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Sudden Aspen Decline 

Contributed by Diane Thompson, July 30, 2018


The aspen decline has been most noticeable in southwestern Colorado, northern Arizona, and parts of Utah and Canada.


More recently, however, "sudden aspen decline" (SAD) has been reported. As of 2007, widespread, severe, rapid dieback and mortality had affected about 13 percent of the aspen in Colorado. SAD is different from what is traditionally referred to as aspen decline. It is occurring on a landscape scale, versus the stand-level changes that are typically noted with aspen forest related to disturbance and succession. The mortality is rapidly occurring over a few years, versus the typical changes occurring over decades. In addition, pathogens and insects associated with SAD are different from those usually associated with aspen mortality in Colorado. Evidence indicates that warm drought conditions earlier in the decade played a primary inciting role, that certain stand and site factors predisposed aspen to damage, and that the pathogens and insects are killing stressed trees.

An accelerated dieback of aspen was also observed across northern Arizona following two defoliation events and several years of drought. A secondary disease and/or insect was found to be associated with the dying trees as well, including canker fungi, wood borers, and clear wing moths. Aspen regeneration by suckering was observed after the dieback, but little of it remained after heavy ungulate browsing.




How do you know if an aspen grove is healthy?

A healthy aspen grove includes sprouts, saplings (middle-aged trees), and mature trees. The best rule of thumb to know for figuring out whether an aspen grove is healthy is whether you can see through it.

If you can see through an aspen grove, check whether the grove is missing the aspen sprout and/or the sapling layer. If either of these layers are missing and there are mature aspen trees, grass, shrubs, and/or young conifers present, the replacement of the aspen grove may be underway. The grass, shrubs, and/or conifers have a competitive advantage, in the absence of disturbance such as fire, as they use valuable resources needed by the aspen sprouts and seedlings, such as sunlight, water, and soil nutrients.

If no sprouts or saplings are present and there are only a few mature trees in an aspen grove, the grove is not healthy, especially if it is no longer producing sprouts. Unfortunately, it may be just a matter of time before an aspen grove dies off and disappears from the landscape.

The key to maintaining a healthy aspen grove are a continuing source of sprouts. New sprouts are the best indicator that the grove is healthy.

The suppression of forest fires has led to natural succession replacing aspen with conifers and other vegetation. Some aspen forests in the West may eventually be replaced by coniferous forest or other non-forest vegetation.

Sudden Aspen Decline in Colorado -- Forest Health Protection Rocky Mountain Region USDA Forest Service   -2011 Feb. 1 

What happened to aspen in the southern Rockies?  Aspen forests in many areas of western Colorado and adjoining states experienced widespread, severe, rapid branch dieback and mortality.  This phenomenon, termed “sudden aspen decline” (SAD), was first noticed in 2004 and increased rapidly through 2008.  There was no notable increase in size of the affected area after 2008.  

Is this any different from change that has always happened to forests?  Aspen forests are dynamic, and have always changed in response to climate, disturbance, and succession.  However, the recent event is different from the usual changes that have been seen during the last hundred years for a number of reasons:  Landscape scale.  The change occurred on a landscape scale, as opposed to the individual stand-level changes we have typically seen in the past. Rapidity of mortality.  The damage increased dramatically over a few years, as opposed to the typical changes that we see over decades.  Mortality agents.  The relative importance of pathogens and insects associated with SAD are different from those typically associated with aspen mortality in Colorado.   

What causes SAD?  Three interacting groups of factors are involved: Predisposing factors.  Low elevations, upper slope positions, south to west aspects, and open stands are vulnerable to warm drought.   Inciting factors.  Hot, dry conditions of 2000-2003 weakened vulnerable stands.   Contributing factors.  Secondary insects and diseases can kill trees under stress.  These include Cytospora canker, poplar borer, bronze poplar borer, and two aspen bark beetle species.

Is SAD related to climate change?  The impacts of SAD are consistent with projected effects of climate change on aspen.  The inciting drought was called a “global-change-type drought” because it was both unusually hot and dry.  SAD occurred mostly in areas projected to become climatically unsuitable for aspen early in the 21st century (Fig. 2), in areas with the most severe moisture deficits.  In Colorado, 2/3 of the aspen-suitable area is projected to become unsuitable by 2060, and the lower elevation of suitable climate is projected to rise nearly 2,500 feet by 2090. 

Is there anything we can do to stop it, or to help stands recover? In stands with active mortality, SAD may continue, and practical methods to protect the overstory are not available.  Where clones still retain some vigor and energy, but are deteriorating, regeneration may be stimulated by burning, cutting or other stand manipulation before root systems are too weak to respond.  Work in Utah suggests that stands with less than 50% mortality may be vigorous enough to respond to such disturbance. Preliminary results of treatments in Colorado generally support this.  In the long run, more young stands on the landscape will increase the likelihood of aspen presence following future warm droughts of this sort.

Why is aspen so important? 

  1. Water.  Aspen forests often yield more and higher-quality water than conifer forests.  They favor riparian and aquatic habitats, reduce erosion, and moderate streamflows. 

  2. Biodiversity and wildlife habitat.  Aspen forests are very diverse.  Many species are specifically associated with aspen.  Aspen provides unique wildlife habitat and good forage for elk and other ungulates. 

  3. Beauty and Tourism.  Esthetically, aspen contribute a major share of Colorado’s scenic beauty.  Tourism contributed $7.3 billion and 200,000 jobs to Colorado’s economy in 2004 (Colorado Tourism Office).  

  4. Wood products.  Several communities have industries that depend on aspen wood, with products such as paneling and excelsior.

Also Recommended


Spring fertilization of the roots to promote new growth with Fertilome Tree and Shrub Food or Fox Farm all-purpose food


Systemic Insecticide in the Spring and Fall with Bayers Tree and Shrub Protect or Tree and Shrub Insect Control

Regulating Factors

The only practical control for canker diseases is to avoid wounding residual stems during stand entries. Infection of wounded live trees, and subsequent tree mortality can increase dramatically in managed stands. In one study in Colorado and New Mexico, 20% of residual trees in partially cut stands died 5 years after the stand was harvested. Trunk cankers developing from infected logging injuries were the major cause of tree death. Two years later, 40% of the remaining residual trees were infected with various cankers, indicating that the mortality would increase.

Stands with a high proportion of cankered stems should be clearcut to stimulate sucker production and regeneration of the stand.

Because aspen is very susceptible to injury and infection by canker fungi, the installation of developed recreation sites in aspen stands should be discouraged.

High-Value Trees

Pruning diseased, dying, and dead branches on high-value trees should aid in reducing fungus spread from infected branches into the stem. Pruning tools should be disinfected with alcohol or laundry bleach after each cut to prevent infection of subsequent pruning wounds.

Small cankers on stems can be removed by excising all infected bark so there are clean edges around the infection to stimulate callus growth. Wounds should be shaped into an ellipse to promote rapid healing.

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