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Effect of Simulated Browsing on Aspen Regeneration: Implications for Restoration

Authors:

Abstract

Aspen (Populus tremuloides Michx.) is a disturbance-dependent, fire-resilient, shade-intolerant, clonal species that is in decline throughout western North America. The objective of this study was to examine the effects of intensity and season of browsing on annual height growth of aspen suckers. The goal was to aid development of livestock grazing strategies to restore stands in decline due to excessive livestock browsing. We implemented 33 combinations of intensity and season of browse on aspen suckers in three aspen stands on Eagle Lake Range District, Lassen National Forest, California, USA, during 2003 and 2004. Greatest growth was on suckers with no terminal leader browse and <or=25% of biomass removed from branches. Lowest growth occurred when 90% of terminal leader length and 50% of branch biomass was removed. Growth was most negatively affected by browse on terminal leader. Growth was lowest for suckers browsed midseason only and suckers browsed both early and midseason. Occurrence of conifer in the stand overstory significantly reduced sucker growth. Managers should minimize browse on terminal leaders, midseason browse over consecutive years, and repeated browse during a growing season.
Effect of Simulated Browsing on Aspen Regeneration: Implications for Restoration
Bobette E. Jones,
1
David F. Lile,
2
and Kenneth W. Tate
3
Authors are
1
District Ecologist, United States Forest Service, Eagle Lake Ranger District, Lassen National Forest, Susanville, CA 96130, USA;
2
Livestock
and Natural Resources Advisor, University of California Cooperative Extension, Lassen County, Susanville, CA 96130, USA; and
3
Rangeland Watershed
Specialist, Dept of Plant Sciences, University of California, Davis, CA 95616, USA.
Abstract
Aspen (Populus tremuloides Michx.) is a disturbance-dependent, fire-resilient, shade-intolerant, clonal species that is in decline
throughout western North America. The objective of this study was to examine the effects of intensity and season of browsing
on annual height growth of aspen suckers. The goal was to aid development of livestock grazing strategies to restore stands in
decline due to excessive livestock browsing. We implemented 33 combinations of intensity and season of browse on aspen
suckers in three aspen stands on Eagle Lake Range District, Lassen National Forest, California, USA, during 2003 and 2004.
Greatest growth was on suckers with no terminal leader browse and #25% of biomass removed from branches. Lowest growth
occurred when 90% of terminal leader length and 50% of branch biomass was removed. Growth was most negatively affected
by browse on terminal leader. Growth was lowest for suckers browsed midseason only and suckers browsed both early and
midseason. Occurrence of conifer in the stand overstory significantly reduced sucker growth. Managers should minimize browse
on terminal leaders, midseason browse over consecutive years, and repeated browse during a growing season.
Resumen
Alamo (Populus tremuloides Michx.) es una planta que depende del disturbio, es resistente al fuego, e intolerante a la sombra;
clones de estas especies esta´n desapareciendo a trave´s de toda la parte oeste de Norte Ame´rica. El objetivo de este estudio fue
examinar los efectos de intensidad y temporada de ramoneo sobre el aumento anual de la altura de reton˜ os de los a´lamos
durante la e´poca de crecimiento. El objetivo fue desarrollar estrategias de pastoreo para restaurar las poblaciones en
disminucio´n debido al excesivo ramoneo. Se realizaron 33 combinaciones de intensidad y e´poca de ramoneo sobre los reton˜os
de los a´lamos en tres poblaciones de a´lamos en Eagle Lake Range District, Lassen National Forest, CA, USA, durante los an˜os
2003 y 2004. El mayor crecimiento se obtuvo en reton˜ os en que no se ramoneo´ la hoja terminal y se removio´ #25% de la
biomasa de las ramas. El menor crecimiento se presento´ cuando el 90% de la longitud de la hoja terminal y el 50% de la
biomasa de las ramas fue removido. El crecimiento fue negativamente afectado por el ramoneo en hojas terminales. El
crecimiento de los reton˜ os se afecto´ mas ligeramente cuando fueron ramoneadas so´ lo a la mitad de la temporada de crecimiento
y los reton˜ os ramoneados en al principio y a la mitad de la temporada. La ocurrencia de conı
´feras en la poblacio´n en la parte
superior redujo significativamente el crecimiento de los reton˜ os. Los manejadores deben minimizar el ramoneo en hojas
terminales, el ramoneo en la mitad de la temporada en an˜ os posteriores, y repetir el ramoneo durante una temporada de
crecimiento.
Key Words: Cascades, frequency, grazing, intensity, Populus tremuloides Michx., season
INTRODUCTION
Aspen (Populus tremuloides Michx.) is a disturbance-depen-
dent, fire-resilient, shade-intolerant, clonal species that relies on
vegetative reproduction to maintain stands between episodic
seeding events (Eriksson 1993; Romme et al. 1997; Shepperd et
al. 2006). As Shepperd (2001) summarizes, successful aspen
regeneration is dependent upon 1) release of apical dominance
and subsequent hormonal stimulation of root buds to initiate
suckering (Schier et al. 1985; Frey et al. 2003); 2) a growth
environment that provides sunlight and warm soil temperatures
(Doucet 1989; Fraser et al. 2002); and 3) protection of aspen
suckers from excessive browsing (Bartos and Campbell 1998;
Kay 2001; Rolf 2001).
Aspen declines have been reported throughout western
North America (Mueggler 1985; Bartos and Campbell 1998;
White et al. 1998; Rogers 2002; Frey et al. 2004; Worrall et al.
2008). Bartos (2001) estimated a 50–96% reduction of aspen
acreage among seven states in the western United States
compared to presettlement acreage. Di’Orio et al. (2005) found
spatial extent of aspen in the Warner Mountains of northeast-
ern California declined by 24% over a 48-yr period. On Eagle
Lake Ranger District, Lassen National Forest, 80% of aspen
stands were found to be deteriorating because of shading by
conifers and poor regeneration (Shepperd et al. 2006).
However, other observers report areas of aspen persistence in
the western United States (Kulakowski et al. 2004, 2006; Zier
and Baker 2006; Binkley 2008; Sankey 2008). Kashian et al.
(2007) assessed 91 northern Colorado Front Range aspen
stands, found both declining and persisting stands, and
concluded that stand decline was spatially and temporally
heterogeneous across the landscape.
Correspondence: Kenneth Tate, Dept of Plant Sciences, University of California, Mail Stop
One, One Shields Ave, Davis, CA 95616, USA. Email: kwtate@ucdavis.edu
Manuscript received 16 June 2009; manuscript accepted 8 September 2009.
Rangeland Ecol Manage 62:557–563
|
November 2009
RANGELAND ECOLOGY & MANAGEMENT 62(6) November 2009 557
Factors known to incite aspen decline include altered fire
regime, excessive browsing by livestock and native ungulates,
severe drought, disease, and insect damage (DeByle 1985;
Mueggler 1985; Chong et al. 2001; Frey et al. 2004; Kaye et al.
