Twig Demography of True Mountain
Mahogany (Cercocarpus montanus Scop.)
Deborah Turley
Bruce A. Roundy
Scott C. Walker
Abstract—True mountain mahogany (Cercocarpus montanus Scop.)
is a valuable forage species for wild ungulates and livestock in
mountain brush communities of Utah, Wyoming, and Colorado.
Considered good to excellent browse, true mountain mahogany is
utilized heavily. To determine response to browsing, twig demography was studied on shrubs inside and outside an exclosure (approximately 30 m by 30 m), in Gardner Canyon, just northeast of Nephi,
Utah. Long shoot branches were measured, diagrammed, and
monitored in 1996 and 1997 to record the effects of browsing on
seasonal growth. Ratios of annual, second year wood, third year
wood, and seeds to total length of twigs were compared for browsed
and unbrowsed shrubs. Seed ratios were significantly higher in the
exclosure (unbrowsed 0.135, browsed 0.332 seeds/cm), browsing
was moderate to heavy (78.5 ± 6.0% of annual twig growth), and
annual growth ratios were significantly greater outside than inside
the exclosure in 1997. Ratios of 1997 to 1996 total twig length were
0.92 for browsed and 1.12 for unbrowsed shrubs. Increased annual
growth of browsed twigs compensated for twig length removed by
browsing.
True mountain mahogany shrub communities are found
throughout the Great Basin and Rocky Mountains in elevations between 1,070 and 3,050 m (Greenwood and Brotherson
1978). This shrub has the second largest distribution in its
genus, after curlleaf mountain mahogany (Cercocarpos ledifolius), and is centrally located in Utah, Colorado, and
Wyoming with the edges of its range extending to South
Dakota, Nebraska, Oklahoma, New Mexico, Arizona, and
Nevada (Davis 1990; Medin 1960). True mountain mahogany is usually found on foothills and mountain slopes
and grows from 1 to a maximum of 4 m tall (Davis 1990). On
average the plants are 1 m in diameter and 1.5 m tall but can
be much lower and wider when grazed (Stutz 1990). This
species is usually closely associated with other species such
as sagebrush (Artemisia spp.), pinyon-juniper (Pinus spp.
Juniperus spp.), aspen (Populus tremuloides), and mountain brush and mixed conifer communities (Davis 1990).
Brotherson and others (1984) reported that true mountain
mahogany was in greatest abundance on southern exposures at higher elevations but could be found on all aspects.
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.
Deborah Turley is Graduate Student and Bruce A. Roundy is Professor and
Chair, Department of Botany and Range Science, Brigham Young University,
Provo, UT 84602. Scott C. Walker is Wildlife Biologist and Acting Project
Leader, Great Basin Research Center, Utah Division of Wildlife Resources,
Ephraim, UT 84627.
168
Physical site factors influencing its growth are soil depth
and stoniness, with greater soil depth increasing growth and
greater stoniness decreasing growth (Medin 1960).
True mountain mahogany is known to have good to excellent palatability and to provide a major amount of winter
forage for mule deer (Odocoileus hemionus). The shrub also
is considered good to excellent for livestock (Range Plant
Handbook 1931) as well as highly valuable for Rocky Mountain elk (Cervus canadensis) as a food source (Kufeld 1973).
Rominger and others (1988) found Cercocarpus montanus
leaves to comprise 56 to 92% of the summer diet of Rocky
Mountain bighorn sheep (Ovis canadensis). Other wildlife
such as antelope (Antilocapra americana), mountain goats
(Oreamnos americanus), and small mammals may use this
shrub as forage (Wasser 1982). This shrub was known to be
hedged between 40 to 60% on one site by deer and elk or
cattle in the Uintah Basin (Applegate 1995). This species
shows good tolerance to winter browsing but may be much
reduced in size and productivity by repeated use during its
growing season.
A study done on bitterbrush (Purshia tridentata) and
sagebrush (Artemisia tridentata) in exclosures showed that
stagnation in the plants occurred after a few years of no
browsing (Tueller and Tower 1979). Waugh (1990) conducted research on a big game exclosure in southeastern
Wyoming. He found no significant difference in biomass
between sagebrush in and outside the exclosures. However,
current year’s production was much less for true mountain
mahogany within the exclosure compared to outside the
exclosure. He suggested that some utilization was necessary
for optimum production of mountain mahogany, and stagnation and decadence would occur if this shrub was not browsed
over time.
Nixon (1977) measured a mountain mahogany growth
after 10 years in Nebraska. Shrubs on the northeast-facing
slope increased 37 cm in height but these shrubs on the
southwest facing slope stayed the same height. There was an
overall decrease in number of base stems on shrubs on both
slopes. Reproduction was found only in one location on the
southwest-facing slope.
Bilbrough and Richards (1993) studied twig demography
to determine growth response to browsing for sagebrush and
bitterbrush. Simulated winter browsing treatments were
applied and responses recorded. The production of the stem
was recorded for short shoots, long shoots, mortality, and
flowering stems. Simulated browsing reduced long-shoot
growth of sagebrush but increased that of bitterbrush. Short
shoots of both species produced long shoots at a similar rate
on both browsed and unbrowsed shrubs. Shoot biomass and
node production in bitterbrush equaled or increased with
USDA Forest Service Proceedings RMRS-P-11. 1999
simulated browsing, whereas sagebrush had a greater frequency of long shoots but biomass was the same as the
unclipped plants. Node production of sagebrush also decreased with simulated browsing.
Roundy and Ruyle (1989) studied twig demography to
determine effects of grazing pressure on jojoba (Simmondsia
chinensis). Twigs of heavy, moderate, and ungrazed shrubs
were diagrammed. Regrowth from buds on grazed branches
was sufficient to replace grazed biomass. Lateral buds produced shoots when apical buds were grazed. Both female and
male flowers were less abundant on grazed shrubs.
