Melting Snow Conundrum: Mild Climate Forests Vs. Open Sunny Areas
November 14, 2013

Melting Snow Conundrum: Mild Climate Forests Vs. Open Sunny Areas

April Flowers for - Your Universe Online

Dense tree cover at the Cedar River Municipal Watershed allows little light to seep into the understory, yet research by University of Washington’s Susan Dickerson-Lange and colleagues suggests that these dense forests are capable of melting snow much faster than the open areas around the forest.

In a study to better understand this phenomenon, Dickerson-Lange and her team have utilized a small device, called an iButton, which is pushed into the leaf-litter soil, to collect temperature readings of the ground under the second-growth coniferous forest every hour for 11 months. The research team plans to use this tool and other instruments to map winter temperatures throughout the watershed. This will allow them to track snow accumulation and melt.

The current study's fieldwork builds on recent findings by Jessica Lundquist, a UW associate professor of civil and environmental engineering, and her lab. Lundquist found that, despite what you may think, tree cover actually causes snow to melt more quickly on the western slopes of the Pacific Northwest’s Cascade Mountains and other warm, Mediterranean-type climates around the world. In contrast, she found that open, clear gaps in the forest tend to keep snow on the ground longer into the spring and summer.

Lundquist's research was published online in Water Resources Research.

Common sense dictates that snow exposed to sunlight in open areas will melt, and snow beneath the shade of a tree will retain snow for a longer time. In regions where winter temperatures are below freezing -- such as the Northeast, Midwest and most of central and eastern Canada -- this is typically the case. Mediterranean climates, however, where average winter temperatures are usually above 30 degrees Fahrenheit, have a different experience. Under the tree canopy in these climates, snow tends to melt, while it stays more intact in open meadows or gaps in the forest.

The research team found a partial answer in the trees. Trees in warmer, maritime forests radiate heat in the form of long-wave radiation to a greater extent than the sky does.

“Trees melt our snow, but it lasts longer if you open up some gaps in the forest,” Lundquist said. “The hope is that this paper gives us more of a global framework for how we manage our forests to conserve snowpack.”

Lundquist performed a literature review of all relevant published research around the world that listed paired snow measurements in neighboring forests and open areas. She then plotted those locations and noted their average winter temperatures. Places such as parts of the Swiss Alps, western Oregon and Washington, and the Sierra Nevada range in California had similar winter climates, and similar outcomes: snow lasted longer in open areas.

“It’s remarkable that, given all the disparities in these studies, it did sort out by climate,” Lundquist said.

Rolf Gersonde, who designs and implements forest restoration projects in the Cedar River Watershed, said that even in the rainy Pacific Northwest, the yearly snowpack is necessary for drinking water and healthy river flows for fish. In the western Cascades, the reservoirs hold approximately a year's supply of water. This means when the snowpack is gone - usually by the summer solstice - the region's water supply depends on often meager summer rainfall to make it into the fall season. Watershed managers care about how forest changes due to management decisions or natural disturbances may impact the melting timetable because snowpack is a key component of the Northwest’s reservoir storage system.

The research performed by UW in the Cedar River Watershed has been a beneficial partnership for the City of Seattle, because the 90,000-acre area provides drinking water to 1.4 million people. Although the land is currently closed to recreation and commercial logging, more than 80 percent of the land was logged during the early 20th century, and a large swath of dense, second-growth trees grows there now. Management techniques that include thinning and cutting gaps in parts of the forest to encourage more tree and plant diversity, that then leads to more diverse animal habitat, have offered the UW team a variety of sites to monitor.

Although temperature is a very broad predictor of snowmelt behavior, the team expects their theory to hold true as they look more closely at the relationship between climate and snowmelt throughout the Pacific Northwest. Their continuing research is a collaborative effort with researchers at Oregon State University and the University of Idaho, and they are ramping up a citizen science project asking hikers and snowshoers to share their observations.

“This is really just a start,” said Dickerson-Lange, a doctoral student in Lundquist’s lab who is coordinating the citizen-science observations. “The plan is to refine this model. With climate change, a cold forest now might behave more like a warm forest 100 years from now. We want to be able to plan ahead.”