Climate change, winter warming harms forests | The lip

Climate change often conjures up images of heat, drought and hurricanes. But according to the latest US National Climate Assessment, released on November 23, 2018, winters have warmed three times faster than summers in the northeast in recent years. These changes also have significant effects.

Historically, more than 50 percent of the Northern Hemisphere had snow cover in winter. Now warmer temperatures are reducing the depth and duration of winter snow cover. Many people assume that winter is a period of inactivity for organisms in cold climates, but decades of research now show that winter climatic conditions—particularly snow—are important regulators of the health of forest ecosystems and the organisms that live in them.

Specifically, us work over the past decade shows that declining snow cover can harm tree health and reduce the ability of forests to filter air and water. Us last study finds that continued winter warming could significantly reduce snow cover in the northeastern United States, causing large reductions in tree growth and forest carbon storage.

Snow like a blanket

We study boreal hardwood forests, dominated by confectioner's, yellow birch, and American beech, and spanning 85,000 square miles, from Minnesota and south-central Canada east to the Canadian Maritimes and northeastern United States. These forests are famous for their vivid fall colors. They generate revenue by attracting tourists, hikers, hunters and campers and support the timber and maple syrup industries. They also provide important ecological services such as carbon storage and maintaining water and air quality.

When winter invades this region, with temperatures often dropping well below freezing, every species needs insulation to cope. Tree roots and soil organisms such as insects rely on deep snowpack for protection from the cold – literally a blanket of snow. Even in sub-zero temperatures, if the snow is deep enough, soils can remain unfrozen.

Six decades of research from Hubbard Brook Experimental Forest in New Hampshire – one of the longest-running studies of its kind – show that winter snowpack is declining. Research carried out by other scholars shows that if this trend continues, it will increase the probability of ground freeze-thaw cycleswith detrimental effects on forest health.

Scientists have used the Hubbard Brook Experimental Forest as a living laboratory to study environmental problems since 1963. Video from FactFile

Because the northern forests need snow

For more than 10 years we have been manipulating winter snow at Hubbard Brook to study the impacts of projected climate change on boreal hardwood forests. In early winter, we head outdoors after each snowfall to remove snow from our experimental plots. We then analyze how the loss of this insulating layer affects the trees and the soil.

We found that in plots where we remove snow, frost penetrates a foot or more into the ground, while it rarely extends more than two inches deep in nearby reference plots with an undisturbed snowpack. And just as freeze-thaw cycles create potholes in city streets, ground freezing grinds and kills tree roots and damages those that survive.

This radical damage causes a series of ecological reactions. Dead roots decompose and stimulate the loss of carbon dioxide from the soil. Trees absorb less nutrients from the soil, they accumulate the toxic element aluminum in their leaves and produce less branch growth. Nitrogen, an essential nutrient, can be leached from the soil. Soil insect communities become less abundant and diverse.

The reduction of the snow pack affects the growth of trees

In ours most recent paper, our climate and hydrologic models show that the area of ​​forest in the northeastern United States receiving insulating snowshoe could decline by 95 percent by the year 2100. Today, 33,000 square miles of forest in northern New York and New England have usually snowpack for several months in winter. By the year 2100, that area could shrink to a sliver of less than 2,000 square miles—about one-fifth the size of Vermont.

This fall will undoubtedly hurt her ski and snowmobiling industries and expose northeast roads to more freeze-thaw cycles. It will also significantly affect tree growth.

To assess the relationship between snowpack and tree growth, we used a specialized hollow drill called a growth auger to remove straw-sized cores of wood from multiple sugar maple stems. Each of these trees experienced either a natural winter snowpack or five consecutive years in which we removed the early winter snowpack. When we scraped the cores and viewed them under a microscope, they revealed annual growth rings that we could use to understand how each tree responds to its environment.

Within the first two years, our analyzes showed a 40 percent reduction in sugar maple growth from plots without snowpack. Growth rates remained compressed by 40 to 55 percent over the next three years. In contrast, there was no reduction in sugarcane growth in control plots where snow covered tree roots in mid-winter. These results are comparable to root mortality observed by other researchers in a previous snow removal experiment in Hubbard Brook.

Carnivores can account for more than half of the annual forest biomass accumulation in Hubbard Brook. Consequently, climate changes that reduce winter snowpack and increase ground freezing could reduce forest growth rates in the northern hardwood forest region by 20 percent through their effects on these trees alone. But we know this yellow birch it also undergoes root damage in response to soil freezingtherefore our estimate of changes in total forest growth is likely to be low.

Warmer growing season temperatures could at least partially offset this damage stimulation of tree growth rates, as some research suggests? Very little work has been done to understand how forests in seasonally snow-covered areas will respond to the interactive effects of climate change across seasons. To help fill this gap, we created the Experiment on climate change through the seasons at Hubbard Brook in 2013.

In this project we use buried heating cables to heat forest soils by 9 degrees Fahrenheit (5 degrees Celsius) during the snow-free season from April to November. In the winter we use a combination of buried heating cables and snow shoveling to induce freeze-thaw cycles in the ground. Our results so far show that root damage and reduced tree growth caused by winter soil freeze-thaw cycles they are not compensated by soil heating during the growing season.

Our work shows how often overlooked changes in winter climate can affect forest ecosystems. Snow loss can reduce forest growth, carbon sequestration and nutrient retention, which will have significant impacts on climate change and year-round air and water quality.

This work was supported by the Agricultural Research Service, the National Science Foundation, the US Environmental Protection Agency, and the Andrew W. Mellon Foundation.

Pamela Temler is a professor of biology at Boston University and Andrew Reinman is an assistant professor of environmental sciences at CUNY Graduate Center and postdoctoral research associate at Boston University.

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