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Growth Response of Wyoming Big
Sagebrush to Heavy Browsing by Wild
Ungulates
Trista L. Hoffman
Carl L. Wambolt
generally northeast aspect, and average annual precipitation is 305 mm, half of which is snow (Farnes 1975).
A one ha exclosure that was erected in 1957 in a Wyoming
big sagebrush, bluebunch wheatgrass (Agropyron spicatum)
habitat type allowed measurement of unbrowsed shrubs.
This exclosure was constructed after biologists noticed severe impacts on the vegetation, including the sagebrush, by
elk, mule deer and pronghorn antelope. These impacts
continue today (fig. 1), especially from the wintering northern elk herd which is currently estimated at 25,000 to 30,000
animals.
Although the use of an exclosure and plants in close
proximity was designed to minimize environmental influences on plants, some differences existed because of the
exclosure. Wyoming big sagebrush was denser with greater
canopy coverage inside the exclosure. Other vegetation
included large amounts of bluebunch wheatgrass prairie
junegrass (Koeleria macrantha), and clover (Trifolium sp.),
with lesser amounts of greasewood (Sarcobatus vermiculatus), fringed sagewort (Artemisia frigida), and prickly
Abstract-An exclosure in a Wyoming big sagebrush <Artemisia
tridentata ssp. wyomingensis)-bluebunch wheatgrass (Agropyron
spicatum) habitat type on the northern winter range of Yellowstone
National Park near Gardiner, Montana was used to compare browsed
with unbrowsed plants. We tested the hypothesis that Wyoming big
sagebrush plants that were not subjected to 35 years of winter
browsing by elk, mule deer, and antelope would exhibit growth
characters similar to browsed plants. Shrub production, seedhead
number, and leaf morphology were significantly different (P < 0.03)
between browsed and protected plants while leader characters were
not. Although there were some intrinsic differences inside and
outside the exclosure, browsing appears to be the most important
factor influencing the morphological differences found.
Wyoming big sagebrush (Artemisia tridentata ssp.
wyomingensis Beetle and Young) is recognized as an important winter food source for mule deer (Odocoileus hemionus
hemionus), pronghorn antelope (Antilocapra americanus)
andelk(Cerviselaphus),(Welch1981;WamboltandMcNeal
1987). This species is not as well adapted to heavy browsing
as many other shrubs (Wandera and others 1992; Bilbrough
and Richards 1993). Browsing affects plant vigor and changes
plant architecture, resource allocation, growth rate and
reproductive capacity (Maschinski and Whitham 1989;
Bilbrough and Richards 1993). Understanding the interaction between browsing, other environmental influences, and
plant response is a complicated and often confusing process.
The objective of this study was to compare morphological
features of Wyoming big sagebrush that were either protected from ungulate browsing for 28 years or were subjected
to heavy browsing during that period.
Study Area _ _ _ _ _ _ _ __
The study site was located on the Northern Yellowstone
Winter Range, 1 km northwest of Gardiner, Montana (45 N
latitude, 112 W longitude). The elevation is 1,615 m, with a
0
0
Figure 1-The ungulate proof exclosure near Gardiner,
Montana in Yellowstone National Park used for this
study. The exclosure was errected in 1957 after the
Wyoming big sagebrush dominated plant community
had been impacted by large numbers of ungulates. The
shrubs on the outside (left) of the exclosure in this scene
are spiny hopsage (Grayia spinosa). The shrubs inside
the exclosure are Wyoming big sagebrush.
In: Barrow, Jerry R.; McArthur, E. Durant; Sosebee, Ronald E.; Tausch
Robin J., comps. 1996. Proceedings: shrubland ecosystem dynamics in ~
changing environment; 1995 May 23-25; Las Cruces, NM. Gen. Tech. Rep.
INT-GTR-338. Ogden, UT: U.S. Department of Agriculture, Forest Service,
Intermountain Research Station.
Trista L. Hoffman is Research Assistant and Carl L. Wambolt is Professor
of Range Science, Department of Animal and Range Sciences, Montana State
University, Bozeman, MT 59717.
