Warming temperatures, less snowfall 

pose new challenges for northeastern species

Winter can be a challenge. For people. And trees.

It’s no surprise that the tropics and subtropics host a lot more types of trees. And that they grow bigger and faster than in the northern temperate zone. Twenty-five acres of Borneo rainforest can have more than 700 species of trees. New England as a whole has 81 tree species or species groups.

In contrast to warmer climes, where trees get to photosynthesize all year round, northern hardwoods are limited to half that. Even conifers that retain their needles are limited in what they can do in cold weather, with dim days and a sun that skirts the horizon. Plus, below zero cold can kill roots. Sun and cold cause frost cracks in bark. Snow and ice can break branches. Spring frosts threaten new buds.

You might think that the hotter planet we’re engineering (January 2020 was the hottest January on record and the four hottest Januaries have all occurred since 2016) would offer some relief to northern trees.

Not necessarily so. Scientists say warmer winters will, on balance, make life more challenging for not only trees, but the forest products sector and the entire Maine and northeastern economy.

Because the tree species that make up Maine forests evolved to deal with extreme cold, deep snows and a growing season that’s only a few months long.

The big challenge a northern winter poses for northern trees is they “can’t conduct photosynthesis in the winter,” said Jay Wason III, a plant physiologist and assistant professor of forest ecosystem physiology at the University of Maine’s School of Forest Resources.

Lack of sunlight is a problem, but also, trees are mostly composed of water. In winter, their access to it is limited. “You can almost think about winter as a time of drought for trees,” said Wason. “They shut down in the winter and try to just hold out and get through,” hunkering down until the sun climbs higher and temperatures moderate. “You could think of it as a form of hibernation in some ways,” Wason said. “They’re certainly still alive, but they’re mostly just waiting for spring and the opportunity to start photosynthesizing again.”

Northern tree species like white birch, sugar maple, beech and aspen are perhaps most vulnerable in the late winter-early spring period.

Trees sense the coming of winter through dropping temperatures and shortened day length and begin winterizing. Hardwoods drop their leaves and conifers lose their older needles. Water migrates out of cells, as a way to lessen cell damage from freezing and expanding water; and the concentrations of sugars and salts in the cells change, Wason said.

In spring, trees have to “wake up,” but need to time it right: soon enough to get a jump on growth, but not so soon that a late winter frost kills tender buds and sets the tree back. It’s dicey — and getting dicier as spring moves forward on the calendar.

Actually, cold and snow both have their pluses when it comes to forest health.

Sub-zero cold can knock back populations of native and invasive forest pests. Snow insulates the ground, protecting tree roots and the health of the trees’ microbial and fungal partners.

The insulation value of snow varies depending on moisture content. On average, the R value of snow is about 1 per inch. A foot of snow provides about the same R value as a 2 by 4 wall packed with insulation.

If you’ve been saying that winters aren’t what they used to be, well, you’re right.

Northeastern winters are becoming warmer, with less snow and more rain.  And that variability is only expected to become more pronounced as the planet heats up. According to the latest US National Climate Assessment, released two years ago, winters are warming three times faster than summers. And spring is coming earlier.

Historically, over 50 percent of the northern hemisphere has had snow cover in winter, but now warmer temperatures are reducing snow depth and duration, researchers affiliated with Boston University note in a study published in 2018 in the journal Global Change Biology.

They estimate that the area of forest in the northeastern US that gets an insulating snowpack could decline by 95 percent by the year 2100, from some 33,000 square miles to less than 2,000.

That will have serious repercussions for northern tree species.

The research team spent 10 years studying the effects of lack of snowfall in the Hubbard Brook Experimental Forest in New Hampshire’s White Mountains. They painstakingly removed early snow from around trees in some plots while letting it pile up in others. They documented a 40 percent decline in sugar maple growth in the trees not protected by snow and concluded that the northeastern US could see “large declines in tree growth and forest carbon storage” as a result of declining snowfall.

“We have found that in plots where we remove snow, frost penetrates a foot or more down into the soil, while it rarely extends more than two inches deep in nearby reference plots with unaltered snowpack,” the authors wrote in Boston University’s publication The Brink. “And just as freeze-thaw cycles create potholes in city streets, soil freezing abrades and kills tree roots and damages those that survive.

“This root damage triggers a cascade of ecological responses. Dead roots decompose and stimulate losses of carbon dioxide from the soil. Trees take up fewer nutrients from soil, accumulate the toxic element aluminum in their leaves and produce less branch growth. Nitrogen, a key nutrient, can wash out of soils. Soil insect communities become less abundant and diverse.”

University of Maine researchers conducted a similar two-year study in a coniferous forest. They concluded in a paper published in NRC Research Press that “soils under a thin snowpack are more likely to experience freeze-thaw cycles” that affect the soil chemistry, particularly the availability of carbon and nitrogen to trees.

Ivan Fernandez, a soil scientist at the University of Maine’s School of Forest Resources and a member of the university’s Climate Change Institute, was one of the authors of that study. He says winters in Maine are no longer consistently cold and snowy; there’s much more variability in the weather.

Perhaps one of the greatest threats is “concrete frost.” It occurs when unfrozen ground is saturated with water, then frozen hard.

Concrete frost can take a serious toll on fine tree roots and microbial and fungal communities in the soil, said Fernandez. As everyone who’s made an ice cube knows, water expands as it freezes. In unsaturated soils — where there’s, say, a 50-50 mixture of water and air — there’s enough space for ice to expand. In waterlogged soils the ice crushes anything else there, he said.

In normal times a healthy snowpack soaks up the occasional winter rain, Fernandez said. If there’s no snow on the ground and we get a winter rain, it’s a recipe for concrete frost. Fernandez and other scientists say that repeated cycles of snowmelt-rain-hard freezes could place trees under great stress and limit their ability to take full advantage of the warmer months.

Scientists say there are a lot of questions about how warmer and more variable winters will affect northern trees, forests and the region. But there’s little doubt that it will.

In a study published in the journal Ecological Applications last year, another team of researchers, including Fernandez, warns that warming winters will likely have wide-ranging effects, ecological, economic, and social.

“In addition to effects on vegetation, reductions in cold temperatures and snow cover are likely to have negative impacts on the ecology of the northern forest through impacts on water, soils and wildlife,” the 18 co-authors conclude.

Forest pests, no longer held in check by extreme cold periods, would likely proliferate. Warmer winters might make it harder to log in low-lying areas. Ski areas and the entire snow-sports economy would take a hit. Diseases like Lyme and West Nile might increase. What about the maple syrup industry, which depends on a suite of warmish days and cold nights in late winter to get sap flowing?

Economies adapt. So do people.

But the trees? Well, the trees of the Acadian Forest evolved over vast spans of time to deal with cold and snowy winters. Because trees are so long-lived, they can’t evolve quickly enough to deal with a climate warming this rapidly, said Wason. Some may not even be able to migrate to more hospitable latitudes or elevations, he adds.

And make no mistake, said Fernandez: “We’re going to continue to see change” in the climate in Maine and the rest of New England. “We’ll see significant change by the end of the century.”


Joe Rankin writes on forests and forestry, nature and sustainability for websites, magazines and other publications. He lives in New Sharon.





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