2005; Kashian et al. 2007; Worrall et al. 2008). Lengthened fire
return interval allows conifer succession in some aspen stands,
creating a growth environment that cannot support aspen
(Schier 1976; Bartos 2001; Kaye et al. 2005; Kashian et al.
2007). Early reports document loss of heavily grazed aspen
stands and persistence of moderately grazed stands in the
western United States (Baker 1918; Sampson 1919). Excessive
browsing suppresses establishment of new aspen tree cohorts
by maintaining suckers in a hedged growth form below the
herbivore browse line, or total elimination of suckers (Bartos et
al. 1994; White et al. 1998; Kay and Bartos 2000; Kay 2001;
Turner et al. 2003; Dockrill et al. 2004). Studies have found
heavy cattle grazing, particularly mid- to late growing season,
to be an effective means to suppress aspen regeneration
following conversion of aspen parkland to grassland (Fitzgerald
et al. 1984; Bailey et al. 1990).
Aspen restoration in grazed landscapes is a priority for many
resource managers (Jones et al. 2005; Shepperd et al. 2006;
Bartos 2007). Excessive browsing by livestock, and some
wildlife species, can be controlled with exclusionary fencing
(Shepperd and Fairweather 1994; Kay and Bartos 2000; Kay
2001). However, widespread exclusionary fencing of aspen
stands may not be ecologically or economically practical (Rolf
2001). The objective of this study was to examine the effects of
intensity and season of browsing on annual height growth of
aspen suckers. Aspen sucker recruitment, establishment of new
stand cohorts, and stand restoration requires suckers to grow
above the livestock and native ungulate browse line (about
1.5 m). Understanding aspen sucker height growth response to
intensity and season of browse is central to developing livestock
grazing strategies to restore stands that are in decline due to
excessive livestock browsing.
METHODS
Study Area
The study was conducted in northeastern California on Eagle
Lake Ranger District (ELRD), Lassen National Forest. ELRD
has cool moist winters and dry warm summers, with most
precipitation occurring as rain and snow from November
through May. The landscape has broad valleys and meadows
that separate conifer forest–covered buttes dominated by Pinus
ponderosa Laws. and Pinus jeffreyi Grev. & Balf. below
1 850 m, and a mixture of Abies concolor (Gordon & Glend)
Lindley, Pinus lambertiana Douglas, P. ponderosa, P. jeffreyi,
and Calocedrus decurrens Torrey above 1 850 m. Aspen stands
cover approximately 1% of ELRD and are associated with
meadow edges, rock outcrops, riparian areas, and forested
areas with high water tables (aspen survey data, ELRD). Aspen
are found from 1 500 to 2 200 m in elevation, on 0–45% slopes
at all aspects, and on mollisol, inceptisol, and alfisol soils.
Permitted cattle grazing occurs annually on ELRD from
approximately 1 June through 30 September. Grazing allot-
ments range from 4 050 ha to 12140 ha in size and cattle
numbers range from 150 to 800 cow-calf pairs per allotment.
An inventory of over 700 aspen stands on ELRD (Shepperd
et al. 2006) was used to identify stands that were excluded from
livestock grazing, had annual deer utilization on less than 5%
of suckers, were not subject to elk browsing, were larger than
1.5 ha, and had over 1 000 suckers?ha
21
less than 1.5 m in
height. Three stands from this pool were randomly selected for
the study: Ashurst (2.8 ha, 1 950 m), Butte Creek (3.3 ha,
1 706 m), and Martin aspen stand (2.8 ha, 1 731 m). Over the
two-year study period, annual precipitation on ELRD was
53 cm in 2003 and 38 cm in 2004, which was above average
and average, respectively.
Browsing Treatments
Thirty-three simulated browsing treatments were implemented
to represent gradients of browse intensity and seasonal timing
observed across ELRD and northeastern California. Browsing
treatments were randomly assigned to aspen suckers (experi-
mental unit) and applied by hand pruning. Hand pruning
standardized treatment application across suckers, stands, and
years. Simulated browsing, however, does not duplicate all
plant injuries associated with browsing or indirect grazing
effects such as nutrient redistribution.
Treatments were a factorial combination of intensity and
season of browse. Nine levels of browse intensity consisted of
removing a percentage (0%, 20%, 50%, and 90%) of current
year’s growth length from an individual aspen sucker’s terminal
leader (tl), and/or removing a percentage (0%, 10%, 25%, and
50%) of the current year’s growth biomass from a sucker’s
remaining branches (br). Season of browse treatment levels
were early season only (first week of July), midseason only (first
week of August), early and midseason, and late season only
(first week of September). The same treatment was applied to
each sucker in 2003 and 2004. Browsing treatments were
replicated four times at Ashurst and Butte Creek and eight
times at Martin based on abundance of aspen suckers in each
stand. Two permanent line transects were established within
each stand. Aspen suckers ,1.5 m tall and within 2 m of each
line transect were tagged with a unique identification number
and randomly assigned one treatment. There were two
replicates of each treatment per transect at Ashurst and Butte
Creek (66 suckers per transect), and four replicates of each
treatment per transect at Martin (132 suckers per transect).
Thus, 132 naturally growing aspen suckers at Ashurst and
Butte Creek and 264 aspen suckers at Martin were treated for
two years.
The height of the dominant stem (terminal leader) of each
aspen sucker was measured at the end of the growing season in
October 2002, 2003, and 2004. Height (cm) measurements in
October 2002 established initial heights for each sucker. Sucker
heights in October 2002 were subtracted from heights in
October 2003 to determine annual height growth for 2003
(year 1). Sucker heights in October 2003 were subtracted from
heights in October 2004 to determine annual height growth for
2004 (year 2). Potentially important covariates were measured
for each sucker and stand, including 1) sucker originated as a
single stem or as one of multiple stems from the same root
node, 2) sucker had a tree-like or hedged architecture, 3) sucker
was deer browsed at any time during the study, and 4) stand
had conifers present in the overstory or not (Table 1).
558 Rangeland Ecology & Management
Data Analysis
Statistical analysis was first conducted to identify significant
differences (P#0.05) in annual sucker height growth (cm)
among the 33 discrete browse treatments. A second factorial
statistical analysis was conducted to investigate main and
interacting effects of intensity and season of browse on annual
sucker height growth. Linear mixed effects regression analysis
was used for both analyses to address autocorrelation introduced
by repeated measures on individual aspen suckers (Pinheiro and
Bates 2000). Aspen sucker identity was used as a random group
term to account for repeated measures in both analyses. For both
analyses, annual sucker height data were transformed [log
10
(ann-
ual sucker height growth +1)] to normalize residuals, and an
exponential variance function was used to create homogeneity of
variance as determined via evaluation of standard diagnostic
graphs (Pinheiro and Bates 2000).
