Fairbanks, AK - For roughly seven months of the year Fairbanks is cold, icy and blanketed in snow, but May is the time of year when the Golden Heart City’s hillsides shed their winter white for bright shades of green. “When Chena Ridge gets that blush of green, we call it Greenup,” Rick Thoman is a Climate Scientists with the National Weather Service in Fairbanks. He and colleagues have been tracking Greenup for 30 years and they’ve found a way to predict it.
Interior Alaska’s rivers are starting to break up, trails are drying out and there are more than a few early-season mosquitoes navigating the skies above the boreal forest. Rick Thoman sasy the WEather Service's methods to predict when the first leaf buds will bulge on tree limbs is pretty accurate
“We track the amount that the daily high temperatures have been above freezing and we’re looking for a total number of those degrees to get above 650 and 750,” he explains. The count was nearing 600 in the middle of this week with more warm weather on the way. Those warm temperatures led the Weather Service to issue Red Flag warnings for the Interior this week, but Thoman says as soon as Greenup happens, things aren’t likely to remain as dry. “So once the trees leaf out, and they’re putting water into the air through their leaves, we see for instance the relative humidities will be higher during the day and that’s important with the threat of fire weather.”
It’s a process called transpiration. More simply put: trees sweat. “Yeah, you could think about it that way,” says Jessica Young. She’s a scientist at the International Arctic Research Center in Fairbanks.
“I want to know the role of these trees in the water cycle and I want to understand how much water they’re moving, when they’re moving it and the impact on climate and hydrology. This is the most exciting time of year, but it’s also nerve wracking because I want to catch them before they really start doing their thing.”
That 'thing' they do ramps up this time of year. Young leads the way to a tall, tan and green aspen tree and pulls out a drill. She makes a tiny hole in the side of the tree. And then we wait for nothing to happen,” she laughs. So, we walk over to a tall, paper birch tree and she pulls her drill out again. A clear watery liquid almost immediately pours from the little hole.
It’s birch sap made up mostly of water, combined with all kinds of other nutrients the tree uses to produce leaves. “As the trees warm up, they start to move carbon or sugars and this is carbon that they gained last year or maybe the year before," explains Yuong. "They’ll start moving that and then that will actually cause the water to start moving in the plant and it tends to start happening when the nights are below freezing and the daytime gets above zero.”
Even though the aspen tree didn’t drip like its birch counterpart, it’s using water in a similar way. Young recently did some calculations for the Fairbanks North Star Borough. Deciduous trees – the ones with leaves - cover between 15 and 20 percent of the landscape here. Young says these birch and aspen move roughly 32 billion gallons of water each the spring. “And that’s water that we used to think went into ground water recharge, but it’s not," she says. "It’s going into the trees and then when the leaves come out they transpire about 20 percent of that to the atmosphere” She says Birch and Aspen trees also store between 60 and 80 percent of water by volume and “sip” from it all summer long, kind of like the camels of the tree world. "Exactly," she laughs. "I call them aqua accordions because they’ll just - there they go moving all that water. It’s probably, and I’m just speculating, but it can be a drought response. It can be a way that they manage drought.”
So, while a dry spell this summer isn’t easily predictable, how the region’s leafy trees respond might be. And according to Rick Thoman over at the Weather Service, they’re well on their way to being ready for an oncoming warmer season. “So it looks like we’re most of the way there now," he says. "based on the current forecast, the best chances for Greenup would be probably most likely over the weekend.”