrenheit). Two of those were also cleared of snow in the winter. The last two were left alone for comparison.

Year after year, the team returned to measure tree growth using dendrometer bands – spring-loaded metal rings wrapped around trunks that track expansion. These readings allowed them to estimate how much carbon each tree was storing.

Trees in the plots where the soil was artificially warmed and snow was left untouched grew 63 percent more than the trees in the unaltered plots.

But when snow was removed – triggering repeated freeze-thaw cycles – the growth benefit dropped. Those trees only grew 31 percent more than the controls over the same period.

In other words, the lack of snow cut the positive effects of warmer temperatures on tree growth by about half.

“Many Earth system models, which predict how much carbon forests can store, aren’t incorporating the complexities of winter climate change that we’re highlighting here,” said Conrad-Rooney. “This means that models might be overestimating the carbon capacity of these temperate forests.”

What’s happening beneath the ground?

Now, the team is turning its attention underground. Templer suspects that repeated freezing and thawing takes a toll on tree roots, which are adapted to the stable cold of snowy winters.

In 2023, Conrad-Rooney buried mesh cylinders called root ingrowth cores to track how roots are growing in each plot. The results are expected by the end of this year.

“We’re going to keep this work going as long as we can,” said Templer. “We’re so fortunate to have this long-term study because we learn so much the longer we keep going.”

“Right now, we see a warming-induced response from the trees, but maybe that’s temporary – maybe the trees will acclimate and their growth will slow down. We don’t know. That’s really the value of having long-term data.”

A climate puzzle with many pieces

This study is one part of a much larger puzzle. Researchers are also looking at how trees behave in cities, along forest edges, and under threats like air pollution, insect loss, and diseases.

But pulling all these pieces together into one clear forecast for forests remains a huge challenge.

“There are so many global changes happening at the same time,” said Templer. “It’s impossible to get at everything all at once, so we each do what we can.”

“The reason we could do this work at all is because others before us monitored the climate. Getting to contribute to long-term science with this study is just amazing.”

The full study was published in the journal Proceedings of the National Academy of Sciences.

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