Ice storms are extreme winter weather events that inspire wonder and fear in people who live and work in northern temperate and boreal forests around the world. They are major causes of disturbance in northern forests, disrupt road and air travel, and cause closure of businesses and schools. They can also cause billions in damage and lead to loss of life. They have been a big deal in the past and, with a changing climate, they may become an even bigger deal in the years to come.
Despite concern over the increase in frequency and severity of ice storms, surprisingly little research has been conducted about their short- and long-term effects on hardwood forests in the northeastern United States and southeastern Canada. That’s changing, however, thanks to groundbreaking research underway in New Hampshire’s White Mountains.
Ice storms, also known as “glazing events,” form under unique climatological conditions. Moisture-rich warmer air moves over a lower ground layer of freezing air. Rain droplets falling from the warmer upper air super cool — but do not freeze — as they fall through the lower colder air. The droplets freeze on contact as they land on cold objects such as tree branches, roads and power lines.
Light icing events, measured as just a few tenths of an inch of ice, temporarily transform forests into crystalline wonderlands, complete with the tinkling, crackling sounds of ice-covered twigs gently rubbing together. Like a rainbow, light icing events are ephemeral, as the ice typically melts away quickly and quietly under the direct radiation of the sun and rising temperatures, leaving behind few traces and no damage.
Moderate to severe ice storms are a different story. If ice accumulates to one-quarter of an inch or more, branches and even entire treetops begin to sag, sway, bend and eventually snap under the heavy weight of frozen water. The gentle sounds of light ice give way to shudders, booms and cracks as tree limbs crash through the forest canopy and shatter on the frozen ground below. The scene is chaotic, unpredictable and often frightening.
Ecologically, moderate to severe ice storms have immediate and direct effects on forests by causing branches to break and trees to topple under heavy loads. This easy-to-observe damage results in a cascade of indirect and often unseen effects, such as reduced tree growth, increased availability of carbon and other nutrients from decaying twigs and branches, changes in the amount of light reaching the soil (and, consequently, increases in soil temperature and moisture), and changes the composition and dynamics of the microbial community.
Extreme and/or repeated ice storms, or ice storms in combination with other stressing events, can lead to more profound and permanent shifts in ecosystem biodiversity. Forests may begin to favor tree species with stronger wood (which doesn’t break as easily), more vertical branching patterns (which don’t accumulate as much ice), softwoods (which tend to shed ice) over hardwoods, or species that can respond rapidly to take advantage of new light and nutrient regimes.
Changes in the structure, function and species composition of forest trees have further consequences for wildlife, hydrology and forest nutrient cycling. They also affect the ecosystem services we expect from our forests: clean air, clean water, timber, wildlife habitat, recreation and aesthetics.
In the U.S., ice storms are common in an “ice belt” that has historically covered a broad swath of land between east Texas and southern New England. Scientists worry that the warming climate may be changing the area in which these storms typically occur. Southern regions may see fewer ice storms as temperatures warm more consistently above freezing. Northern regions, including northern New England and southeastern Canada, may see more — and more severe — ice storms as warm air moves northward and collides with the southern terminus of Arctic cold fronts.
We really don’t know much about ice storms and their impacts on forests. This is largely because ice storms are meteorologically difficult to forecast, so scientists don’t know when or where they will next occur. However, a new research study funded by the National Science Foundation is working to change that. Instead of taking scientists to ice storms, the scientists are bringing ice storms to them.
In late winter of 2016, a team of scientists from several research institutions (U.S. Forest Service, Syracuse University, Texas Tech University, Cornell University, the Cary Institute of Ecosystem Studies, the University of Southern Maine and the Hubbard Brook Research Foundation) set up ten plots in the U.S. Department of Agriculture’s Hubbard Brook Experimental Forest in the White Mountains of New Hampshire. Each plot was the size of a basketball court. The team spent the summer measuring all the trees and shrubs in the plots, monitoring tree health and bird and insect populations, studying the cycling of carbon, nitrogen and other nutrients through the forest, and learning to operate firefighting equipment.
Then, on four freezing nights between January 18 and February 11, 2016, the team used their firefighting gear to spray frigid water 100 feet into the air. It fell back over the forest as a fine mist, freezing on the already frozen branches on contact. In doing this, they created the first-ever experimental forest ice laboratory.
The team iced eight plots. Two plots received roughly one-quarter of an inch of ice (measured in a radius around a twig or branch), four received one-half of an inch, and two received three-quarters of an inch. Having successfully created this ice lab, the team started the long process of measuring the immediate effects of the icing in terms of branch breakage and fallen trees. They will also monitor the longer-term effects, such as growth response, change in species composition, interactions with invasive species, impacts on wildlife and consequences for forest nutrient cycling.
The team hopes to provide the scientific community, land managers and the concerned public with greater insight into the impacts of these powerful, frightening and curiously aesthetic extreme winter weather events on ecosystem dynamics in northern forests.
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