Antelope Bitterbrush and Scouler’s Willow
Response to a Forest Restoration Project
Donald J. Bedunah
Michael G. Harrington
Dayna M. Ayers
Abstract—Scouler’s willow (Salix scouleriana) and antelope bitterbrush (Purshia tridentata) survival, vigor, and use by ungulates
were monitored following shelterwood cut and prescribed burn
treatments designed to restore pre-European settlement conditions
to a western Montana forest stand. The prescribed burn treatments
resulted in modest willow mortality, substantial bitterbrush mortality, concurrent decreases in canopy cover, but increased plant
vigor in spite of heavy ungulate browsing on treated areas. A no
stand entry option would eliminate mortality of individual shrubs
caused by these restoration treatments, but population fitness may
decline dramatically as overstory density increases, and the threat
of severe wildfire will increase. A wildfire could be highly destructive to bitterbrush specifically, and to the forest in general.
Antelope bitterbrush (Purshia tridentata) and Scouler’s
willow (Salix scouleriana) are two important shrubs occupying grasslands, low-elevation forest and forest ecotones in
the northern Rocky Mountains. In western Montana, these
species are often two of the most preferred browse for wild
ungulates such as mule deer (Odocoileus hemionus) (Wilkins
1957), elk (Cervus elaphus) (Leege 1979), bighorn sheep
(Ovis canadensis) (Welch and others 1982), and moose
(Alces alces) (Pierce 1984), as well as white-tailed deer
(Odocoileus virginianus) and domestic livestock. There is
ample evidence that fires, both natural and human caused,
were a significant ecological component of these forestgrassland ecotones (Gruell and others 1982), and land
management agencies are prescribing fire as an ecological
restoration tool. The ponderosa pine (Pinus ponderosa)
cover type, located on lower elevation areas in western
Montana, is of special interest to land managers because of
the many values of this type. Early photographs provide
evidence that many ponderosa pine stands were more open
and spatially patterned by fire (Gruell and others 1982).
Since the late 1800’s fire suppression and selective logging
in ponderosa pine stands have resulted in forest stands with
dense Douglas-fir (Pseudotsuga menziesii) with largely unknown affects on browse production. The large-scale replacement of ponderosa pine by Douglas-fir may be undesir-
In: McArthur, E. Durant; Ostler, W. Kent; Wambolt, Carl L., comps. 1999.
Proceedings: shrubland ecotones; 1998 August 12–14; Ephraim, UT. Proc.
RMRS-P-11. Ogden, UT: U.S. Department of Agriculture, Forest Service,
Rocky Mountain Research Station.
Donald J. Bedunah is Professor, School of Forestry, University of Montana,
Missoula 59812. Michael G. Harrington is Research Forester, U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station,
Intermountain Fire Sciences Laboratory, Missoula, MT 59807. Dayna M.
Ayers is Research Assistant, Division of Biological Sciences, University of
Montana.
256
able because fir are more susceptible to disease and outbreaks of defoliating insects. Fir forests are usually more
“closed” with subsequent reduction in vigor and biomass of
understory vegetation, and these forests are more prone to
stand-replacement fires.
In 1991 the Bitterroot National Forest and Intermountain
Forest Science Fire Laboratory initiated a project to examine the response of a ponderosa pine/Douglas-fir stand to a
combination of prescribed fire and shelterwood cutting as
ecological restoration management tools. Photographs of
the study area in the early 1900’s reveal an open stand of
large ponderosa pine with little shrub understory. Apparently, with the removal of fire and subsequent stand entry
for harvesting trees, the area became dominated by Douglas-fir with an important component of antelope bitterbrush and Scouler’s willow in the understory. Since the
study area is an important winter range for wild ungulates
(deer, elk, and moose), the influence of the treatments on
browse species was considered very critical. Therefore, our
objective was to determine the effects of the restoration
treatments on antelope bitterbrush and Scouler’s willow
survival, vigor, and use by ungulates following treatments
and to determine variables influencing survival and vigor.
