气候温和时树冠下的积雪比空地融化地快

It's a foggy fall morning, and University of Washington researcher Susan Dickerson-Lange pokes¹ her index finger into the damp soil beneath a canopy²（遮盖） of second-growth conifers（松柏类）. The tree cover is dense³ here, and little light seeps⁴ in among the understory of the Cedar⁵ River Municipal Watershed⁶ about 30 miles east of Seattle. She digs a small hole in the leaf-litter soil, then pushes a thumb-sized device, called an iButton, about an inch beneath the surface. If all goes well, this tiny, battery-powered instrument will collect a temperature reading every hour for 11 months. Researchers hope this tool and a handful of other instruments will help them map winter temperatures throughout the watershed（流域） as they track snow accumulation and melt.

 

This fieldwork piggybacks on a recent finding by Jessica Lundquist, a UW associate professor of civil and environmental engineering, and her lab that shows that tree cover actually causes snow to melt more quickly on the western slopes of the Pacific Northwest's Cascade⁷ Mountains and other warm, Mediterranean⁸-type climates around the world. Alternatively, open, clear gaps in the forests tend to keep snow on the ground longer into the spring and summer. Lundquist and her colleagues published their findings online this fall in Water Resources Research.

 

Common sense says that the shade of a tree will help retain snow, and snow exposed to sunlight in open areas will melt. This typically is the case in regions where winter temperatures are below freezing, such as the Northeast, Midwest and most of central and eastern Canada. But in Mediterranean climates -- where the average winter temperatures usually are above 30 degrees Fahrenheit⁹ -- a different phenomenon occurs. Snow tends to melt under the tree canopy and stay more intact in open meadows（草地） or gaps in a forest.

 

This happens in part because trees in warmer, maritime¹⁰（海的） forests radiate heat in the form of long-wave radiation to a greater degree than the sky does. Heat radiating from the trees contributes to snow melting under the canopy first.

 

"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 conserve¹¹ snowpack."

 

For the study, Lundquist examined relevant published research the world over that listed paired snow measurements in neighboring forested and open areas; then she plotted those locations and noted¹² their average winter temperatures. Places with similar winter climates -- parts of the Swiss Alps, western Oregon and Washington, and the Sierra Nevada range in California -- all had similar outcomes: Snow lasted longer in open areas.

 

"It's remarkable¹³ that, given all the disparities in these studies, it did sort out by climate," Lundquist said.

 

Even in the rainy Pacific Northwest, we depend on yearly snowpack for drinking water and healthy river flows for fish, said Rolf Gersonde, who designs and implements¹⁴ forest restoration projects in the Cedar River Watershed. Reservoirs in the western Cascades¹⁵ hold approximately a year's supply of water. That means when our snowpack is gone -- usually by the summer solstice -- our water supply depends on often meager¹⁶ summer rainfall to get us through until fall, he said. Snowpack is a key component¹⁷ of the Northwest's reservoir storage system, so watershed managers care about how forest changes due to management decisions or natural disturbances¹⁸ may impact that melting timetable.

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