242
pear cactus (Opuntia polycantha). The lichenXanthoparmelia
chlorochroa covered almost every soil interspace leaving
little bare soil. Outside the exclosure, sagebrush cover was
sparse with plants often 20 m apart. Other vegetation also
was sparse and was dominated by prickly pear cactus, and
weedy annuals, with small amounts of crested wheatgrass
(Agropyron cristatum), prairie junegrass, needleandthread
(Stipa comata), and fringed sagewort. The soil interspace
was dry and bare.
Methods
production measured on unbrowsed plants was 44.7 g/plant.
These plants appeared vigorous, while plants outside the
exclosure exhibited large amounts of dead crown and appeared to have little vigor.
Wyoming big sagebrush is susceptible to browsing for a
number of reasons. It has an erect, linear architecture with
many growing points on the ends of the stems (Bilbrough
and Richards 1991). Also, it is heterophyllous, producing
large ephemeral leaves in spring and small perennial leaves
that live approximately one year (Beetle 1960; Miller and
Shultz 1987). Plants with evergreen leaves depend more on
these leaves for storage than plants with deciduous leaves
(Bryant and others 1983). Sagebrush carries 50% of its
nitrogen and total non-structural carbon (TNC) pool in its
leaves (Bilbrough 1990), so each time a leafis removed, this
nitrogen and TNC is lost to the plant. With moderate
browsing, sagebrush growth is reduced, vigor declines, and
plants are unable to replace lost biomass (Bilbrough and
Richards 1993).
Assimilate often moves from discrete portions of the sagebrush canopy to discrete portions of the roots (Watson and
Casper 1984). Parts of a crown that are severely browsed
thereby losing vigor may have a corresponding loss of vigor
in a particular portion of the roots. Plants in this study were
heavily browsed outside the exclosure and appeared to be
losing vigor as portions of every crown were dead. It is likely
that corresponding portions of roots were also dead.
Tiny insect galls were found in large numbers on the
leaves of many unbrowsed plants. The removal of leaves by
ungulates perhaps reduces the success of gall-forming insects outside the exclosure, because many insects overwinter in galls or use galls for several generations (Shorthouse
and Rohfritsch 1992). The galls indicated that plants were
not free of stress by growing in the exclosure. Competition
evidenced by the density of sagebrush, grasses and forbs was
another potential stress to these plants. However, the higher
vigor and subsequent production of the protected sagebrush
indicated that browsing most likely outweighs other stresses.
The high density of unbrowsed sagebrush and associated
plants inside the exclosure increases competition for water
and nutrients. Consequently, growth rates of plants inside
the exclosure may be decreased by competition as well as
insect herbivory. However, while high plant density increases competition, it also results in better developed canopy
coverage. This additional coverage improves soil moisture
content and heat storage, and reduces evaporation (Norman
and Campbell 1991). Surface temperatures of soils are more
variable between sagebrush plants than underneath them
(Pierson and Wight 1991). Less interspace between plants
inside the exclosure may reduce variability in soil temperatures. This has implications for germination and growth of
----------------------------------
Sampling
Ten plants were selected outside the exclosure, a few on
each of the 4 sides ofthe perimeter. These plants were paired
with 10 adjacent plants inside the exclosure to insure similar environmental conditions. Plants were measured for
production according to Wambolt and others (1994).
Seedheads were counted individually ifthere were less than
100 per plant and ocularly estimated ifthere were more than
100 per plant.
Ten current year leaders were clipped from each plant and
refrigerated for transport. Because ofheavy browsing, plants
outside the exclosure had no terminal leaders. Therefore,
axial long shoots were clipped. Inside the exclosure, plants
were dominated by terminal growth, and axial long shoots
were rare. Terminal long shoots were clipped from these
plants. Harvesting took place in mid-September 1992 after
all ephemeral leaves had fallen, but before heavy winter
browsing by ungulates began.
Leaves were removed from leaders and separated into two
categories: lobed and unlobed. Lobed leaves were fully developed in the typical tridentate shape associated with Wyoming big sagebrush. Unlobed leaves were small to fully
extended, but had not developed the tridentate shape. Total
leaf area for each leader was determined using a LI-3100
area meter (LI-Cor, Inc., Lincoln, NE). Leader length was
measured from the top of the previous year's growth scar to
the tip. Leaves and leaders were air dried at 40°C and dry
weight was measured with a Mettler H31AR balance.
