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Impact of Large Ungulates in Restoration of
Aspen Communities in a Southwestern
Ponderosa Pine Ecosystem
Wayne D. Shepperd and M.l. Fairweather1
Abstract - Experience has shown that in some areas of the Southwest,
regenerating aspen suckers require fencing to protect them from browsing
elk. In October, 1991 we removed the fence surrounding a 6.5 ha aspen
sucker stand north of Flagstaff, Arizona to test whether the trees were large
enough to be out of reach of the animals. The site had been fenced for
five yearS following clearfelling of several clones that comprised the original
stand. The regenerated stand averaged 50,000 stems/ha with dominant
stems over 3 m in height. By October, 1992, most stems in one clone had
been severely damaged by elk. Animals broke many stems to reach the
terminal foliage, often infecting the residual stem with Cytospora canker.
Monitoring will continue to determine if the remaining clones will be browsed
in future years. It appears that fencing must remain in place indefinitely in
this ecosystem, given the demand for browse associated with current high
animal populations.
INTRODUCTION
absence of frequent fIre in southwestern ecosystems since
European settlement (Covington and Moore 1991, Dieterich
1980) and extreme browsing pressure from large ungulates.
The impacts of long-term fire control measures upon
vegetation succession in Southwestern ponderosa pine
ecosystems has been well docwnented (Cooper 1960,1961).
Conifer understories have established under many aspen clones.
Young conifers have grown into aspen canopies in many cases
and are gradually crowding the aspen out.
Clearfelling isolated mixed conifer/aspen stands to
re-establish aspen has not been successful in isolated clones.
Fencing trials on both the Apache-Sitgreaves and Coconino NF s
have verified that elk browse aspen suckers and have confmned
the need for elk-proof fencing to allow regenerating aspen
suckers to establish and grow. Although there is no doubt that
clearfelling and subsequent fencing will result in abundant stands
of healthy aspen suckers (Schier et aI. 1985, Shepperd and
Engelby 1983), the question of when fencing can safely be
removed remains. This study was designed to test if aspen
regeneration can be certified as established if fencing is removed
after five years. We had hoped that five-year-old aspen stems
would be dense enough and tall enough to withstand animal
browsing after fences were removed and not succumb to
subsequent diseases introduced from animal wounds.
Aspen is currently a minor component of most southwestern
landscapes. Large, stable aspen communities similar to those on
the plateaus of Colomdo and Utah (SheppenlI990) do not exist
in the Southwest. Instead, most aspen stands are in advanced
stages of succession to conifers. This is especially true in the
ponderosa pine type on the Coconino and South Katbab National
Forests, where the only remaining aspen clones consist of a few
small scattered groups of declining and damaged stems in a sea
of pine. Aspen is approaching "threatened and endangered"
status in these situations, since the few remaining aspen
genotypes will be lost if the surviving stems die without
re-sprouting. The long term survival of aspen in some
southwestern landscapes may be in doubt.
Although an earlier study of techniques to regenerate aspen
in this area reported success (Larson 1959), attempts to
regenerate small clones in recent years have invariably failed,
not from lack of initial suckering, but from subsequent sucker
mortality. Two factors have contributed to this situation: the
1 Research Forester, Rocky Mountain Forest and Range
Experiment Station, USDA Forest Service, Fort Collins, CO; and
Plant Pathologist, Southwestem Region, USDA Forest Service,
Flagstaff AZ.
344
overall live stem density (fig. 1), the data belies the serious
browsing damage suffered by many surviving stems outside the
fence. Nearly all stems less than 0.45 m in height were browsed,
as were nearly half of the surviving mid-size classes (fig. 3).
Swprisingly, about 60% of latge dominant stems (> 2.5 em
dbh) were also browsed. Many larger stems were stripped of
lower branches, or were broken completely off by the elk (fig.
4). In addition, most of the severely wounded stems were
infected with Cytospora canker [Cytospora chrysosperma
(pers.)] (fig. 5). Elk brow,sing was also manifested by a
significant (p = 0.05) reduction in the average height of
dominant stems in I the unfenced, heavily browsed area.
Dominant stem heights averaged 3.47 m inside the fence in
1992, but only 2.67 m outside the fence (fig. 6).
METHODS
The study site was located on the Peaks RD of the Coconino
NF in the NE 114 of Sec 17, T24N, R6E in an area known as
the Hochderffer Aspen Regeneration Project. The 6.5-ha study
area was one of several aspen stands that were clerufelled and
fenced with 2 m hog-wire fencing in 1986. In 1990, a sample
of ten 2.322-m2 plots indicated that the area contained about
51,000 stemslha with dominant stems averaging 2.5 m in height.
No animal-related damage was evident inside the fence and no
suckers SUIVived outside the fence, although there was evidence
that suckering had occurred.
In the fall of 1991 Forest Service crews removed fenc~
from the unit except for an exclos~ at one end. Sixty 4.05-m
circular plots were then established in a unifonnly stocked area
in one genotype within the cut unit. Thirty of the plots were
located in the area where fencing had been removed and 30
were inside the remaining fenced ~. All plots were measured
one year later, in September, 1994 Live aspen stems in each
plot were examined for damage and tallied as undamaged, or
assigned a damage code (based on the damage most likely to
affect stem vigor). Dead stems were tallied by size class only.
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In September, 1992, the fenced exclosure contained an
average stocking of 50,000 live stems/ha while the adjoining
area, where fencing had been removed one year earlier,
contained only 30,000 live stemslha (fig. 1). The greatest
differences in live stem numbers occurred in size classes from
0.45 m in height to 2.5 em diameter breast height (dbh) (fig.