Study Area _____________________
The study site was approximately 50 ha (124 acres) of
second-growth Douglas-fir and ponderosa pine at the Lick
Creek Study Area of the Bitterroot National Forest in
Ravalli County, Montana. Elevation is 1,300 to 1,500 m and
precipitation averages 56 cm/yr, approximately 50 percent
occurs as snow (Gruell and others 1982). The soils are
derived from granitic parent materials and are shallow to
moderately deep. Tree basal area averaged 28 m2/ha and
was predominately even-aged Douglas-fir and ponderosa
pine that established after a 1906 clearcut (Gruell and
others 1982). Habitat types varied from Pseudotsuga
mensiezii/Vaccinium caespitosum on the toe slopes and
benches to Pseudotsuga mensiezii/Symphoricarpos albus,
Agropyron spicatum phase (Pfister and others 1977) on the
drier south aspects.
Experimental Approach
The study site was divided into 12 approximately equal
units of 4 ha (10 acres) by personnel from the Intermountain Fire Sciences Laboratory. Each unit was assigned to
one of the following four treatments: (1) a shelterwood cut,
(2) a shelterwood cut and a high-consumption burn, (3) a
shelterwood cut and a low-consumption burn, and
USDA Forest Service Proceedings RMRS-P-11. 1999
(4) a control. The shelterwood cut was completed in the fall
of 1992. Tree basal area was reduced by 53 percent to 13.1m2.
Prescribed burns were conducted in May 1993.
Before application of the shelterwood cut, we permanently
recorded the location of all bitterbrush and willow within 36
400-m2 circular plots established in the control, shelterwood
cut, low-consumption burn, and high-consumption burn
treatments. Pretreatment measurements of 1,856 bitterbrush and 871 willow permanently located included height,
stem diameter, and a vitality rating. Immediately after the
shelterwood cut, all bitterbrush and willow were relocated to
determine the degree of mechanical damage (all treatments
except control) and proximate fuel quantity (low consumption and high consumption burn treatments). Following
prescribed burning, all bitterbrush and willow were relocated to determine level of fire damage. Each shrub was then
monitored in the summers of 1993 and 1994 to document
survival and vigor. In each treatment, canopy cover (percent) of willow and bitterbrush was estimated in 36 33-m2
circular quadrats.
and bitterbrush was greatest in the burn treatments associated with the combined effects of the harvest and burn. The
combined effects of the stand entry and prescribed burning
is best illustrated using flow diagrams grouping the low and
high consumption burn treatments since there was not a
difference (p >0.05) in mortality between the burn treatments (figs. 1 and 2). Stand entry resulted in 77 percent and
70 percent of the Scouler’s willow and bitterbrush, respectively, receiving plant damage. Only 55 percent of the
bitterbrush plants in the burn treatments exhibited fire
damage; however, of these plants only 28 percent survived
(fig.1). For bitterbrush with mechanical damage, but exhibiting no fire damage, mortality was still high averaging 65
percent (fig. 1). For Scouler’s willow a greater percentage of
plants exhibited fire damage (81 percent) as compared to
bitterbrush, but survival was much greater averaging 82
percent (fig. 2). It is also interesting to note that of the
Scouler’s willow in the burned treatments that received
mechanical damage, but no fire damage, mortality was 50
percent. This relatively high mortality was a result of severe
plant damage associated with skid trails and was not representative of the response of willow to the range of mechanical
damage effects. Scouler’s willow mortality for the shelterwood
cut was only 14 percent compared to 35 percent mortality of
bitterbrush (table 1). This difference in survival between
species is likely associated with differences in plant morphology. The deep root system and multi-stemmed growth of
willow allow for higher tolerance to disturbance than that of
bitterbrush. Willow often resprouts after surface disturbance from a subterranean root crown (Lyon 1966), whereas
bitterbrush can only resprout from a surface caudex (Guinta
and others 1978), which is more easily removed or injured by
disturbance.
