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
  • ASSOCIATE PROGRAM MANAGER
    Associate Program Manager ORGANIZATION BACKGROUND Parks California is a new organization working to ensure that our State Parks thrive. From redwood groves and desert springs...
  • ATTORNEY AD
    Criminal Defense, Code Enforcement, Water Rights, Mental Health Defense, Resentencing.
  • LUNATEC HYDRATION SPRAY BOTTLE
    A must for campers and outdoor enthusiasts. Cools, cleans and hydrates with mist, stream and shower patterns. Hundreds of uses.
  • LUNATEC ODOR-FREE DISHCLOTHS
    are a must try. They stay odor-free, dry fast, are durable and don't require machine washing. Try today.
  • PROFESSIONAL GIS SERVICES
    Custom Geospatial Solutions is available for all of your GIS needs. Affordable, flexible and accurate data visualization and analysis for any sized project.
  • FREE RANGE BISON AVAILABLE
    Hard grass raised bison available in east Montana. You harvest or possible deliver quartered carcass to your butcher or cut/wrapped pickup. Contact Crazy Woman Bison...
  • CONSERVATION ASSOCIATE - OKANOGAN LAND TRUST (NORTH CENTRAL WA)
    Do you enjoy rural living, wild places, and the chance to work with many different kinds of people and accomplish big conservation outcomes? Do you...
  • CARDIGAN WELSH CORGIS
    10 adorable, healthy puppies for sale. 4 males and 6 females. DM and PRA clear. Excellent pedigree from champion lineage. One Red Brindle male. The...
  • A CHILDREN'S BOOK FOR THE CLIMATE CRISIS!!
    "Goodnight Fossil Fuels!" is a an engaging, beautiful, factual and somewhat silly picture book by a climate scientist and a climate artist, both based in...
  • DIGITAL ADVOCACY & MEMBERSHIP MANAGER
    The Digital Advocacy & Membership Manager will be responsible for creating and delivering compelling, engaging digital content to Guardians members, email activists, and social media...
  • DIGITAL OUTREACH COORDINATOR, ARIZONA
    Job Title: Digital Outreach Coordinator, Arizona Position Location: Phoenix or Tucson, AZ Status: Salaried Job ID Number: 52198 We are looking for you! We are...
  • DESCHUTES LAND TRUST VOLUNTEER PROGRAM MANAGER
    The Deschutes Land Trust is seeking an experienced Volunteer Program Manager to join its dedicated team! Deschutes Land Trust conserves and cares for the lands...
  • ASSOCIATE DIRECTOR OF DEVELOPMENT
    The Nature Conservancy in Wyoming seeks an experienced fundraiser to join our team. We're looking for a great communicator who is passionate about conservation and...
  • INDIAN COUNTRY FELLOWSHIP
    Western Leaders Network is accepting applications for its paid, part-time, 6-month fellowship. Mentorship, training, and engaging tribal leaders in advancing conservation initiatives and climate policy....
  • MULESHOE RANCH PRESERVE MANAGER
    The Muleshoe Ranch Preserve Manager develops, manages, and advances conservation programs, plans and methods for large-scale geographic areas. The Muleshoe Ranch Cooperative Management Area (MRCMA)...
  • ARTEMIS PROGRAM MANAGER
    Founded in 1936, the National Wildlife Federation (NWF or Federation) is America's largest and most trusted grassroots conservation organization with 52 state/territorial affiliates and more...
  • ASSISTANT OR ASSOCIATE PROFESSOR OF ENVIRONMENTAL HUMANITIES
    Assistant or Associate Professor of Environmental Humanities Whitman College The Environmental Humanities Program at Whitman College seeks candidates for a tenure-track position beginning August 2023...
  • EXECUTIVE DIRECTOR
    High Country Conservation Advocates (HCCA) in Crested Butte, CO is seeking an enthusiastic Executive Director who is passionate about the public lands, natural waters and...
  • ASSOCIATE DIRECTOR OF VOLUNTEER PROGRAMS
    Are you passionate about connecting people to the outdoors? The Pacific Crest Trail Association (PCTA) is looking for someone with volunteer management experience to join...
  • EXECUTIVE DIRECTOR
    The conservation non-profit Invasive Species Action Network seeks an executive director. We are focused on preventing the human-caused spread of invasive species by promoting voluntary...