Dying Aspens Bring Climate Change Home

Few living things are as emblematic of the Rocky Mountain West as quaking Aspen trees. Rustling in delicate green waves, their fluttering leaves herald the freshness of a mountain morning. Ablaze in gold and crimson in the fall, their bold expanses proclaim the end of summer. Tall and sturdy, their knobby black and tan posts provide quiet guidance to backcountry skiers in a white world.

But Aspen trees are in trouble. Nearly one in every five Colorado aspen trees has died off in recent years. The phenomenon even has a name: Sudden Aspen Decline or SAD. It’s sad indeed and new research is linking the deaths to climate change.

It’s easy to think that climate change is “out there’’ in time or “over there’’ in space. Our tendencies are to focus on more immediate concerns like doing a job or paying the bills.

But the effects are creeping into our lives and onto our doorsteps. Driving along the Interstate 70 corridor between metro Denver and the ski areas, you can’t miss the vast hillsides of grey dead Lodgepole Pines. The mountain pine beetles that have ravaged many forests have also been linked to climate change. Every time I drive through, I fear the spark that could create a conflagration.
And now the aspens are dying.

Research published recently by experts from Stanford University and the University of Utah helps explain at least one major reason. Aspen groves were dehydrated by Colorado’s severe drought between 2000 and 2004. Researchers looked at two main hypotheses about how trees were affected: either photosynthesis was affected and trees eventually starved, or their ability to transport water was compromised and they began dying of thirst.

More frequent severe droughts, wildfires and hotter, drier temperatures are in store for the Rocky Mountain West as man-made climate change intensifies, scientists warn.

"Large scale mortality events, such as we see with aspens, are the dynamite in ecosystem responses to climate change. We know that when they occur, they make a huge difference. But we are at the early stages of being able to predict occurrence," said Chris Field, director of Carnegie's Department of Global Ecology and professor of biology and of environmental Earth systems science at Stanford, in a statement.

The research by the Stanford team favors the theory that Aspens  die of thirst because they lose the ability to conduct water in a drought.  Researchers found that about 70 percent of the vascular systems of affected trees were blocked. Tiny air bubbles, akin to embolisms in people, prevented water from traveling through the “veins’’ and root structures of the affected trees.  Roots appear to be especially vulnerable to long-lasting damage from lack of water. "Root death may also explain the lack of substantial resprouting in SAD forests , which has significant implications for the regrowth of aspen ecosystems, effects on biodiversity,and long-term carbon losses,'' the authors wrote in the journal PNAS. 

Much work remains to be done to better understand how climate stresses affect forest health.
"Our study provides a snapshot of what future droughts could hold for the emblematic tree of the American West. Our results indicate an impaired ability to transport water due to drought damage plays an important role in the recent die-off of aspens," said researcher William Anderegg in a statement.

Drought is not the only cause of climate-related tree stress.  A global overview of recent research by Craig D. Allen and others suggests that forests around the world are at risk.

The abstract of their study from 2010 published in Forest Ecology and Management says in part: “Although episodic mortality occurs in the absence of climate change, studies compiled here suggest that at least some of the world’s forested ecosystems already may be responding to climate change and raise concern that forests may become increasingly vulnerable to higher background tree mortality rates and die-off in response to future warming and drought, even in environments that are not normally considered water limited.’’

It is sad to envision a world with far fewer forests, which almost certainly will be our generation's legacy.


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