To examine annual sucker height growth differences among
the 33 browse treatments, initial fixed independent variables
were browse treatment (33 levels), year (2003, 2004), stand
(Ashurst, Butte Creek, Martin), conifer occurrence in stand
overstory (yes, no), sucker architecture (tree-like, hedged), deer
browse on sucker (yes, no), initial sucker height (cm), number
of suckers per root node (single, multiple), treatment by year
interaction, treatment by stand interaction, and stand by year
interaction. A final significant model was determined in a
backwards stepping approach. A conditional t-test pairwise
comparison of all browse treatments was used to determine
which treatments were significantly different (P#0.05; Pin-
heiro and Bates 2000).
For the factorial analysis of main and interacting browse
treatment effects on annual sucker height growth, a backward
stepping approach was employed to identify a final linear
mixed effects regression model reflecting relationships between
annual sucker height growth: browse treatment factors; two-
way interactions between treatment factors, stand, and year;
and covariates. Nonbrowsed control suckers were excluded
from this analysis to allow for examination of interactions
between browse factors. Initial fixed independent variables
introduced during analysis were intensity of browse on terminal
leader (tl); intensity of browse on branches (br); season of
browse (early, mid-, early and mid-, late); year (2003, 2004);
stand (Ashurst, Butte Creek, Martin); conifer occurrence in the
stand overstory (yes, no); sucker architecture (tree-like,
hedged); deer browse on sucker (yes, no); initial sucker height
(cm); number of suckers per root node (one, multiple); and
two-way interactions between browse treatment factors, year,
and stand. A conditional ttest was used to determine which
fixed effects were significantly correlated with aspen sucker
height growth, and regression coefficients were estimated using
restricted maximum likelihood (Pinheiro and Bates 2000).
RESULTS
Differences Between Browsing Treatments
Pairwise comparisons of all treatments and ranking of these
treatments from highest to lowest annual sucker height growth
are reported in Table 2. There were significant differences
(P#0.05) in annual height growth among the 33 browsing
treatments. Occurrence of conifer was a significant predictor
(P,0.001) of annual sucker height growth, with less growth in
stands with conifers. Year, stand, sucker architecture, deer
browse on sucker, initial sucker height, number of suckers per
root node, and all two-way interactions were not significantly
correlated to annual sucker height growth (P.0.1 in all cases).
Six treatments that included browse on branches had appar-
ently, not significantly, greater annual sucker height growth than
nonbrowsed control treatment suckers (Table 2). Greatest annual
height growth was observed on suckers with no terminal leader
browse and no more than 25% of biomass removed from
branches regardless of season. Suckers with lowest height growth
had 90% of terminal leader removed and 50% of biomass
removed from branches regardless of season. The remaining
treatments were intermediate, and among these treatments less
growth occurred when browse was early and midseason or
midseason only compared to early season only or late season only.
Browse Treatment Factor Analysis
Annual sucker height growth was significantly affected by
browse intensity and season, interactions of these factors, and
the occurrence of conifer in the stand (Table 3). Season of
browse, intensity of browse on terminal leader, intensity of
browse on branches, quadratic form of intensity of browse on
terminal leader, intensity of browse on terminal leader by season
of browse interaction, and intensity of browse on terminal leader
by intensity of browse on branches interaction were significant
predictors of annual aspen sucker height growth (P#0.05).
Annual sucker height growth decreased as intensity of browse on
terminal leader increased, but magnitude of decrease depended
on season of browse (Fig. 1). Intensity of browse on terminal
leader also interacted with intensity of browse on nonterminal
leader branches to determine annual sucker height growth
(Figs. 2 and 3). Presence of conifers in aspen stands reduced
annual sucker height growth regardless of intensity or season of
browse (Table 3). Year, stand, sucker architecture, deer browse,
initial sucker height, number of suckers per root node, and all
two-way interactions were not significant (P.0.1 in all cases).
DISCUSSION
Aspen sucker height growth response to browsing treatments
ranged from +19 to 288% of nonbrowsed control suckers
Table 1. Summary of covariates measured in three study aspen stands
on Eagle Lake Ranger District, Lassen National Forest, CA, USA, during
2003 and 2004. Conifer 5conifer occurrence in stand overstory;
Single 5percentage aspen suckers growing as a single sucker per
root node; Multiple 5percentage aspen suckers growing as multiple
suckers per root node; Tree-like 5percentage aspen suckers with
uninterrupted vertical growth; Hedged 5percentage aspen suckers with
shrub-like appearance due to repeated browse; Deer 5percentage aspen
suckers browsed by deer at least once during the study;
Mortality 5percentage aspen suckers that died during the course of
the study.
Stand Conifer Single Multiple Tree-like Hedged Deer Mortality
Ashurst, n5132 Yes 50 50 84 16 4 8
Butte, n5132 No 34 66 71 29 7 7
Martin, n5264 Yes 48 52 93 7 12 5
62(6) November 2009 559
(Table 2). Results of this study agree with reports that both
intensity and season of browse affect aspen regeneration
(DeByle 1985; Kays and Canham 1991; Campa 1992; Dockrill
et al. 2004). Intensity of browse on terminal leader was
negatively correlated with annual sucker height growth
(Tables 2 and 3; Figs. 1 and 2). Annual sucker height growth
was more negatively affected by browse on terminal leader than
browse on branches. Effect of intensity of browse on branches
interacted with intensity of terminal leader browse (Table 3;
Figs. 2 and 3). Positive growth response to browse on branches
was observed as long as terminal leader browse was #20%
(Tables 2 and 3, Fig. 3). At 50% browse on terminal leader
there was no response of growth to increased browse on
branches. At 90% terminal leader browse there was a reduction
in growth with increased browse on branches (Fig. 3). Annual
growth may have increased with browse on branches when
terminal leader browse was low because of hormonal
stimulation to elongate the remaining leader, and reduced
competition among remaining buds for root carbohydrates
(Maini 1966). Growth eventually decreased with increasing
browse on both branches and terminal leader, most likely
because of substantial reduction in photosynthetic area.
Season of browse interacted with intensity of terminal leader
browse to determine annual sucker height growth (Table 3;
Fig. 1). Overall, growth was lowest for suckers browsed
midseason only and early and midseason. There was little
difference in growth for suckers browsed early season only,
midseason only, and early and midseason when terminal leader
browse was 20%. However, as browse on terminal leader
increased, early season only suckers outgrew midseason only and
early and midseason browsed suckers (Fig. 1). Late season only
browse suckers had greater growth compared to all other seasons
of browse when terminal leader browse was 20% or 50%.
Table 3. Results of linear mixed effects regression analysis predicting
the effect of intensity and season of browsing treatments and occurrence
of conifers on log
10
(annual growth of aspen suckers +1) in three aspen
stands on Eagle Lake Ranger District, Lassen National Forest, California,
USA, during 2003 and 2004. Season of browse treatments were early
(first week of July), mid- (first week of August), early and mid-, and late
(first week of September) growing season. Browse intensity was the
percentage of current year’s growth length removed from terminal
leader, and percentage of current year’s growth biomass removed from
branches. Conifer occurrence was presence or absence of conifer in the
aspen stand overstory.
