Jim Worrall*, Roy Mask*, Tom Eager*, Leanne Egeland* and Wayne Shepperd†
Sudden aspen decline
in southwest Colorado
* US Forest Service, Rocky Mountain Region, Forest Health Management, Gunnison, Colorado, USA
† US Forest Service, Rocky Mountain Experiment Station (retired), Fort Collins, Colorado, USA
(INT-EM-07-01, year 1)
Note that regenerated
clearcuts are healthy,
surrounded by dead and
dying residual overstory
Intensive Survey
In the second phase, we began an
intensive field survey. We completed
76 plots on four National Forests, with a
randomly selected damage plot and a
neighboring, paired healthy plot.
We initiated a project in FY07 with the objectives of
determining the severity, site/stand factors and causes of
the recent, rapid mortality of aspen in Colorado.
300,000
As of 2007, ~13% of
Colorado’s aspen acreage
was affected
25
-1
Background and Objectives. In 2004, foresters in southwest Colorado began to report unusual dieback and
mortality of aspen. Mortality has increased rapidly in
Colorado and has attracted intense media interest and
concern from wildlife officials, forest managers, forest
products industry, environmentalists, and others.
200,000
100,000
0
Work in the first year was in two phases. The results of
the first phase, reported in this section, are in press§:
2005
2006
2007
20
15
10
5
0
Year
0
Landscape Analyses. We used GIS-DEM to analyze aerial
survey and cover-type data. We found:
aspen
cover type
• Damage generally increases
as elevation decreases
(see also gradient in photo above)
Root mortality varied tremendously, from
0 to >90% of root volume. Several analyses indicated a connection between
crown loss and poor root condition (low
volume/ high mortality). Healthy plots
generally had low root mortality. However, damaged plots had root mortality
from <10% to >90%.
A result of concern, confirming those at left,
is that regeneration did not increase in
response to crown loss. Where there was
a suckering response, it occurred on plots
with a large volume of healthy roots and/or
on mollic and especially pachic mollic soils.
Regeneration (10³ stems ha )
Aspen damage in CO (aerial survey acres)
Background, Landscape/Stand Analyses
20
40
60
% Crown Loss
80
100
a
SAD
• Damage tends to be on flat slopes
Regeneration
Anomaly
assoc. with
droughty soil
SAD
b
• Five biotic agents occurred in some
combination in all examined SAD locations:
•Cytospora canker,
•poplar borer,
•bronze poplar borer,
•two aspen bark beetle species
Regeneration (10 3 stems ha-1)
• Relative size of mortality is strongly
correlated with amount of mortality
Ratio of DBH of current mortality to healthy
Stand Analyses. We analyzed stand exam data from 31
stands in two areas of the San Juan NF and found:
• Regeneration is poor in damaged stands
1.4
1.2
1
14
12
Average regeneration
one year after
clearcutting aspen in SW
Colorado: 76,600 ha-1
10
8
Average regen.
in uncut, intact
stands in SW
Colorado:
2,500 ha-1
6
4
2
0
0.8
0
R = 0.64, P < 0.001
0
10
20 30 40 50 60
Current mortality (%)
70
25
50
75
Mortality (%)
0.6
80
100
Live aspen regen. (10 3 stems ha -1)
aspen
cover
type
20
Damaged plots
r = 0.36, P = 0.027
15
Healthy plots
r = 0.16, P = 0.34
Overstory
Root condition is correlated with
overstory crown loss and
(in damaged plots only)
regeneration, but the
latter two are not
correlated with
one another
10
Roots
5
80
Crown loss (%)
• Damage is focused on south
and southwest aspects
r = 0.47
P < 0.001
60
40
20
0
0
0
20
40
60
80
Root Mortality (%)
100
0
0.2 0.4 0.6 0.8
Root index (a measure of
root volume and vitality)
Preliminary Conclusions
We propose a hypothesis on causal factors in a decline context:
Predisposing factors: Low elevations, south to west aspects,
low density, mature age distribution on the landscape.
Inciting factors: Warm, dry conditions 2000-2005 and possibly
earlier.
Contributing factors:
above.
Secondary, biotic agents mentioned
1. Sudden aspen decline continues to increase rapidly in Colorado.
2. Two sources of data indicate that damaged stands are not
responding to crown loss and mortality with regeneration.
3. Poor regeneration in damaged stands is associated with dying
root systems.
4. Warm, dry conditions appear to drive SAD as inciting factor, but
predisposing and contributing factors also play important roles.
§Worrall JJ, Egeland L, Eager T, Mask RA, Johnson EW, Kemp PA,
and Shepperd WD. In press. Rapid mortality of Populus tremuloides
in southwestern Colorado, USA. Forest Ecology and Management.
http://dx.doi.org/10.1016/j.foreco.2007.09.071