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
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