Model term
Regression
coefficient P-value
Intercept 0.059 ,0.0001
Conifers occurrence in aspen stand
Present 0.000 —
Absent 0.065 ,0.0001
Season of browse
Early 0.00 —
Early and mid- 20.00035 0.965
Mid- 0.01 0.199
Late 0.019 0.026
Browse intensity on terminal leader 20.00068 0.004
Browse intensity on branches 0.00059 0.054
(Browse intensity on tl)
2
0.0000057 0.007
Season of browse 3browse intensity on terminal leader
Early 3browse on terminal leader 0.00
Early and mid- 3browse on terminal leader 20.00017 0.214
Mid- 3terminal browse on leader 20.00037 0.009
Late 3browse on terminal leader 20.00026 0.070
Browse intensity on terminal leader 3browse
intensity on branches 20.000013 0.001
Table 2. Mean annual aspen sucker growth (cm) response to 33
browsing treatments applied to suckers in three aspen stands on Eagle
Lake Ranger District, Lassen National Forest, California, USA, during
2003 and 2004. Each browse treatment was a combination of season of
browse (early 5first week of July, mid- 5first week of August,
late 5first week of September), percentage of current year’s growth
length removed from terminal leader, and percentage of current year’s
growth biomass removed from branches.
Mean
separation
1
Browse treatment
Height growth
(cm)
2
Season of
browse
% terminal
leader removed
% branches
removed
A Late 0 10 32.2
A Late 0 25 30.9
AB Mid- 0 25 29.4
AB Early and mid- 0 25 26.8
ABC Mid- 0 10 29.7
ABC Early 0 25 28.0
ABCD Control-none Control-none Control-none 27.0
ABCD Early 50 0 23.1
BCDE Early 0 10 22.7
BCDE Mid- 20 0 22.0
BCDEF Late 20 10 22.9
BCDEFG Late 20 0 22.0
BCDEFG Early and mid- 20 0 21.4
BCDEFG Early and mid- 0 10 19.8
CDEFGH Early 50 25 19.1
DEFGHI Mid- 20 10 18.0
DEFGHI Early 20 0 16.7
DEFGHI Late 50 25 16.6
DEFGHI Late 50 0 16.4
DEFGHIJ Early and mid- 20 10 15.5
EFGHIJK Late 90 0 14.9
EFGHIJK Early and mid- 90 0 14.6
FGHIJK Early 90 0 16.1
GHIJK Early 20 10 12.4
HIJK Mid- 50 25 12.2
HIJK Mid- 50 0 10.9
IJK Early and mid- 50 0 12.4
IJK Mid 90 0 11.1
IJK Early and mid- 50 25 9.1
JKL Late 90 50 7.8
KL Early 90 50 8.2
L Early and mid- 90 50 4.0
L Mid- 90 50 3.1
1
Browse treatments with the same letter are not significantly different from one another
(P$0.05) based on conditional t-test pairwise comparison.
2
Mean annual aspen sucker height growth for suckers receiving each treatment across all
years and stands.
560 Rangeland Ecology & Management
Season of browse affects amount of carbohydrates available
to support sucker growth and duration of growing season
remaining for regrowth after browse (Schier and Zasada 1973;
Kays and Canham 1991; Dockrill et al. 2004). Our results
agree with Fitzgerald et al. (1984) that intense browse in
August (midseason) reduced aspen growth more than intense
browse in June (early season). Kays and Canham (1991) found
that browse after initiation of sucker growth in spring, but
before cessation of above-ground growth in late summer,
resulted in minimal sucker growth. This contrasts with
conventional theory that browsing just after leaf emergence,
when carbohydrate reserves are lowest, would reduce sucker
growth (Tew 1970; Schier and Zasada 1973). Early season only
browse provides a relatively long regrowth period that may
compensate for reduced carbohydrates. Suckers browsed late
season had greater growth when terminal leader was browsed
#20%, likely because they were able to benefit from a full
season of growth, photosynthesis, and translocation prior to
browse. However, late season only browse suckers had less
growth when terminal leader was browsed .20%, likely due
to insufficient growing season remaining for regrowth.
Controlling intensity of mid- and late season browse may be
problematic because cattle tend to prefer aspen to other forages
during the mid- and late season (Fitzgerald et al. 1986).
Suckers in stands without conifer had approximately 16 cm
more annual height growth than suckers in stands with conifers
in the overstory (Table 3). Shading from conifers creates low-
light intensities and reduces soil temperatures, which can
diminish growth of aspen suckers (Farmer 1963; Gifford 1966;
Shepperd 2001; Fraser et al. 2002; Frey et al. 2003). Reduced
annual sucker growth due to shading by conifers can prolong
the period that suckers are below the browse line and
susceptible to height growth suppression by browsing.
MANAGEMENT IMPLICATIONS
Several factors are driving aspen declines across western North
America. Where excessive livestock grazing is contributing to
stand decline, protection of suckers from browse is a critical
component of aspen restoration. Fencing stands can eliminate
livestock browsing and is a good option for extremely degraded
stands at risk of extinction. Across the broader landscape,
managers can use grazing practices such as herding-water-
supplement distribution activities, rest-rotation systems, and
seasonal grazing strategies to actively manage the season,
Figure 1. Product of linear mixed effects regression illustrating
relationships between current year’s growth length removed from aspen
sucker terminal leader and annual sucker height growth (cm) for suckers
browsed early season only (1 July), midseason only (1 August), early
and midseason, and late season only (1 September). Current year’s
growth biomass removed from sucker branches and conifer occurrence
were set to zero and not present.
Figure 2. Product of linear mixed effects regression illustrating the
interaction between percentage of current year’s growth length removed
from aspen sucker terminal leader, percentage of current year’s growth
biomass removed from sucker branches (br), and annual sucker height
growth (cm) for suckers browsed early season only (1 July) and without
conifer present.
Figure 3. Product of linear mixed effects regression illustrating
relationships between percentage of current year’s growth biomass
removed from aspen sucker branches and annual sucker height growth
(cm) for suckers with 0%, 20%, 50%, and 90% of current year’s growth
length removed from terminal leader (tl). Season of browse and conifer
occurrence were set to early season only (1 July) and not present.
62(6) November 2009 561
intensity, and frequency of browse on aspen suckers. Intensity
of terminal leader browse is a major determinant of annual
aspen height growth and should serve as a monitoring indicator
for making livestock management decisions to enhance aspen
regeneration. A key management goal should be to minimize
browse on sucker terminal leaders. Suckers with #20% of
terminal leader growth removed averaged over 50% of height
growth observed on nonbrowsed control suckers and should
grow above the browse line within several years. Midseason
browse should be avoided over consecutive years. Intensity of
terminal leader browse should be minimized during mid- and
late season. Repeat browsing of suckers within a growing
season should be avoided. Evidence of repeat browsing should
be a signal to managers to adjust grazing management,
primarily the amount of time livestock have access to a stand.
