Autopsy of an Aspen

 

Cross-posted from The Last Word on Nothing.

In the rural Rocky Mountains where I live, we disagree about a lot of things -- politics, religion, water, Tim Tebow -- but we all agree on aspen. We love them, especially when they turn blaze-yellow in the fall, and we’d like them to stick around. So in 2004, when aspen throughout the Rockies started dying wholesale, the public reaction was fierce. What the heck was happening to our trees?

Since 2008, the dieoff has slowed, but so-called sudden aspen decline, or SAD, has hit nearly one-fifth of the aspen stands in the Rockies. It turns out that aspen decline was driven by drought -- namely a prolonged, region-wide dry spell that peaked in the early 2000s, and was thought to be a harbinger of the more frequent and severe droughts expected as the climate changes. In a grim sign of the times, though, it's no longer enough to know why the trees are dying. In order to predict future die-offs, it's important to know how.

So Bill Anderegg, a doctoral student at Stanford and a native of southwestern Colorado, performed an autopsy on aspen. Anderegg and his colleagues assumed, from previous studies of pinyon pines, that aspen killed by SAD essentially starved to death. When stressed by drought, the thinking went, the trees closed the pores on their leaves to keep water from escaping, interrupting photosynthesis and, over time, running down their carbohydrate supply. But in a paper published yesterday in the Proceedings of the National Academy of Sciences, Anderegg and his colleagues report that the aspen are dying not of hunger, but of thirst.

When the researchers studied dying aspen in the field in Colorado, and induced drought stress in both potted aspen and full-grown trees, they found that the aspen hung on to plenty of carbohydrates. The problem was that the water-delivery systems in the trees' roots and branches were blocked with air bubbles, like straws trying to pull water from too-shallow pools. (This condition is called cavitation, and researchers who have held sensitive microphones up to drought-stressed trees -- don't ask me why -- have recorded the pinging sound of air bubbles shooting into the water-carrying xylem tubes.) When trees lose 50 percent of their water-delivery capacity, they start to drop their leaves, no matter the season; the dying aspen in the study had lost 70 to 80 percent. And the more root blockage, the researchers found, the more root death. Aspen are a clonal species, and without healthy roots, they're slow to resprout and recover.

Like most of the casualties of climate change, the aspen are dying of multiple causes. Thirst weakens the trees, but as Forest Service entomologist Jim Worrall points out, insects and fungal infections play a big part too, often finishing the job. Some of these afflictions appear to be climate-change opportunists, encouraged by hot, dry conditions, vulnerable trees, or both: The aspen bark beetle, for instance, was unfamiliar to most entomologists before biologists studying SAD started finding them in almost every diseased stand.

The results of the aspen autopsy will help scientists model the response of forests to future climate change, predicting how forest types will shift, shrink, expand or simply blink out. Meanwhile, we in the Rockies still have our signature tree. But the view is bittersweet.

Michelle Nijhuis is a contributing editor at High Country News.

Image courtesy Stephanie Paige Ogburn

High Country News Classifieds
  • PART-TIME EXECUTIVE DIRECTOR
    Mitchell Museum of the American Indian Location: Evanston, IL Salary Range: $45,000 @ 24 hours per week. send resume: [email protected] www.mitchellmuseum.org
  • OUTDOOR PROGRAM - ASSISTANT DIRECTOR
    St. Lawrence University seeks to fill the position of Assistant Director in the Outdoor Program. To view the complete position description, including minimum qualifications required,...
  • PUBLIC LANDS DIRECTOR
    Job Announcement Conserve Southwest Utah is seeking a dedicated advocate for conservation and public lands Public Lands Director a "make a difference" position Conserve Southwest...
  • FOR SALE
    Yellowstone Llamas Successful Yellowstone NP concession Flexible packages
  • DIRECTOR OF DEVELOPMENT & MARKETING
    Grand Staircase Escalante Partners is seeking a full-time Director of Development & Marketing. This is a senior position responsible for the development of all marketing...
  • LEGAL DIRECTOR
    The Legal Director will work closely with the Executive Director in cultivating a renewed vision at NMELC that integrates diversity, equity, and justice. Black, Indigenous,...
  • VICE PRESIDENT, LANDSCAPE CONSERVATION
    The Vice President for Landscape Conservation leads Defenders' work to promote landscape-scale wildlife conservation, focusing on four program areas: federal public lands management; private lands...
  • NOVA SCOTIA OCEAN FRONT
    Camp or Build on 2+ acres in Guysborough. FSBO. $36,000 US firm. Laurie's phone: 585-226-2993 EST.
  • COMMUNITY FORESTER
    The Clearwater Resource Council located in Seeley Lake, Montana is seeking a full-time community forester with experience in both fuels mitigation and landscape restoration. Resumes...
  • GUNNISON BASIN ROUNDTABLE
    The Gunnison Basin Roundtable is currently accepting letters of interest for ten elected seats. Five of the elected members must have relevant experience in the...
  • PCTA TRAIL CREW TECHNICAL ADVISORS IN WASHINGTON'S NORTH CASCADES
    Seasonal Positions: June 17th to September 16th (14 weeks) - 3 positions to be filled The mission of the Pacific Crest Trail Association is to...
  • WE'RE LOOKING FOR LEADERS!
    As we celebrate 50 years of great Western journalism, High Country News is looking for a few new board members to help set a course...
  • MEMBERSHIP DIRECTOR
    Job Announcement Job Title: Membership Director Supervisor: Executive Director Salary: Up to $65,000/year DOE Benefits: Generous benefits package — health insurance, Simple IRA and unlimited...
  • UTAH PUBLIC LANDS MANAGER
    Who we are: Since 1985, the Grand Canyon Trust has been a leading voice in regional conservation on the Colorado Plateau. From protecting the Grand...
  • EXECUTIVE DIRECTOR
    Executive Director Walker Basin Conservancy Reno & Yerington, NV Background The Walker Basin Conservancy (Conservancy) leads the effort to restore and maintain Walker Lake while...
  • WIND RIVER WRITERS AND PHOTOGRAPHERS RETREAT BY THE NATIONAL BIGHORN SHEEP CENTER
    Enhance your writing or photography skills with world-class instructors in the beautiful Wind River Mountains. All skill levels welcome. Continuing education credits available.
  • EARTH CRUISER FX FOR SALE
    Overland Vehicle for travel on or off road. Fully self contained. Less than 41,000 miles. Recently fully serviced Located in Redmond, OR $215'000.
  • ENVIRONMENTAL GEOPHYSICS
    identifies suspect buried trash, tanks, drums &/or utilities and conducts custom-designed subsurface investigations that support post-damage litigation.
  • SMALL FARM AT BASE OF MOUNT SHASTA, CALIF.
    Certified organic fruit/berry/veggie/flower farm. Small home, 2 barns (one has an apartment), and more. Approx. two acres just in the City limits. Famously pure air...
  • TAOS HORNO ADVENTURES
    A Multicultural Culinary Memoir Informed by History and Horticulture. Richard and Annette Rubin. At nighthawkpress.com/titles and Amazon.