2). Although this was a significant difference (p = 0.05) in
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FENCING EFFECT BY SIZE CLASS
Figure 2. - Ninety-five percent Tukey HSD intervals for a
Two-way Analysis of Variance of live six-year~ld aspen
stem densities grouped by stem size and fencing treatment.
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Figure 1. - Analysis of Variance of live aspen stem densities for
six-year~ld aspen sampled from 60 4.06 m2 plots. Bars are
96% Tukey HSD intervals for stem density in unfenced and
fenced treatments a year after fence removal.
STEM SIZE CLASS
Figure 3. - Ninety-five percent Tukey HSD intervals comparing
the percent of browsed aspen stems by size class.
345
Figure 4. - Effect of elk browsing in a six-year-old aspen population, one year after fence removal. Note the distinctive browse line in
the background and the stems broken by elk to access upper foliage.
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Figure 8. - Ninety-five percent Tukey HSD intervals comparing
the height of dominant stems in unfenced and fenced
treatments a year after fence removal.
Figure 5. - Fruiting bodies of Cytospora chrysosperma canker.
Many broken stems were infected with this disease.
346
pattern of elk browsing obselVed here are indicate a larger
underlying ecosystem problem. Restoring aspen may ultimately
require reduced elk populations as well as altering forest
conditions within the landscape to support remaining animals
and favor aspen establishment. Otherwise, some aspen
genotypes in this area may have to exist like other endangered
species - behind zoo-like fences.
DISCUSSION
It is clear from the striking change in appearance of the
previously fenced area a year after fence removal that elk
severely impacted the health and vigor of this five-year-old
aspen sucker population A sea of dense, vigorous aspen suckers
was reduced to a scattering of severely damaged stems in the
most severely affected area (fig. 4). The high percentage of large
dominant stems that were browsed by elk (fig. 3) is disturbing.
Most of these stems must survive intact if a new generation of
aspen is to survive and prosper. Heavy browsing, destruction of
tenninalleaders, and canker infections in the largest stems only
forecast regeneration failure. As long as elk go to these extremes
to reach live leaves, aspen stems will have to be much larger
to resist breakage and foliage browsing. Ten, or perhaps 15 years
of continued fencing protection may be necessruy.
One encouraging factor is that not all genotypes in the study
area were heavily browsed the first year after fence removal.
Ironically, the clone where the plots~ were located received most
of the damage. This suggests that elk have exhibited a genotypic
taste preference for the foliage of one aspen clone over others
in the study area. If so, there may be compounds present in the
foliage of the other clones in the area that discourage herbivory.
Remeasurement in future years should detennine if this effect
is pennanent, or if the elk will browse less tasty genotypes once
the preferred clone is depleted.
The existence of a genotype over a hectare in size that is
highly preferred by elk indicates that browsing intensities
obselVed here were not likely to have occurred in the past. This
suggests that there is either a lot less aspen, or a lot more elk
in the landscape today than in the past. Both cases are likely to
be true.
Fire suppression has resulted in a reduction of openings
within southwestern ponderosa pine forests (Cooper 1960,1961;
Covington and Moore 1991). Accompanying the increased
density of ponderosa pine has been a reduction in the light and
fire-disturbance regimes favorable to the regeneration of aspen
(Schier et al. 1985; Shepperd and Engelby 1983). This trend has
also reduced other forage available to browsing animals and
further exacerbated the impacts of elk upon regenerating aspen
in this area.
This study has clearly demonstrated that fences will have to
remain around aspen regeneration in this southwestern
ponderosa pine ecosystem longer than five years to guarantee
sUlVival of genotypes that are preferred by elk. The severity and
LllERATURE CITED
Cooper, C. F. 1960. Changes in vegetation, structure, and growth
of southwest pine forests since white settlement. Ecological
Monographs. 30:129-164.
Cooper, C. F. 1961. Pattern in ponderosa pine forests. Ecology
42:493-499.
Covington, W. W.; Moore, M.M. 1991. Changes in forest
conditions and multiresource yields from ponderosa pine
forests since European settlement. Final Report, Water
Resources Operations, Salt River Project, Phoenix, Arizona.
51.
Dieterich, J. H. 1980. Chimney Spring forest fire hiStOlY. USDA
Forest SeIVice Research Paper RM-220. Fort Collins, CO:
U.S. Department of Agriculture, Forest SeIVice, Rocky
Mountain Forest and Range Experiment Station 9.
Larson, Merlyn M. 1959. Regenerating aspen by suckering in
the southwest. Research Note 39. Fort Collins, CO: U.S.
Department of Agriculture, Forest SeIVice, Rocky Mountain
Forest and Range Experiment Station 2.
Schier, George A.; Shepperd, Wayne D.; Jones, John R. 1985.
Regeneration. In: DeByle, Norbert V.; Winokur, Robert P.,
eds. Aspen: Ecology and management in the western United
States. Gen Tech. Rep. RM-1l9. Fort Collins, CO: U.S.
Department of Agriculture, Rocky Mountain Forest and
Range Experiment Station 197-208.
Shepperd, Wayne D. 1990. A classification of quaking aspen in
the central Rocky Mountains based on growth and stand
characteristics. Western Journal of Applied Forestry
5(3):69-75.
Shepperd, Wayne D. ; Engelby, OIVille. 1983. Rocky Mountain
aspen In: Silvicultural systems for the major forest types of
the United States. Agriculture Handbook 445. Washington
D.C.: U.S. Department of Agriculture. 77-79.
347