A Bitterbrush Dieback in the Upper
Gunnison River Basin, Colorado
J. Arthur Hayes
Abstract—During a significant drought period (1988-1993), an
extensive dieback of antelope bitterbrush (Purshia tridentata
[Pursh] DC.) occurred in the Upper Gunnison River Basin of
Colorado. The area affected by the dieback includes at least 500
square miles. The dieback symptom was a necrosis of leaves and
stems. Fifty-four sites were examined. The amount of dieback at
a site ranged from 88% to 1% with an average of 41%. A canker
fungus, tentatively identified as Coniochaeta ligniaria (Grev.)
Cook, was found on stems of affected bitterbrush. Prolonged
drought stress could be the factor that predisposed shrubs to
fungus-caused disease development.
A period of significant drought began in October 1987
and continued through 1993. The Palmer Drought Severity Index, which is compiled by the Colorado Climate Center of Colorado State University, uses precipitation and
temperature records to model soil moisture (Palmer 1965).
The model is a simple soil moisture budget which describes drought severity in four categories: mild, moderate, severe and extreme. Data from six cooperative stations in the Upper Gunnison Basin is used to model soil
moisture to determine drought severity. During the 72
months between October 1987 and September 1993, 24
months were in the mild drought category, 18 were in the
moderate category, 18 were in the severe category and 12
months were in the extreme category. The Palmer Index
indicates that this drought period was the most severe in
terms of duration and depth or severity of drought since
1950 (Figure 1). Comparison over a longer period indicates this drought probably equals or exceeds the most
severe in the last 100 years (Nolan Doesken, pers. comm.).
The first occurrence of bitterbrush dieback was observed in 1990 in the Willow Creek area west of the city of
Gunnison, north of Blue Mesa Reservoir and in the Wood
Gulch Area, approximately 15 miles east of Gunnison.
Plants in both areas were observed to be dry and brittle
and appeared to have an unusual amount of dead
branches. Soil moisture monitoring in 1990 and 1991 revealed two areas in the Gunnison Basin that were among
the most drought affected in terms of soil moisture; the
Chance Gulch Area, 3 miles southeast of Gunnison, and
the lower Antelope Creek Valley, 3 miles northwest of
Gunnison. In May of 1991, an area of at least five hundred acres of big sagebrush (Artemisia tridentata Nutt.) in
the Chance Gulch area was observed to be defoliated, with
Figure 1—Palmer Drought Severity Index for the
Upper Gunnison River Basin, Colorado for the period January 1950 to September 1993. Source:
Colorado Climate Center, Ft. Collins, Colorado.
plants partially or completely dead. In July of 1991, observations in the Antelope Creek Valley revealed several
hundred acres where bitterbrush was affected, with the
majority of plants having more than 50% dead stems.
The appearance of the plants (the bark appearing to have
recently separated from the stems and the presence of
small, dead stems of a size which would be removed by
weathering in two or three years) suggest that the stems
had recently died. This study was undertaken to determine the extent of the bitterbrush dieback, the areas affected and to describe the sites where bitterbrush occurs
in the Upper Gunnison River Basin.
Study Area
The upper Gunnison Basin ranges in elevation from
about 7,500 feet to over 13,000 feet. The average annual
precipitation at Gunnison (7,700 ft. elevation) is 9.7
inches. Annual precipitation at 10,000 feet elevation
could be expected to be in the 20 to 25 inch range. Table 1
shows the monthly precipitation and temperature averages for Gunnison.
In: Roundy, Bruce A.; McArthur, E. Durant; Haley, Jennifer S.; Mann,
David K., comps. 1995. Proceedings: wildland shrub and arid land
restoration symposium; 1993 October 19-21; Las Vegas, NV. Gen. Tech.
Rep. INT-GTR-315. Ogden, UT: U.S. Department of Agriculture, Forest
Service, Intermountain Research Station.
J. Arthur Hayes is a Natural Resource Specialist with the Bureau of
Land Management in Gunnison, Colorado 81230.
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Table 1—Monthly averages for precipitation and temperature for Gunnison, Colorado.
Precipitation (inches)
Temperature (°F)
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
0.74
8.9
0.62
14.8
0.64
27.0
0.54
38.2
0.64
47.4
0.56
55.6
1.37
61.6
1.54
59.0
1.03
51.8
0.80
41.3
0.56
28.3
0.77
14.4
Source: Colorado Climate Center, Dept. of Atmospheric Science, Colorado State University (25 years of record).
Methods
Eighty-three percent (45/54) of the sites examined had
an average plant dieback of at least 25%. Fifty percent
(27/54) of the sites had an average plant dieback of at
least 40%. A total of 812 bitterbrush plants were examined with 1.7% (14/812) of all plants completely dead.
Although a multivariate analysis was not done, it appears that there was little correlation between elevation
or aspect and the amount of dieback at a site. The apparent lack of correlation between elevation or aspect and
dieback was likely because of the variability in precipitation and micro-climates within the basin.
The age of the dead stems ranged from one to 45-years
old. The age of a 1/4 inch diameter stem was generally
between 5 and 10 years. Stems 1/2 inch in diameter were
between 8 and 15 years old.
The amount of hedging of bitterbrush by big game animals between sites ranged from none to severe. There appeared to be little correlation in the degree of hedging and
the amount of dieback at a site.