Analysis
Normality testing indicated that most of the variables
measured had non-normal distributions. Thus, non-parametric statistics were used (Sokal and Rohlf 1981). The
Kolmogorov-Smirnov test, (SAS Institute 1988), was used to
determine if the distributions between browsed and
unbrowsed variables were similar, and the Wilcoxon test
determined the significance of the distribution medians.
Results and Discussion
Table 1-Average differences between browsed and unbrowsed
Wyoming big sagebrush plants.
Growth and Forage Production
Unbrowsed plants had consistently higher production
than browsed plants (table 1). Only 1 browsed plant produced more than 10.0 g of forage per plant, while 9 of 10
unbrowsed plants exceeded this amount. The maximum
Production (g/plant)
Seedheads per plant
Leader length (mm)
Leader dry weight (g)
Browsed
10.0a
o.oaa
22.9a
0.02 a
Unbrowsed
44.7 b
60.3 b
22.3a
0.02a
Values followed by different letters are significantly different (P < 0.05).
243
sagebrush seedlings whose timing mechanisms depend on
soil and air temperature (Meyer and others 1990; Meyer and
Monsen 1992).
Canopy structure also slows soil evaporation by shading
and by slowing wind. The presence of lichen on the soil
interspace also reduces surface evaporation. Campbell and
Harris (1977) found that soil evaporation was inversely
related to site production. At our study site, soil evaporation
was expected to be lower inside the exc10sure and indeed,
production was higher there.
Plants that are browsed replace lost carbon by increasing
shoot production compared to root production (Chapin and
others 1987). Browsed plants probably have difficulty acquiring nutrients due to lost tissue and lack of new root
growth. Further, nutrients are quickly depleted near existing roots making the investment in new roots a necessity.
The availability of nutrient resources is directly related to
the ability ofa plant to compensate for herbivory (Maschinski
and Whitham 1989), and low resource availability reduces
growth rate (Bryant and others 1983; Coley 1988). Therefore, browsed plants must balance vegetal production requirements with the need to produce new roots.
Average leader length and dry weight were not found to
be significantly different (P < 0.05) between browsed and
unbrowsed plants (table 1). Browsed plants produced leaders averaging 22.9 mm in length, while unbrowsed leaders
were 22.3 mm. Average dry weight was identical at 0.02 g.
Although terminal leaders were compared with axial leaders, both leader types were long shoots as described by
Bilbrough and Richards (1991). It is suggested that with the
terminal leader removed, the axial shoot adopts the function
of the former at least in length and weight. Most plants
outside the exc10sure were browsed so heavily that it was
difficult to find leaders. These plants had leaders growing
only where they were protected by masses of woody dead
crown. Inside the exc1osure, plants had hundreds ofleaders
growing on all parts of the crown.
No flowering stems were found on axial leaders, an observation similar to that of Bilbrough and Richards (1991).
However it is not known iflack of seedheads was due to the
axial position of the leader or to the prevention of flowering
by browsing stress.
Seed head Production
Leaves
Perhaps the most striking difference between browsed
and unbrowsed plants was in seedhead production (table 1).
The maximum number ofseedheads on browsed plants was 3,
and over half of the browsed plants had none. One of the
unbrowsed plants had no seedheads, but it was more common to find 60 to 100 seedheads per plant. Kay and Chaddie
(1991) found similar results in heavily browsed willows
(Salix spp.) growing in exc10sures not far from those in this
study. No aments were observed on stems that were within
reach of ungulates.
Reproduction depletes plant resources as photosynthate
is diverted from growth and maintenance to flower and seed
production (Bazzaz and others 1987; Reekie and Reekie
1991). Studies indicate that stress such as herbivory may
delay or prevent flowering for several years (Bazzaz and
others 1987; Maschinski and Whitham 1989). In antelope
bitterbrush (Purshia tridentata)and chamise (Adenostoma
fasciculatum), grazing is inversely related to flower production (McConnell and Smith 1977). The lower overall production of browsed sagebrush plants may indicate that the
allocation to replace vegetal biomass results in reduction to
reproductive ability.
