Southwestern Region

Southwestern Region
333 Broadway SE [map]
Albuquerque, NM 87102
(505) 842-3292
TTY: (505) 842-3198

FOREST HEALTH: Aspen Decline in Northern Arizona

HISTORY

For decades forest managers in the Southwest considered a decline in aspen communities the result of two main factors: 1) a change in fire regimes since European settlement, which allowed succession to conifer forests; and 2) heavy browsing by large ungulates, which prevented the regeneration of aspen in burned or harvested forests. Over the past several years, however, we have observed an accelerated dieback of aspen across northern Arizona following two defoliation events and several years of drought.

The following map shows areas of aspen decline detected through aerial surveys from 2000 - 2005. The map on the left shows the decline areas throughout the state in yellow. The right map shows a magnification of the area north of Flagstaff around the San Francisco Peaks.

MONITORING PROJECTS

We began an evaluation and monitoring project of affected aspen stands on the Coconino National Forest (NF) in 2003, followed by the Apache-Sitgreaves NFs in 2004. Our objectives are to describe mortality levels, regeneration condition, and stand and site variables influencing dieback and decline of aspen on these National Forests.

There are some differences in stand characteristics between the National Forests. On the Coconino NF, elevation of the aspen stands varies from 6,800 to 9,300 feet and is a key factor in both tree species composition and severity of decline. Lower elevation sites (< 7,500 feet) are on northerly aspects, dominated by aspen, and have a ponderosa pine and Gambel oak component. In general, these sites have a higher ratio of dead aspen to live aspen and more than one-half of the overstory aspen trees have died since 2001. Death is gradual and mortality is expected to continue as many live trees currently have only 10 to 30 percent of their original crown remaining. Higher elevation sites are found on various aspects, have mixed conifer component and a higher ratio of live to dead aspen trees. On the Apache-Sitgreaves NFs, elevation of the aspen stands varies from 7,600 to 9,100 feet, and mortality is severe in only two of the nine surveyed sites. On these sites, aspect seems to play a bigger role than elevation in where the mortality has occurrred.

A secondary pathogen and/or insect has been found to be associated with the mortality on all the sites, and includes canker fungi, wood borers, and clear wing moths. The presence and abundance of these agents varies by site.

Aspen regeneration by suckering following the overstory dieback has been observed on all sites by varying amounts. On the Coconino NF, timing of regeneration appears to be associated with elevation. Sprouting on the lower elevation sites was most abundant in 2003 and for higher elevation sites the spike was in 2004. Currently, there is little aspen regeneration remaining on any site due to heavy ungulate browsing. All sites have some level of browse damage; the percent of trees browsed ranges from 50 to 100 percent. Drought-stressed, overstory trees are expected to sprout suckers only within a year or two of tree death. The large die-off of mature aspen trees in many lower elevation sites coupled with browsing by ungulates is expected to result in forest type conversions, from aspen to conifer, in many of these ecologically unique and important sites across the state.

The following sites provide additional information on aspen in the Western U.S.:

Quaking Aspen Forest - Biotic Communities of the Colorado Plateau
Information on aspen forests of the Colorado Plateau

Aspen restoration in the Western United States
Forest Service research on aspen ecology and restoration