Field Notes: For the Love of Tree Rings

Aug. 30, 2011 | KUOW
Ashley Ahearn


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  • Mountain Hemlocks at around 4,000 feet elevation. This grove started growing about 100 years before the American Revolution. credit: Ashley Ahearn
  • Jeremy Littell hikes across the path of an avalanche, now pretty melted in late August.
  • A beautiful core sample. credit: Ashley Ahearn
  • Chris Robertson cores an Alaskan yellow cedar credit: Ashley Ahearn
  • Jasper the snow dog. credit: Ashley Ahearn
  • A grove of Alaskan Yellow Cedar credit: Ashley Ahearn
Mountain Hemlocks at around 4,000 feet elevation. This grove started growing about 100 years before the American Revolution. | credit: Ashley Ahearn | rollover image for more

(Editor’s note: EarthFix Field Notes are reporters’ personal impressions and experiences from their coverage of the Pacific Northwest. In this entry, Ashley Ahearn describes her excitement at learning about the scientific secrets hidden within the growth rings of trees.)

Jeremy Littell doesn’t hike, he shops. I followed the researcher from University of Washington’s Climate Impacts Group into the Mt. Baker/Snoqualmie National Forest the other day on a shopping trip for old trees – but this was no flea market bargain basement expedition. Littell is constantly on the hunt for old high-elevation-loving trees (mountain hemlock and larch mainly) because they’re priceless treasure troves of climate data. That’s right – Jeremy Littell can drill into a tree trunk, pull out what’s called a core sample about the width of a drinking straw, and tell you roughly how old that tree is and what the climate patterns have been like during the course of that tree’s life. Now, if you’re a 900-year-old tree, that’s a whole heck of a lot of climate data. And if you’re a paleoclimatologist, finding that 900-year-old tree is like finding a piece of furniture from Versailles at your local flea market.

“When I’m hiking I’m almost always shopping,” Littell explains. “When thousand-year-old trees go on sale I’m there. I’m the first in line. Some people camp out in Kmart® for 36 hours, I’m willing to walk 36 hours.”

As I huff and puff behind Littell on the steep, rocky trail leading across avalanche paths and boggy meadows full of melting snow, he explains that for the most part, a tree’s growth rings correspond in size with how good the growth year was. So on a good year – i.e. a nice long growing season with proper amounts of rain and sun – those growth rings are going to be wider because the tree sequestered more carbon and put on more girth. I envision it a little bit like freshman year of college for most of us – an abundance of food and beer lead to a great year for growth in the horizontal direction. Ok, not exactly the same thing, but you get the picture.

Trees that grow at high elevations where they’re regularly covered in snow have shorter growth seasons that are highly dependent on snowfall. That allows scientists like Littell and Robertson, of Washington State’s Department of Natural Resources (who also is hiking with us), to tell, by looking at the growth rings in these trees, how snowfall has changed over the years.

And it’s not looking good. Littell co-authored a paper in Science earlier this summer which showed that across the West growth rings are getting wider as we have less snow (and therefore longer growth seasons for high elevation trees). “Trees that are most snow limited appear to be growing more now than they did in the decades prior and certainly than in some of the centuries in the past when snowfall was more abundant,” Littell says. This past winter was an exception in that we had heavy snowfall late in the season, but the warming trend overall is stronger than one irregular year.

Today, Littell and Robertson are “shopping” for old mountain hemlock and Alaskan yellow cedar. About a month ago, Littell was on a backpacking trip in the area with his 4-year-old daughter (whom he usually carries on his shoulders) when he stumbled upon some Alaskan yellow cedar, and he’s been jonesing to come back to find out how old these trees are.

Littell and Robertson don’t see trees the way the rest of us see trees. They talk about them like they’re old friends. (“Oh, this grove reminds me of that grove of yellow cedars over near Mt. Pilchuck.”). They remember their ages almost the way you or I would remember birthdays. (“That was a really old grove of hemlocks up on the ridge there, you could tell because of their gnarled trunks and crowns.”). They get excited to visit the trees, remembering unmarked mountain paths and trail mileages to obscure sites they daydream about revisiting. When Robertson describes his tree, the Alaskan yellow cedar, he uses his hands. “They have this stripy white bark, almost hairy looking,” he says. “And the needles hang elegantly with a yellowish tinge.”

I didn’t go up into the mountains just to learn about old trees, however. The data paleoclimatologists like Littell and Robertson collect is useful to policymakers and utility companies around the country who will be charged with managing dwindling water resources in the coming years. Littell is working with an organization called TreeFlow, which collects tree ring data and uses that information to help water managers plan for climate change in the future.

“There’s a big question about what the future holds,” Littell explains. “We need to know what the variability has been in the past and we need to have some idea of what we can expect in the future, particularly if climate continues to change the way that it has in the last several decades.”

The question is, are utility companies listening? 85 percent or more of our annual precipitation in this region comes in the winter, much of that as snow. And snow operates as a water bank for the West, dispensing water throughout the dry summer months as the snow slowly melts away at higher elevations and makes its way into rivers which power dams. Climate models for the region predict less snow and more rain – that means less snowy storage capacity. What might that mean for a dam manager? How might he or she plan for extended droughts or lower stream flow later in the summer? That will be the focus of my reporting in the coming days, but I wanted to share this adventure now, while I’m still soggy and wet from a day in the mountains, and really excited about tree rings and the information they hold for all of us.

© 2011 KUOW
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