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

Jeremy Christensen
Jeremy Christensen
Dec 13, 2011 12:38 PM
Important to also consider the effects of browsing by wild and domestic ungulates (Cows, elk) on the recruitment and regeneration of aspen stands. This is clearly having just as much effect on aspen decline as drought or infestation.
Jonathan Gelbard
Jonathan Gelbard Subscriber
Dec 13, 2011 10:18 PM
Nice work, Bill - fascinating (and scary) story...
DollySue Armstrong
DollySue Armstrong
Dec 18, 2011 11:19 AM
You didn't mention air pollution, what would be the effects of vaporized TCE or Benzene on our Aspen trees? Doesn't the carbon crap feed the fungus and extend the insect life? Please put my question to your scientists. Thankyou
Tim Baker
Tim Baker Subscriber
Dec 18, 2011 01:15 PM
DollySue -- the pollutants you mention (TCE and benzene) are highly unlikely to be culprits in SAD because they aren't found in high enough concentrations across a broad spectrum of the aspen range to be the cause. The only pollutant commonly occurring at high enough levels to be a contributor would be tropospheric ozone (O3) which does cause problems across broad regions of the country and is known to cause damage to various trees including aspen.

As for 'the carbon crap', I'm assuming you mean carbon dioxide which isn't a pollutant from the perspective of trees, at least not directly. In many cases, elevated CO2 levels are thought to reduce the impact of drought by allowing trees to photosynthesize more efficiently while keeping their stomates closed more of the time, hence losing less water. The fungus feeds on the trees, as do the insects, and neither are directly affected by CO2 levels (ignoring the impact of CO2 on temperature which is an indirect effect).

Hope that helps!
DollySue Armstrong
DollySue Armstrong
Dec 18, 2011 03:48 PM
Thankyou No, I was referring to Carbon Monoxide and the air pollution that hurts. Thanks for clarifying and adding to my vocab with Tropospheric Ozone. So you do agree this is a major problem (that wasn't mentioned in article) and do you have access to in situ monitoring? radiosonde? anemometer? And what effect does the tropospheric ozone have on the trees,fungus and insects? Thankyou for your time to explain in detail, I am learning alot.
Tim Baker
Tim Baker Subscriber
Dec 18, 2011 05:42 PM
Carbon monoxide (CO) is very short-lived (combines with O2 to form CO2 and O3) in the environment and unlikely to cause any widespread problems to trees.

There is a nationwide network of O3 monitors (maps are available if you search the EPA) and the subject of ozone levels has been a contentious debate nationally. Most recently the EPA tried to stiffen regulations of ozone to increase protection of plants but was rebuffed by the Obama administration. I don't want to side-track the discussion away from SAD though, so suffice it to say that the ecological impacts of O3 (and atmospheric N deposition for that matter) have been mostly lost in the clamor around climate change.