Tree species shape boreal forest water cycle

December 15, 2016

University of Alaska Fairbanks scientists are presenting their work at the American Geophysical Union’s fall meeting in San Francisco this week. Here are some highlights of their research, as shared at the world’s largest Earth and space science meeting.

The water cycle in boreal forests can vary enormously, depending on whether deciduous or coniferous trees cover the landscape.

Jessica Young-Robertson, a researcher at the University of Alaska Fairbanks’ International Arctic Research Center, said a two-year study indicates there is wildly different water usage among different tree types in Alaska and Western Canada. She discussed the project on Thursday during a presentation at AGU.

Little has been known about the hydrology of boreal forests, which dominate the region. The study, conducted in 2013-14, determined that deciduous trees — primarily birch and aspen — consume 20-25 percent of snowmelt each season. They occupy 60 percent of the boreal forest, growing on relatively warm, dry soils. That water consumption could total as much as 20 billion cubic meters of water per year, equivalent to about 10 percent of the Yukon River drainage.

Coniferous trees, such as black and white spruce, are much less thirsty. They consume less than 1 percent of snowmelt. They typically grow in colder, permafrost-laden soils, comprising 38 percent of the boreal forest.

Those stark differences could have big implications on the future water cycle as climate change shifts the sub-Arctic landscape. Warming temperatures are expected to bring more wildfires and permafrost thaw to the region, which could result in a gradual shift toward more deciduous tree abundance.

Not only do deciduous trees consume more water, they also transpire as much as 20 percent of it back into the environment during leaf-out. That additional moisture could create conditions more favorable to thunderstorms, another factor that could increase the frequency of wildfires.

Young-Robertson showed that an expansion of deciduous tree cover of 1-15 percent will increase the consumption of snowmelt by as much as 3 billion cubic meters. She said the next stage of the UAF-led research project will look at how the environment affects deciduous tree use of snowmelt water, the effect on the water cycle and larger environment. This includes understanding how this research can help reduce the winter air pollution in Fairbanks caused by burning wet wood.

“We’re trying to dig in to look at the impact of climate,” she said.