ACKNOWLEDGMENTS
We thank Gyna Ridenour, Shannon Cler, and Betsy Huang for their help
with treatment implementation; Yukako Sado for data entry; Fred Kent for
earlier discussions relating to aspen range management; Dale Bartos and
Wayne Shepperd for sharing their expertise regarding aspen ecology and
management; Tuyeni Mwampamba for reviews and comments on this
manuscript; and Eagle Lake Ranger District, Lassen National Forest.
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62(6) November 2009 563
... Researchers and land managers have addressed the impacts of ungulates on aspen by altering the grazing regimes of domestic ungulates (Jones et al., 2009), culling native herbivores (Houston, 1982;Olmsted, 1979), reintroducing apex predators to control native herbivore populations (Beschta et al., 2018;Ripple & Beschta, 2007), and erecting fences around aspen stands (Kay, 2001a;Kay & Bartos, 2000). In semiarid ecosystems used for livestock production, managers often erect fences designed to exclude only domestic ungulates, thereby reducing browsing intensity on aspen while preserving valuable habitat for wild ungulates (Kay & Bartos, 2000). ...
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Herbivory by wild and domestic ungulates can influence tree recruitment and understory forest communities throughout the world. Herbivore‐driven declines in tree recruitment have been observed for quaking aspen (Populus tremuloides), a foundation species whose health and management is recognized as a critical priority throughout much of its range. Livestock fencing is commonly used to promote aspen regeneration, but its effectiveness is rarely assessed, especially across large spatial scales. Using a livestock‐reduction experiment, we evaluated the effects of ungulate herbivory on aspen in the Great Basin and southern Cascades, an expansive and environmentally heterogeneous region where aspen faces the interacting threats of climate change, conifer encroachment, and herbivory. We found that livestock fencing only reduced the intensity of herbivore browsing on aspen when wild ungulate abundance was low and did not increase stem densities of aspen recruits. Contrary to expectations, wild ungulate abundance was a strong driver of browsing intensity on juvenile aspen within fenced, but not unfenced, aspen stands, and when the abundance of these herbivores was high, browsing intensity in fenced stands exceeded that in unfenced stands. The density of aspen recruits decreased with browsing intensity on juvenile aspen and with the density of both adult aspen and conifers, suggesting that both herbivory and intra‐ and interspecific competition are important drivers of recruitment. Fire history was also an important driver of recruitment, with stands that burned 10–20 years ago having the greatest density of aspen recruits. Finally, in the stand understory, we found that livestock fencing decreased forb cover, increased shrub species richness, and increased the cover of exotic annual grasses, a group dominated by Bromus tectorum. This latter finding suggests that livestock fencing may not be appropriate in areas where controlling the spread of this invader is a priority. In sum, our findings indicate that aspen recruitment is limited by browsing by both wild and domestic ungulates, is mediated by competition with neighboring trees and fire history, and will require management actions beyond livestock fencing, as this approach does not control browsing by wild ungulates.
... Briske et al. 2011;Hunt et al. 2014), appropriate seasons of use (e.g. Jones et al. 2009), and monitoring indicators of successful implementation and resource response (e.g. Clary and Webster 1990). ...
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Optimising the spatial distribution of free-ranging livestock is a significant challenge in expansive, grazed landscapes across the globe. Grazing managers use practices such as herding (i.e. droving), strategic placement of off-stream livestock drinking water and nutritional supplements, and strategic fencing in attempts to distribute livestock away from sensitive streams and riparian areas. We conducted a cross-sectional survey of 46 cattle-grazed riparian areas and associated stream reaches embedded in rugged range landscapes to examine relationships between implementation of these management practices, stocking rate, and riparian health. We determined in-stream benthic invertebrate assemblages at each site to serve as an integrative metric of riparian health. We also collected information from the grazing manager on stocking rate and implementation of livestock distribution practices at each site over the decade before this study. Off-stream livestock drinking-water sources were implemented at just two sites (4.3%), indicating that this was not a common distribution practice in these remote management units. We found no significant relationship of riparian health (i.e. invertebrate richness metrics) with stocking rate (P ≥ 0.45 in all cases), or with the simple implementation (yes/no) of off-stream nutritional supplements, fence maintenance, and livestock herding (P ≥ 0.22 in all cases). However, we did find significant positive relationships between riparian health and managerial effort (person-days spent per year for each individual practice) to implement off-stream nutritional supplements and fence maintenance (P ≤ 0.017 in all cases). Livestock herding effort had an apparent positive association with riparian health (P ≥ 0.2 in all cases). Results highlight that site-specific variation in managerial effort accounts for some of the observed variation in practice effectiveness, and that appropriate managerial investments in grazing distributional practices can improve riparian conditions.
... The growth of trees such as birch, beech and especially oak and fir is negatively affected by roe deer and red deer browsing (Van Hees et al. 1996;Cote et al. 2004;Kupferschmid 2018). The effects of browsing are greatly influenced by the quality of the habitat, the time when the damage occurs (Canham et al. 1994), the age of the seedling (Harmer 2001;Jones et al. 2009), the landscape mosaic (Ward et al. 2008), and the intensity of browsing (Kupferschmid 2018) as well as the population density of ungulates (Bergquist and Orlander 1998;Gill 2000). Constantly increasing deer populations in many countries suggest that in the future, the problem of ungulate pressure on forest ecosystems will increase (Cutini et al. 2011). ...
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One of the most important ecosystem processes, especially in temperate and boreal forests, is herbivore browsing. In this study, we tested five hypotheses: (H1) oak tree density would positively affect their height; (H2) despite severe browsing pressure, the height of oaks would increase with the age after planting; (H3) oaks growing among the planted pines would be taller than those among the sown ones; (H4) the growth of single oak saplings or of oak nests would be positively affected by the height of the pines growing in the immediate neighbourhood of the oaks; and (H5) the oak height would be affected by the location within a nest and oaks growing in the central zone would be taller than the others. The study was conducted in the Piska Forest in the north-eastern part of Poland. We established three variants of experiment. In the first variant, oaks were planted individually. In the second and third variants, oaks were planted in small nests. The second and third variants differed in the spacing of the nests: 8 × 8 m and 6 × 6 m, respectively. The division into the sub-variants referred to type of pine regeneration. In the first sub-variant, Scots pine was planted. In the second, pine was sown. In general, oaks growing within nests were significantly taller than those planted individually in the Scots pine rows. Despite heavy browsing, oak height consistently increased during the course of the study. Oak height also depended on the type of pine regeneration, and the mean height of oaks surrounded by planted pines was significantly higher than that of oaks surrounded by sown pines (which, in average, was shorter). That was because the planted pines were taller than the sown ones. We found that Spearman’s correlation coefficients were positive and significant between pine and oak heights in variants 2 and 3. Growth of oaks did not depend on their location within a nest. The results of the regeneration method of oak (single vs nest and few vs many nests) are modified by browsing. The best method found in the heavy browsing circumstances was planting oaks in many nests within successful pine regeneration (providing the oaks with protection against the browsing). The pine will provide the most efficient protection if regenerated a few years before the oaks.