Short leader growth and dead stems on big sagebrush,
serviceberry and aspen suggests that these species had
also been affected by the drought.
A large portion of the Upper Gunnison Basin was surveyed to locate areas where bitterbrush was a significant
component of the plant community. Within each area, a
representative site was selected by ocular estimate and
sampled to determine canopy cover of bitterbrush and the
percentage of dead bitterbrush. A total of 54 sites were
selected for sampling in 1993. One 200 foot long transect
was run at each site to measure the canopy cover of bitterbrush. The percent of dead branches (branches that had
the appearance of having died within the last 3 or 4 years)
on each bitterbrush plant along the transect was estimated. Aspect, elevation and a description of the plant
community were noted at each site. Sample bitterbrush
branches were sent to the Forest Service, U.S. Department of Agriculture, Shrub Sciences Laboratory in Provo,
Utah for the identification of possible causal fungus.
Results
The elevational range of bitterbrush in the Gunnison
Basin was found to be from 7,800 feet to 9,600 feet. Bitterbrush was found most commonly associated with plant
communities dominated by big sagebrush, and forest
types dominated by ponderosa pine (Pinus ponderosa
Lawson) and Douglas-fir (Pseudotsuga menziesii [Mirbel]
Franco). While bitterbrush was found on all aspects, it is
more commonly found on southeast, south and southwest
aspects. Bitterbrush was also found on a wide variety of
soil types, indicating it could be a component of many ecological types. The canopy cover of bitterbrush on the 54
sites examined ranged from 2.2% to 33%, with an average
for all sites of 13%, indicating that bitterbrush is often a
significant component of plant production on the sites
where it occurs.
A possible canker fungus, the characteristics of which
place the fungus “near the genus Coniochaeta, perhaps C.
ligniaria” (Nelson 1993) was found on live and dead stems
and was restricted to the vicinity of cankers. Many cankers on live stems appeared to have originated from cicada (Fam. Cicadidae) ovipositing wounds (Nelson 1993).
The variation in the amount of dieback among sites was
high. The most severely affected area was found to have
an average of 88% of each plant defoliated with many
whole branches dead. Two sites that burned about 10
years ago had the least dieback (1% and 2%). Excluding
the burned sites, the range of average individual plant
dieback for the 52 remaining sites was 13% to 88%.
Discussion
The prolonged deficiency in soil moisture and the occurrence of the dieback in bitterbrush strongly suggest that
moisture stress could have been a factor in the dieback.
Ordinarily weak or secondarily pathogenic fungi may become active when plants are predisposed or weakened by
environmental stress (Nelson et al. 1990). The model proposed by Houston (1992) describes dieback-decline diseases in forest systems as a “Host-Stress-Saprogen” relationship where environmental stress initiates disease.
Drought initiated declines of oak (Quercus sp.) and ash
(Fraxinus sp.) are discussed by Houston (1992). It is
likely that the drought and subsequent moisture stress
predisposed bitterbrush to a secondarily pathogenic fungus such as Coniochaeta.
While identification of the cause of plant disease often
requires intensive investigation, climate monitoring using
indices such as the Palmer Drought Severity Index is
comparatively simple and can be useful in predicting and
identifying areas where environmental stress of plants
can be expected. The ability to predict possible environmental stress to wildland plant communities would be
useful for land managers for wildlife habitat and livestock
forage management.
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Acknowledgments
Houston, D.R. 1992. A Host-Stress-Saprogen Model for
Forest Dieback-Decline Diseases. In: Manion, P.D. and
Lachance, D., eds. 1992. Forest Decline Concepts. Am.
Phytopathological Soc., St. Paul, MN: 3-25.
Nelson, D.L., Weber, D.J., Garvin, S.C. 1990. The Possible
Role of Plant Disease in the Recent Wildland Shrub
Dieoff in Utah. In: Proceedings-Symposium in Cheatgrass Invasion, Shrub Die-off, and Other Aspects of
Shrub Biology and Management. 1990. Las Vegas, NV,
April 5-7, 1989. U.S. Department of Agriculture, Forest
Service, Intermountain Research Station. General
Technical Report INT-276: 84-90.
Nelson, D.L., USDA Forest Service Plant Pathologist. Intermountain Research Station. Shrub Sciences Laboratory. Personal communication to Pete Angwin, USDA
Forest Service Plant Pathologist, Gunnison Service
Center, Dec. 28, 1993.
Palmer, W.C. 1965. Meteorological Drought. U.S. Dept.
of Commerce, Weather Bureau, Wash. D.C., Research
Paper No. 45.
The initial study to evaluate the dieback of bitterbrush
was funded by the Gunnison Basin Habitat Partnership
Program. I would like to thank Shawn Miller for his efforts in the data collection and Sandy Hayes, Barry
Johnston and Scott Sherwood for their help in the design
of this study, identifying areas of shrub dieback and in
reviewing this report. I would also like to thank Pete
Angwin and David L. Nelson for their efforts in the
identification of the canker fungus and for reviewing
this report.
References
Doesken, Nolan. 1993. Assistant State Climatologist,
Colorado Climate Center, Department of Atmospheric
Science, Colorado State University, Ft. Collins, Colorado.
Personal Communication to J. Arthur Hayes.
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