Miller and Schemske (1990) positively correlated flower
number with biomass in common rape (Brassica rapa).
While overall biomass was not measured in the present
study, unbrowsed shrubs visually appeared to have far more
biomass than browsed shrubs. Production, which may reflect biomass, was consistently higher in unbrowsed shrubs.
Bilbrough and Richards (1991) found that buds for flowering stems on mountain big sagebrush (Artemisia tridentata
ssp. vaseyana) were located on short shoots at the distal end
of the terminal leader. Because almost all terminal leaders
were removed on browsed plants in this study, flowering
stems on these plants would have to be initiated from
elsewhere.
Although browsed and unbrowsed plants had virtually
the same total number of leaves (43 and 44 leaveslleader,
respectively), unbrowsed plants had significantly (P < 0.05)
more unlobed leaves, while browsed plants produced more
that were lobed (table 2). Unbrowsed plants averaged 20
lobed and 24 unlobed leaveslleader while browsed plants
produced 28 lobed and 15 unlobed.
The total leaf area was identical for plants in and outside
of the exc10sure (354 mm2lleader,P < 0.04). However, plants
that were browsed had significantly (P < 0.0001) larger lobed
leaves and smaller unlobed leaves (table 2). Unlobed leaves
on browsed plants averaged 273 mm21leader while there
were 226 mm21leader on protected plants. Lobed leaves,
in contrast averaged 81 mm21leader on browsed plants and
128 mm21leader on unbrowsed.
The mass of lobed leaves was not significantly different
between treatments (table 2). However, browsed plants
produced unlobed leaves averaging 0.01 gIleader, which was
significantly less (P < 0.05) than the 0.03 gIleader average of
unbrowsed plants. Total dry weight was also significantly
(P < 0.03) less for browsed plants.
Leaders
Table 2-Average differences among leaves per leader on browsed
and unbrowsed Wyoming big sagebrush plants.
Browsed
Lobed Unlobed Total
Leaf number
288
Leaf area (mm2) 2738
Leaf dry wt. (g)
.05a
SLA (mm2/g)
54608
158
81 a
.01 a
8100a
438
354a
.06 a
5900a
Un browsed
Lobed Unlobed Total
20b
226b
.05 a
4520b
448 .
24b
b
128
354a
.03b
.08b
4267b 5027b
Values followed by different letters in like columns (ie.lobed, unlobed, total) are
significantly different (p < 0.05).
244
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The area of a leaf divided by its dry weight is known as
specific leaf area (SLA), and is often used as an indicator of
photosynthetic efficiency or resource allocation by plants
(Reekie and Reekie 1991). Browsed sagebrush had a significantly (P < 0.0001) higher SLA value than unbrowsed shrubs
(table 2) which may indicate thinner leaves with a lower
investment in leaf biomass (Porter and Remkes 1990). Increased SLA may help compensate for the decrease in leaf
allocation due to herbivory (Reekie and Reekie 1991). SLA
was especially high among the unlobed leaves of browsed
plants at 8100 mm2/g/leader, while the same type ofleaves
on unbrowsed plants had only half that value at 4,267 mm2/g/
leader. Browsed plants also had higher SLA values for lobed
leaves and total leaves (5,460 and 5,900 mm2/g/leader,
respectively versus 4,520 and 5,027 mm2/g/leader for
unbrowsed plants).
Species may allocate assimilate toward increased photosynthesis or structural production, but not both (Reich and
others 1991). Thin leaves are a means of maximizing leaf
area with a minimal amount of resources. It appears that by
producing leaves with high SLA, browsed plants used resources efficiently to compromise between photosynthesis
and regrowth.
Conclusions
-------------------------------
Shrubs that were protected from long-term browsing were
larger and consistently produced significantly more forage
than browsed shrubs. Seedhead production was also severely reduced by browsing. This may result in stands that
have difficulty regenerating over the long term. Leaders on
browsed shrubs have similar numbers of leaves and total
leaf area as unbrowsed shrubs. However, browsed shrubs
produce more lobed leaves as well as thinner leaves, probably to compensate for herbivory.
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