... Although effects of herbivory vary, the negative influence of animal browsing on aspen tree regeneration and recruitment has been welldocumented (e.g. Jones et al., 2009;Seager et al., 2013;Rogers and Mittanck, 2014;Shinneman and McIlroy, 2019). Aspen are a nutrientrich resource for ungulates (DeByle, 1985) and browsing can decrease sucker survival, restrict growth (e.g. ...
Article
Altered climate and changing fire regimes are synergistically impacting forest communities globally, resulting in deviations from historical norms and creation of novel successional dynamics. These changes are particularly important when considering the stability of a keystone species such as quaking aspen (Populus tremuloides Michx.), which contributes critical ecosystem services across its broad North American range. As a relatively drought intolerant species, projected changes of altered precipitation timing, amount, and type (e.g. snow or rain) may influence aspen response to fire, especially in moisture-limited and winter precipitation-dominated portions of its range. Aspen is generally considered an early-seral species that benefits from fire, but increases in fire activity across much of the western United States could affect the species in unpredictable ways. This study examined post-fire aspen stands across a regional climate gradient spanning from the north-central Great Basin to the northeastern portion of the Greater Yellowstone Ecosystem (USA). We investigated the influence of seasonal precipitation and temperature variables, snowpack, and site conditions (e.g. browsing levels, topography) on density of post-fire aspen regeneration (i.e. all small trees ha⁻¹) and recruitment (i.e. small trees ≥2 m tall ha⁻¹) across 15 fires that occurred between 2000 and 2009. The range of post-fire regeneration (2500–71,600 small trees ha⁻¹) and recruitment (0–32,500 small trees ≥2 m ha⁻¹) densities varied widely across plots. Linear mixed effects models demonstrated that both response variables increased primarily with early winter (Oct-Dec) precipitation during the ‘fire-regen period’ (i.e., fire year and five years after fire) relative to the 30-year mean. The 30-year mean of early winter precipitation and fire-regen period snowpack were also positively related to recruitment densities. Both response variables decreased with higher shrub cover, highlighting the importance of considering shrub competition in post-fire environments. Regeneration and recruitment densities were negatively related to proportion browsed aspen leaders and animal pellet densities (no./m²), respectively, indicating the influence of ungulate browsing even at the relatively low levels observed across sites. A post-hoc exploratory analysis suggests that deviation in early winter precipitation during the fire-regen period (relative to 30-year means) varied among sites along directional gradients, emphasizing the need to consider multiple spatiotemporal scales when investigating climate effects on post-fire successional dynamics. We discuss our findings in terms of dynamic management and conservation strategies in light of changing fire regimes and climate conditions.
... Intense browsing by ungulates can exert a major influence on aspen dynamics, especially regeneration, by inhibiting recruitment (Bartos and Mueggler, 1981;Bartos et al., 1994;Romme et al., 1995;Baker et al. 1997;Hessl, 2002;Larsen and Ripple, 2003;Jones et al., 2009;Seager et al., 2013;Rogers and Mittanck, 2014). Such chronic herbivory can lead to vegetation type conversions, as well as deleterious effects on overall species diversity (Bailey and Whitham, 2002;Griffis-Kyle and Beier, 2003;Martin and Maron, 2012;Rogers et al., 2010). ...
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https://www.sciencedirect.com/science/article/pii/S2351989419305803
... However, most research has focused on differential space use and selection of different forage plant species McNaughton 1998, Du Toit andCumming 1999). Fewer studies have investigated whether different ungulates use a single plant species in different ways (but see Hester et al. 2004, Jones et al. 2009, Bork et al. 2013, Rhodes et al. 2018. Different ungulate species may select different tissues of the same plant or may prefer to consume the plant in different seasons. ...
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Article
Timing of herbivory or selection of specific plant tissues (mode of herbivory) by different ungulate herbivore species are likely to have important influences on plant defense strategies. In this study, we devised two different modes of simulated herbivory, representing a selective ungulate feeding strategy (defoliation: leaf tissue removal only) and a bulk feeding strategy (clipping: leaves, twigs and meristems taken together). We applied these contrasting herbivory treatments to juvenile aspen suckers (Populus tremuloides Michx.) regenerating underneath aspen stands in early summer (June), late summer (August) or at both times to determine the effects of herbivory mode, timing and frequency on regenerating aspen. In response to the simulated herbivory treatments, we measured traits related to three plant defense strategies: tolerance (aboveground biomass and stem diameter), resistance (foliar phenolic glycosides) and vertical escape (sucker height and average leader length). There was no evidence that mode, timing or frequency of simulated herbivory induced or repressed phenolic glycoside production. Early summer herbivory was more detrimental than late summer herbivory on aspen tolerance and escape. Repeat herbivory in late summer did not amplify the negative effects of early summer herbivory. Clipping and defoliation tended to have similar effects on tolerance but clipping was more detrimental than defoliation on vertical escape. These results suggest that different ungulate herbivore species may have disparate impacts on the plant communities by selecting different tissues of the same plant, or browsing the plant at different times in the growing season.
... Development of permanent pastures for grazing gives farmers access to other means of income from cattle and reduces animal feeding costs. Continual heavy-grazing pressure can drastically harm environmental resources (Holechek, 1981); however, appropriate stocking rates and rotational/seasonal grazing management have potential to sustain or even improve environmental resources (Jones et al., 2009;Roche et al., 2013;Freitas et al., 2014)), promote desirable wildlife species (McIlroy et al., 2013), and increase available forage quantity and quality for livestock. Potential success of a future Myanmar beef industry as well as empowerment of its rural population can be greatly influenced by its National Land Use Policy, and development of long-term, sustainable grazing land management. ...
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Article
Natural resources and value-added opportunities exist in Myanmar for improved food security and beef industry development. Nutrition currently limits animal reproduction and growth; reassessment of Myanmar land use policy and development of grazing opportunities are needed for reduced feed and labor costs, and dietary improvement. Standardized cattle trait recording along with live animal and carcass classification would improve value-added marketing and management opportunities throughout the supply chain. Education of cattle farmers and farm youth is needed for fundamental animal and business management principles; technical information should be delivered through "Train-the- Trainer" type programs.
... Trees that were browsed more frequently and severely had slower growth rates (Chapters 2 and 3). This is common across forested landscapes , Puettmann and Saunders 2001, Jones et al. 2009) and is often due to a depletion of carbohydrate reserves (Jameson 1963). The season that browsing occurs in will have varying consequences for a trees' ability to respond to damage in interior Alaska (Chapter 3). ...
