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What is killing the aspen trees in our forests?

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This is an excellent question and one that researchers are still trying to answer.  It is estimated conservatively that about 10% of the aspen trees are dying or in decline—a number that is staggering and a cause for concern.  And this is not just Utah, but all areas where aspen grows:  from Arizona to Colorado to Alberta, Canada.  The fact that aspen regenerate primarily through suckers rather than seed is exacerbating the problem.  Aspen, as a species, is not very genetically diverse, with large stands of a single clone.  Lack of genetic diversity means that there is less of a chance that a tree will be resistant to whatever is causing the decline.  In addition, as the “parent trees” are weakened by this disease complex, their root systems are unable to send up new shoots.  Good news is that preliminary research has shown that aspen may be more genetically diverse than originally suspected.

Researchers admit that there is certainly no one causal factor in the aspen decline.  Many theories have been raised, including several years of drought combined with heat, cytospora canker, and poplar borer.  Over-browsing by elk and cattle may also be a contributing factor.  Another theory is based on our land use history.  Widespread timber harvesting and wildfires in the late 1800s and early 1900s allowed aspen—which is a colonizer and depends upon disturbance—to grow in wider swaths than ever before.  The reduction in landscape disturbance starting in the 1920s, including fire suppression, is simply causing natural succession to speed up, and hence a decline in early-successional species such as aspen.

As research continues, forest managers will certainly adapt their practices to allow for restoration of the aspen—a tree species that symbolizes the “wild west.”

Posted on 22 Feb 2007

Marion Murray
Intergrated Pest Management Project Leader

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