The objective of this study is to determine response of true
mountain mahogany to winter browsing from deer and elk.
The approach was to compare plant growth responses between browsed and unbrowsed shrubs growing under the
same climatic and ecological conditions. Diagramming
branches allows the determination of growth characteristics
of a shrub and can be done to monitor responses to herbivory
on a specific site. It can determine precisely where shoot
growth occurs, which buds form shoots, what part of the
plant is reproductive, and where second-year and third-year
wood are found. This allows determination of the effects of
browsing, the number of available growing points, and the
amount of growth from those points.
Methods _______________________
An exclosure located in Gardner Canyon a few miles
northeast of Nephi, Utah, was first built in the 1930’s and
then reinforced in 1946. The area of the exclosure supports
a dominant true mountain mahogany community. The portion of the exclosure keeping out wildlife and cattle still
stands presently. Due to the long-term rest from browsing
for shrubs within the exclosure, the site seemed satisfactory
to compare browsed versus nonbrowsed shrubs of mahogany.
The exclosure covers both a south- and north-facing slope
and an area approximately 30 m by 30 m.
A 30 m line transect was set up both inside and outside the
exclosure. Ten mature shrubs were randomly chosen along
the transect line. Four branches on each shrub were then
chosen for diagramming. Three branches contained and
were chosen to measure the amount of long-shoot growth, or
new growth, found on the branch. The fourth branch was
chosen to measure the short-shoot growth. These branches
were then diagrammed with measurements of length of
annual growth, second-year wood, third-year wood, and
locations of buds, flowers, and seeds. The branches were
marked with browse tags and first diagrammed in the fall of
1996 to get a baseline of the branch initially observed. Each
branch was drawn free hand to scale. The drawings were
photocopied and used to diagram new growth in the summer
of 1997. Any missing branches previously drawn but now
absent were recorded so that an accurate diagram of the
branch persisted and browsing effects would be known. The
site was visited in early summer to determine amount of
browsing and number of flowers produced on the branches
diagrammed. Shrubs on this site are mainly browsed during
winter by deer and elk. The site was visited again in late
summer to diagram new growth and number of seeds
produced.
USDA Forest Service Proceedings RMRS-P-11. 1999
Because each branch varied in size, ratios of seasonal
growth to total branch length were calculated to standardize
the data. Analysis of variance was used to determine significant differences between browsed and unbrowsed shrubs
over time.
Results ________________________
Only results of long-shoot measurements will be presented in this paper. Browsed and unbrowsed shrubs averaged 0.14 and 0.33 seeds/cm of twig length, respectively. The
seed ratios were significantly different (p <0.05). For the
winter of 1996-1997, the percent browsed length of total
twig length was 78.5 ± 6.0 for annual growth, 29.3 ± 5.3 for
second-year wood, 6.8 ± 3.8 for third-year wood, and 41.3 ±
4.0 for total twig length.
There were no significant differences in annual, secondyear wood, and third-year wood between browsed and
unbrowsed shrubs in 1996 (table 1). Annual growth was
significantly higher for browsed than unbrowsed shrubs in
1997. Second-year and third-year growth were both significantly less on browsed than unbrowsed shrubs in 1997.
For unbrowsed shrubs within the exclosure, the annual
growth ratio was significantly lower in 1997 than in 1996.
Second-year wood was also significantly less and third-year
wood was significantly higher in 1997 than in 1996.
Outside the exclosure, annual growth ratios of browsed
shrubs were not significantly different from 1996 to 1997.
Second-year wood was significantly less in 1997 than in
1996. Third-year wood was significantly higher in 1997 than
in 1996. All categories in table 2 were significantly different
from each other comparing browsed to unbrowsed.
Table 1—Ratios of specific twig growth of true mountain mahogany to
total twig length.
Category
Year
Annual
growth
Second-year
wood
Third-year
wood
Unbrowsed
Browsed
Unbrowsed
Browsed
1996
1996
1997
1997
0.235aa
0.291a
0.093b
0.342a
0.546a
0.553a
0.209a
0.050b
0.115a
0.103a
0.496a
0.376b
a
Means followed by the same letter within a column and year but between
categories are not significantly different (p <0.05).
Table 2—Ratios of twig length in 1997 to that in 1996 for true mountain
mahogany.
Category
Total twig
length
Annual
growth
Second-year
wood
Browsed
Unbrowsed
0.923a
1.121
1.379
0.538
0.099
0.522
a
All values within a column differed significantly (p < 0.05).
169
Discussion _____________________
Branches inside and outside the exclosure were similar in
growth parameters in 1996. By 1997, branches outside the
exclosure were significantly different from those inside the
exclosure due to the amount of browsing and regrowth that
had occurred. The ratio for annual growth on the unbrowsed
branches had dropped dramatically, whereas annual growth
on the browsed branches remained similar to that of the year
before. Second-year wood was decreased on browsed branches
indicating the high utilization that had occurred over the
winter.
Seed ratios were much higher for shrubs inside than
outside the exclosure. Seeds of true mountain mahogany are
produced from second-year or older growth. Ratio differences may be a result of greater second- and third-year twig
length on unbrowsed than browsed branches in 1997.
The amount of annual growth diagrammed that was
browsed was high at 78.5%. This, and the fact that 41% of the
total length diagrammed was browsed, indicates moderate
to heavy winter browsing. The ratio of total twig length in
1997 to 1996 is 0.92 for browsed twigs (table 2), indicating
branch length is similar for both years. For the 1996 to 1997
browsing and growing seasons, true mountain mahogany
compensated well with new growth for the amount browsed
even under high utilization on this site.
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USDA Forest Service Proceedings RMRS-P-11. 1999