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Thesis
Boreal forest vegetation is likely to change in response to warming and associated changes in disturbance regimes. Recently, high severity fires have caused a shift from pre-fire mature black spruce stands to early assemblages of broadleaf trees. However, interactions between plants and animals may modify the outcomes of early forest succession. Selective herbivory by mammalian herbivores can alter the relative dominance of forest canopy species and influence successional pathways. However, woody plants have evolved multiple strategies to maximize chances of survival and long-term fitness after herbivory. In this thesis, I explore the dynamics of moose-tree interactions and their consequences for early succession in boreal forests of interior Alaska. I assessed the interaction of moose browsing and fire severity on tree growth and canopy composition and found that moose slow the rate of trembling aspen growth, but only in severely burned areas. Black spruce showed no direct or indirect growth responses to moose browsing, indicating that moose browsing on aspen did not alter the initial trajectory to an aspen-dominated canopy within my study area. In lightly burned areas, moose may benefit from longer durations of forage availability and accelerate the development of a mixed spruce-aspen canopy. Alaskan paper birch is the other dominant broadleaf species colonizing early post-fire forests and I determined saplings’ ability to tolerate simulated summer leaf stripping by moose. I found decreases in woody growth and carbon-based defenses while individual leaf area increased in response to simulated leaf stripping. My results are consistent with the carbon-nutrient balance hypothesis suggesting that the loss of growing points during leaf stripping decreased competition for nutrients, which were then available for leaf regrowth. Birch saplings have the potential to compensate for herbivory, but resilience of individuals will depend on the intensity, season, and frequency of herbivory. I set up long-term exclosures to test if mammalian herbivores can alter patterns of canopy succession in early post-fire sites that ranged in broadleaf dominance. Removal of natural moose herbivory inside exclosures led to species-specific responses after two years. Alaskan paper birch (dominant broadleaf species) grew larger when safe from herbivores while the opposite was true for black spruce. However, impacts of natural moose browsing on the dominant birch were insufficient to eliminate the competitive hierarchies that supported birch dominance of the canopy and suppressed growth of black spruce in the understory. I conclude that even with the negative impacts of moose on broadleaf species growth, their tolerance to browse damage enables their persistence as the dominant canopy species after large and severe fires in the boreal forest of Alaska
... Similarly, summer-browsed (removal of 100% of the first flush of the current year's growth) Quercus rubra seedlings had negligible height growth, while seedlings browsed during the dormant period did not affect growth (Woolery and Jacobs 2014). In addition, height, height increment and/or above-ground biomass were found to be clearly smaller after mid -late summer clipping compared to earlier clipping dates (Bell et al. 1999;Guillet and Bergström 2006;Jones et al. 2009;Welker and Menke 1990). Thus, there is some evidence that deciduous trees have less growth after mid -late summer than after winter leader shoot clipping. ...
Chapter
Habitat preferences are a crucial facet in the management of endangered wildlife species, but only recently have wildlife managers begun to fully realize the potential of information on species' habitat selection to solve pressing problems in the conservation and management of protected areas. Habitat choice is particularly important in extreme environments like hyperarid deserts, in which resources like food, water and cover are limited. A number of studies suggested that endangered Arabian gazelles (Gazella arabica) prefer wooded, mountainous terrain on the Arabian Peninsula, while avoiding open plains and sand deserts. However, this pattern could also be an artefact created by the tremendous hunting pressure experienced by the species and a retreat into areas inaccessible to humans. We studied habitat choice of the largest indigenous population of Arabian gazelles in the world, persisting on the Farasan Islands in the Red Sea, off the coast of Saudi Arabia. We measured the availability of different habitat types using GIS-based habitat mapping and the degree of utilization by gazelles, assessed via biannual transect route counts in the protected area. Our results suggest that former fields (beside Acacia grooves) are the preferred habitat type of Arabian gazelles, especially during the hot summer months. Our study further suggests that Acacia-wooded habitats are the originally preferred habitat type of this gazelle species (especially during the cooler winter months) rather than secondary habitats in remote mountainous areas on the Arabian Peninsula.
... Similarly, summer-browsed (removal of 100% of the first flush of the current year's growth) Quercus rubra seedlings had negligible height growth, while seedlings browsed during the dormant period did not affect growth (Woolery and Jacobs 2014). In addition, height, height increment and/or above-ground biomass were found to be clearly smaller after mid -late summer clipping compared to earlier clipping dates (Bell et al. 1999;Guillet and Bergström 2006;Jones et al. 2009;Welker and Menke 1990). Thus, there is some evidence that deciduous trees have less growth after mid -late summer than after winter leader shoot clipping. ...
Full-text available
Chapter
The range of wild ungulates, and principally that of red deer and wild boar, has expanded throughout the Mediterranean Basin in recent decades. The abundances of these species have also increased dramatically, and they are now attaining densities that often exceed ecologically admissible thresholds. This situation has been favoured by several factors: i) The increase in shrub and forest areas owing to the cessation of marginal agricultural practices, ii) the growing demand for big game hunting and its associated intensive management actions, iii) the loss of large native predators and iv) climatic change, among others. The increase in some ungulate populations has led to agricultural damage, road accidents and the spread of shared diseases among livestock and ungulates, in addition to causing important dysfunctions at various levels of the ecosystem. In this chapter, we assess the peer-reviewed literature concerning the effects of wild ungulates on the Mediterranean Basin ecosystems. The aim of this chapter is to synthesize the knowledge regarding the ecological effects of current wild ungulate abundance on Mediterranean ecosystems, paying particular attention to the cascade effects in them. Several works have reported the negative impacts of wild ungulates, from the basic levels of the ecosystem (soil properties, nutrient cycles, microorganisms), to higher levels (vertebrates) through a series of intermediary effects on the plant community or the structure of arthropods and vegetation. These impacts on the ecosystem also promote alterations in the populations of some species whose conservation is a cause for concern, and may even lead to modifications of natural processes. All groups involved in wild ungulate management (hunters, gamekeepers, conservationists, researchers, competent authorities) should actively participate in efforts to understand, monitor, and reduce the impact of wild ungulates on ecosystems, highlighting common priorities for future research and action plans. Furthermore, addressing impacts at the landscape scale requires management policies that will integrate information regarding the negative impacts of ungulates in order to ensure an appropriate and holistic management.