Mortality of willow and bitterbrush with burn damage
was best explained by the degree of burn severity (burn
class) (table 2). Bitterbrush was notably impacted by any fire
damage, whereas willow was not markedly affected until it
suffered deep charring of the root crown. Vigorous resprouting
is consistent with other research (Leege and Hickey 1971;
Leege 1979), and willow has been reported to increase in
both biomass and vigor by as much as 100 percent following
a burn (Noste and Bushey 1987; Leege 1969; Mueggler
1965). For willow and bitterbrush without fire damage,
mortality was best explained by the degree of mechanical
damage (table 2). Of the bitterbrush plants receiving any
mechanical damage, almost 70 percent sustained severe
damage and 86 percent of those died. Willow survival was
Data Analysis ___________________
Differences in pre- and post-treatment canopy cover and
density were evaluated using one-way analysis of variance.
We also grouped bitterbrush and willow into categories of
burned and unburned, since not all plants within the
burned treatments received fire damage, and then used
logistic regression to determine which variables were most
significant in predicting mortality for each group using the
variables of height, diameter, vitality class, mechanical
damage, fuel class, and burn class. These variables were
also correlated with post-treatment vigor using Kendalltau non-parametric correlation analysis (Sokal and Rohlf
1995). Differences in vigor among treatments were compared using an X 2 test. Differences were considered significant at p <0.05 for all analyses. Assumptions of equal
variance and normality were tested and, if necessary,
variables were transformed to meet assumptions.
Results and Discussion __________
The shelterwood cut and prescribed burn treatments
resulted in modest willow mortality, substantial bitterbrush
mortality, concurrent decreases in canopy cover, but increased plant vigor (table 1). Mortality of Scouler’s willow
Table 1—Change in cover, mortality, and plants with high vigor for antelope bitterbrush (Purshia tridentata)
and Scouler’s willow (Salix scouleriana) by treatment in the Lick Creek Demonstration Area, 1994.
Changes are relative to pre-treatment conditions. Average pre-treatment cover was 0.94 percent
for bitterbrush and 0.77 percent for willow.
Treatment
Antelope bitterbrush
Cover
High vigor
reduction Mortality
plants
Cover
reduction
Scouler’s willow
High vigor
Mortality
plants
- - - - - - - - - - - - - - - - - - - - - - - - - - percent - - - - - - - - - - - - - - - - - - - - - - - - - - Control
Harvest-only
Low consumption
High consumption
2
75
83
92
USDA Forest Service Proceedings RMRS-P-11. 1999
4
35
62
68
28
70
88
78
1
33
62
58
3
14
22
26
15
60
71
69
257
Pre-treatment
bitterbrush
Pre-treatment
willow
N = 1169
N = 401
Shelterwood cut
Shelterwood cut
No mechanical
damage
Mechanical
damage
No mechanical
damage
Mechanical
damage
N = 352
N = 817
N = 91
N = 310
Prescribed burn
Prescribed burn
Prescribed burn
Prescribed burn
Without
fire
damage
N = 95
Survival
89%
With
fire
damage
N = 257
Survival
32%
Without
fire
damage
N = 435
Survival
35%
With
fire
damage
N = 382
Survival
26%
Figure 1—Mechanical damage, fire damage, and
mortality of antelope bitterbrush (Purshia tridentata)
for the low-consumption and high-consumption burn
treatments combined.
Table 2—Significant variables in antelope bitterbrush (Purshia
tridentata) and Scouler’s willow (Salix scouleriana) mortality
in Lick Creek Demonstration Area, 1995.
Exposure
Bitterbrush
Fire
No fire
Fire
No fire
Number
dead (N)
Variable
Variance
explained
percent
Willow
258
459 (639)
425 (1217)
65 (337)
68 (534)
With
fire
damage
N = 89
Survival
82%
Without
fire
damage
N = 62
Survival
50%
With
fire
damage
N = 248
Survival
82%
Figure 2—Mechanical damage, fire damage, and
mortality of Scouler’s willow (Salix scouleriana) for
the low-consumption and high-consumption burn
treatments combined.
greater than 94 percent, except for those plants severely
damaged on skid trails, where survival decreased to 58
percent.