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In two separate studies, the suckering of trembling aspen (Populus tremuloides Michx.) roots was assessed in response to different daily maximum soil temperatures and in relation to available nutrients (CaSO4 and NH4NO3). In the first study, aspen root sections were incubated under high:low temperature regimes of 12:8, 14:8, 16:8, 18:8, or 20:8°C until 124 degree-days above 8°C had been reached. Daily maximum temperature did not affect the number of suckers produced per square centimetre of root surface area or per root section. However, more time was required to initiate suckers on root sections grown under the 12:8°C temperature regime compared with those grown at 20:8°C. Furthermore, when calculated from a base temperature of 5°C, the number of degree-days needed to initiate aspen suckers was not different across the temperature regimes. In the second study, CaSO4, NH4NO3, or distilled water was added to aspen root sections. While nutrients did not affect the number of suckers produced, the addition of CaSO4 or NH4NO3 significantly increased sucker dry mass. Nutrient addition appeared to stimulate sucker growth and mobilization of stored reserves to support this growth.Le drageonnement du peuplier faux-tremble (Populus tremuloides Michx.) en réponse à différentes températures quotidiennes maximum du sol et en relation avec les nutriments disponibles (CaSO4 et NH4NO3) a été évalué dans le cadre de deux études distinctes. Dans la première étude, des sections de racine de peuplier ont été incubées sous différents régimes de température maximum : minimum de 12 : 8, 14 : 8, 16 : 8, 18 : 8 ou 20 : 8 °C jusqu'au moment d'atteindre 124 degrés-jours au-dessus de 8 °C. La température maximum durant le jour n'a pas affecté le nombre de drageons produits par centimètre carré de surface racinaire ou par section de racine. Cependant, il fallait plus de temps pour initier la formation des drageons sur les racines conservées au régime de température 12 : 8 °C comparativement au régime 20 : 8 °C. De plus, lorsqu'il était calculé sur la base de 5 °C, le nombre de degrés-jours requis pour initier la formation des drageons était le même quel que soit le régime de température. Dans la seconde étude, CaSO4, NH4NO3 ou de l'eau distillée a été ajouté aux sections de racine de peuplier. Bien que les nutriments n'aient pas affecté le nombre de drageons produits, l'addition de CaSO4 ou de NH4NO3 a significativement augmenté le poids sec des drageons. L'addition de nutriments semble stimuler la croissance des drageons et la mobilisation des réserves pour supporter cette croissance.[Traduit par la Rédaction]
Article
A widely held tenet of Rocky Mountain ecology is that trembling aspen (Populus tremuloides Michx.) reproduces almost exclusively by vegetative root sprouting. However, a rare episode of sexual reproduction in aspen occurred following the extensive Yellowstone fires in Wyoming (USA) in 1988. Great numbers of aspen seedlings became established in burned areas in 1989, apparently because of the unusual combination of high seed production, favorable weather conditions, and suitable substrate conditions that existed that year. The new aspen genets are elongating and are developing lateral root systems with numerous sprouts. Genetic diversity of seedling populations on the Yellowstone Plateau is greater than the diversity of mature clones that were sampled in northern Yellowstone National Park. Aspen has become established in many portions of the park where it was absent before the 1988 fires. Thus, we appear to be witnessing an unusual ecological event in which a widespread, ecologically important species has very rapidly expanded its geographic range and genetic variability in response to a large-scale natural disturbance.
Article
Conflicts may arise between cattle and aspen fibre production if both occur on the same landbase. The effect of cattle on aspen regeneration was evaluated by determining the effect of four treatments (No grazing, June-only grazing, July-only grazing and continuous June-July grazing on five variables (aspen height and density, percentage of aspen trampled, foraged and dead). Continuous June-July grazing impeded aspen regeneration relative to the control. The effect of cattle on aspen mortality was likely indirect, rather than a consequence of foraging and trampling. Reducing stocking levels or delaying cattle grazing may be necessary to reduce the adverse effect of cattle grazing on aspen regeneration.
Article
Aspen (Populus tremuloides) forest occupies potentially useful grazing land in the aspen parkland of western Canada, and is expanding. The replacement of forest with grassland involves the removal of trees and the control of suckers which invariably emerge following overstory removal. The control of aspen suckers by heavy browsing with cattle may be a useful technique especially in the presence of logs and stumps. In order to evaluate the effectiveness of browsing by cattle, aspen forest was burned and seeded to forages, after which the regrowth was heavily grazed by cattle either after emergence of suckers (early) or just prior to leaf fall (late). Grazing treatments were conducted over two growing seasons. A single heavy late grazing practically eliminated aspen regeneration, and two quite different plant communities resulted from the two grazing regimes. After the first year, the plant biomass in early-grazed plots consisted of 29% aspen and 28% grass (mainly sown species), while late-grazed plots had only 2.5% aspen and 18% grass, with a higher proportion of shrubs, especially snowberry. Trends established after the first year were still evident after the second year. The results indicated that heavy browsing by cattle in August may be an effective technique for control of aspen suckers following initial top kill.
Article
Q uaking aspen (Populus tremuloides) are unique because, in contrast to most western forest trees, they reproduce primarily by suckering from the parent root system. Generally disturbance or dieback is necessary to stimulate re-generation of aspen stands. These self-regenerating stands have existed for thousands of years. If they are lost from the landscape, they will not return through normal seeding processes as do other tree species. Aspen landscapes in the West provide numerous benefits, including forage for livestock, habitat for wildlife, watershed protection, water yield for downstream users, esthetics, sites for recreational opportunities, wood fiber, and landscape di-versity. Loss, or potential loss, of aspen on these lands can be attributed primarily to a combination of successional processes, reduction (or elimination) of fire, and long-term overuse by ungulates. Existing cond~t~ons ind~cate that most aspen stands will eventually be replaced by conifers, sagebrush, or possibly other shrub communities. The decline of aspen results in loss of water, forage, and biodiversity. Numerous landscapes throughout the West that were once dominated by aspen are in late successional stages dominated by mixed-conifer. If restoration treatments are to be successful, action must be taken soon. Fig. 1. Aspen clones on the Fishiake National Foresf have sparse regeneration (upper third) and show aspen being repiaced by conifer (middle third). Photograph, Kreig Rasmussen.
Article
The role of livestock grazing and big-game browsing in the decline of aspen (Populus tremuloides Michx.) in the Intermountain West has long been questioned. All known aspen exclosures (n=8) on the Dixie and Fishlake National Forests in south-central Utah were measured during late summer of 1995 and 1996 to determine aspen stem dynamics, successional status, and understory species composition. Fire of the exclosures mere of a 3-part design with a total-exclusion portion, a livestock-exclusion portion, and a combined-use portion which permitted the effects of deer (Odocoileus hemionus) and elk (Cervus elaphus) herbivory to be measured separately from those of livestock. Aspen within all total-exclusion plots successfully regenerated and developed multi-aged stems without the influence of fire or other disturbance. Aspen subjected to browsing by wildlife, primarily mule deer, either failed to regenerate successfully or regenerated at stem densities significantly lower (2,498 stems ha(-1)) than that on total-exclusion plots (4,474 stems ha(-1)). On combined wildlife-livestock-use plots, most aspen failed to regenerate successfully, or did so at low stem densities (1,012 stems/ha(-1)). Aspen successfully regenerated on ungulate-use plots only when deer numbers were low. Similarly, ungulate herbivory had significant effects on understory species composition. In general, utilization by deer tended to reduce shrubs and tall palatable forbs while favoring the growth of native grasses. The addition of livestock grazing, however, tended to reduce native grasses while promoting introduced species and bare soil. Thus, communities dominated by old-age or single-age trees appear to be a product of ungulate browsing, not a biological attribute of aspen as has been commonly assumed. There was no evidence that climatic variation affected aspen regeneration. Observed differences are attributed to varied histories of ungulate herbivory.