For surviving bitterbrush and willow we found an increase in the proportion with high vigor in the burn and the
harvest-only treatments compared to the control (p <0.05)
(table 1). This increase in vigor apparently increased the
palatability of these plants to browsers. Elk, moose, and
mule deer were observed on the study site throughout the
year. Heavy browsing of willow occurred during summer
immediately after burning; whereas, browse utilization of
bitterbrush was most extensive on the shelterwood cut
treatment, probably associated with higher available bitterbrush on this treatment compared to the burn treatments.
The heavy browsing before initial reference measurements
probably resulted in an underestimation of intensity of
browse use and most likely resulted in loss of new growth
and subsequently lower vigor for these plants. Other research findings have reported an increase in current annual
Species
Without
fire
damage
N=2
Survival
100%
Burn class
Mechanical damage
Burn class
Mechanical damage
72
91
84
90
growth of bitterbrush following logging for up to 7 years after
overstory removal (McConnell and Smith 1970; Stuth and
Winward 1976; Edgerton 1982) and immediately after spring
burning (Demarchi and Lofts 1985; Cook and others 1994).
Management Implications ________
Our results show significant bitterbrush mortality and
modest willow mortality associated with overstory removal
and prescribed burning associated with the restoration
treatments. The restoration treatments were a positive
stimulus to willow productivity with survival greater than
75 percent, and the percentage of high-vigor plants increased from 15 percent pre-treatment to 70 percent posttreatment. The reduction in bitterbrush density by an average of 65 percent for the burn treatments would certainly be
of concern to land managers interested with the maintenance of this important browse species. However, these
treatments also increased vigor of surviving bitterbrush,
and because its regeneration strategy requires almost competition-free, mineral seedbeds for seed germination from
rodent caches (Sherman and Chilcote 1972; Clark and others 1982) long-term effects on this species may still be
positive. Also, the restoration treatments would lower the
risk of a stand-replacement wildfire that would likely be
highly detrimental to the bitterbrush in this stand. By
reducing impacts of harvesting with widely spaced skid
trails using low-impact equipment and prescribing low fuel
consumption burns with variable fire coverage, plant mortality should be minimized, especially for bitterbrush. With
the return of open stand conditions, mineral seedbeds, and
USDA Forest Service Proceedings RMRS-P-11. 1999
more vigorous plants, the potential for natural regeneration
to replace fire-killed plants should be high (Gruell 1986).
References _____________________
Clark, R. G.; Britton, C. M.; Sneva, F. A. 1982. Mortality of bitterbrush after burning and clipping in eastern Oregon. Journal of
Range Management. 35: 711-714.
Cook, J. G.; Hershey, T. J.; Irwin, L. L. 1994. Vegetative response to
burning on Wyoming mountain-shrub big game ranges. Journal
of Range Management. 47: 296-302.
Demarchi, D. A.; Lofts, S. 1985. The effects of spring burning on the
productivity and nutrient concentration of several shrub species in
the southern Rocky Mountain trench. MOE Technical Report 19.
Victoria, BC: British Columbia Ministry of Environment. 89 p.
Edgerton, P. J. 1982. Response of the bitterbrush understory of a
central Oregon lodgepole pine forest to logging disturbance. In:
Tiedemann, A. R.; Johnson, K. L., comps. Proceedings—research
and management of bitterbrush and cliffrose in Western North
America; 1982 April 13-15; Salt Lake City, UT. Gen. Tech. Rep.
INT-152. Ogden, UT: U.S. Department of Agriculture, Forest
Service, Intermountain Forest and Range Experiment Station:
256-265
Gruell, G. E.; Schmidt, W. C.; Arno, S. F.; Reich, W. J. 1982. Seventy
years of vegetative change in a managed ponderosa pine forest in
western Montana—implications for resource management. Gen.
Tech. Rep. INT-130. Ogden, UT: U.S. Department of Agriculture,
Forest Service, Intermountain Forest and Range Experiment
Station. 42 p.
Gruell, G. E. 1986. Post-1900 mule deer irruptions in the Intermountain West: principle cause and influences. Gen. Tech. Rep.
INT-206. Ogden, UT: U.S. Department of Agriculture, Forest
Service, Intermountain Research Station. 37 p.
Guinta, B. C.; Stevens, R.; Jorgenson, K. R.; Plummer, A. P. 1978.
Antelope bitterbrush—an important wildland shrub. Publication
78-12. Salt Lake City, UT: Utah State Division of Wildlife
Resources. 48 p.
Leege, T. A. 1969. Burning seral brush ranges for big game in
northern Idaho. North American Wildlife and Natural Resources
Conference Transactions. 34: 429-437.
Leege, T. A.; Hickey, W. O. 1971. Sprouting of northern Idaho
shrubs after prescribed burning. Journal of Wildlife Management. 35: 508-515.
USDA Forest Service Proceedings RMRS-P-11. 1999
Leege, T. A. 1979. Effects of repeated prescribed burns on northern
Idaho elk browse. Northwest Science. 53: 107-113.
Lyon, L. J. 1966. Initial vegetative development following prescribed burning of Douglas-fir in south-central Idaho. Res. Pap.
INT-29. Ogden, UT: U.S. Department of Agriculture, Forest
Service, Intermountain Forest and Range Experiment Station.
17 p.
McConnell, B. R.; Smith, J. G. 1970. Response of understory vegetation to ponderosa pine thinning in eastern Washington. Journal
of Range Management. 23: 208-212.
Mueggler, W. F. 1965. Ecology of seral shrub communities in the
cedar-hemlock zone of northern Idaho. Ecological Monographs.
35: 165-185.
Noste, N. V.; Bushey, C. L. 1987. Fire response of shrubs of dry forest
habitat types in Montana and Idaho. Gen. Tech. Rep. INT-239.
Ogden, UT: U.S. Department of Agriculture, Forest Service,
Intermountain Forest and Range Experiment Station. 21 p.
Pfister, R. D.; Kovalchik, B. L.; Arno, S. F.; Presby, R. C. 1977. Forest
habitat types of Montana. Gen. Tech. Rep. INT-34. Ogden, UT:
U.S. Department of Agriculture, Forest Service, Intermountain
Forest and Range Experiment Station. 174 p.
Pierce, J. D. 1984. Shiras moose forage selection in relation to
browse availability in north-central Idaho. Journal of Canadian
Zoology. 62: 2403-2409.
Sherman, R. J.; Chilcote, W. W. 1972. Spatial and chronological
patterns of Purshia tridentata as influenced by ponderosa pine.
Ecology. 53: 294-298.
Sokal, R. R.; Rohlf, F. J. 1995. Biometry. 3rd ed. New York: W. H.
Freeman and Co. 887 p.
Stuth, J. W.; Winward A. H. 1976. Logging impacts on bitterbrush
in the lodgepole pine-pumice region of central Oregon. Journal of
Range Management. 29: 453-456.
Welch, B. L.; Monsen, S. B.; Shaw, N. L. 1982. Nutritive value of
antelope and desert bitterbrush, stansbury cliffrose and apacheplume. In: Tiedemann, A. R.; Johnson, K. L., comps. Proceedings—research and management of bitterbrush and cliffrose in
Western North America; 1982 April 13-15; Salt Lake City, UT.
Gen. Tech. Rep. INT-152. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station: 173-185.
Wilkins, B. T. 1957. Range use, food habits, and agricultural
relationships of the mule deer, Bridger Mountains, Montana.
Journal of Wildlife Management. 21: 159-169.
259