EFFECTS OF GRAZING AND TRAMPLING BY ROCKY
MOUNTAIN ELK (Cervus elaphus nelsoni) ON
THE VEGETATIVE COMMUNITY OF BANDELIER
NATIONAL MONUMENT, NEW MEXICO
by
SUSAN P. RUPP, B.S.
A THESIS
IN
WILDLIFE SCIENCE
Submitted to the Graduate Faculty of Texas Tech University in
Partial Fulfillment of the Requirements for
the Degree of
MASTER OF SCIENCE
Approved
December, 2000
8'^S ACKNOWLEDGMENTS /^A/I^ ~9Z^^
c-;, I would like to extend my utmost gratitude to the National Park Service,
t^i J ,1^ specifically Bandelier National Monument, and Texas Tech University for providing
funding and the opportunity to conduct this research. I would also like to thank my co-
advisors. Dr. Mark Wallace and Dr. Rob Mitchell, for their vast knowledge, support and
valuable insight. You have instilled in me direction, purpose, and dedication through
your example and generosity. My other committee members, Dr. David Wester and Dr.
Andy Herring, have also provided considerable guidance. Specifically, Dr. Wester has
opened his office door more times than I deserve. His statistical assistance, insight, and
support have tmly enlightened me.
I would also like to thank the personnel of Bandelier National Monument. They
have served as wonderful mentors and friends at my "home away from home." Mr.
Stephen Fettig and Dr. Craig Allen have provided knowledge of the Pajarito Plateau
which has greatly enlightened my perceptions of the difficulties endured by managers in
this area. I am also greatly indebted to Mr. Brian Jacobs, vegetation specialist at
Bandelier, who identified and confirmed numerous unknown plant specimens that were
collected in the field. Finally, Ida Formea, Kay Beeley, John Hogan, and Vicki Estes
illuminated my days with good conversations, and many laughs.
Without the assistance of my various field technicians, data collection would have
been impossible. Erika Hersch, Cindy Caplen, Doug Krantz, Christine Evans and Salinda
Daley endured many hours of long, tedious data collection and I thank them for their hard
work and patience throughout this project. With their help, I have learned more about
ii
managing and supervising field technicians than I ever thought I could learn in such a
short amount of time. Their friendship also helped me to endure the monotonous hours in
the field.
My deepest heartfelt gratitude goes out to my husband, Paul Rupp, whose love
and companionship has provided strength when I felt little left to give. His vast computer
knowledge has saved me (and my data!) in moments of potential catastrophe and I cannot
thank him enough for creating the database on which I stored my data. The faith, love,
and support he has shown me have been my inspiration, as a professional and as a human
being, and words cannot express how richly blessed I am to be loved by such a wonderful
person.
Finally, I would like to thank Our Heavenly Father for providing this incredible
opportunity to complete my graduate education and for answering our prayers under what
appeared to be insurmountable circumstances. It is to Him I dedicate this work.
ni
TABLE OF CONTENTS
ACKNOWLEDGEMENTS ii
ABSTRACT vi
LIST OF TABLES viii
LIST OF FIGURES x
CHAPTER
I. INTRODUCTION AND LITERATURE REVIEW 1
IL BACKGROUND INFORMATION 7
Study Area 7
Justification 10
Objectives 14
III. MATERIALS AND METHODS 21
Experimental Design 21
Parameters Measured 23
IV. RESULTS 30
Intensity Study 30
Frequency Study 35
Reference Area Analysis 39
V. DISCUSSION AND MANAGEMENT IMPLICATIONS 63
LITERATURE CITED 71
IV
APPENDIX A: EXCLOSURE MAPS 78
APPENDIX B: SUMMARY STATISTICS FOR PLOT A 92
APPENDIX C: SUMMARY STATISTICS FOR PLOT B 97
ABSTRACT
The increase in the Jemez region elk (Cervus elaphus nelsoni) population is a
concem to local managers. Threats to archeological resources and naturally fijnctioning
ecosystems through accelerated soil erosion, degrading plant communities, and unnatural
vegetation change rank as the highest management priorities at Bandelier National
Monument, New Mexico.
In summer 1998, Bandelier National Monument erected a series of ungulate
exclosures and paired reference areas to evaluate elk impacts on the vegetative
community in piilon-juniper (PJ), ponderosa grassland (PG), and mixed-conifer (MC)
habitat types. A two-factor factorial randomized block design was established to evaluate
simulated grazing/trampling treatment combinations applied at different intensities and
frequencies from January through May of 1999 and 2000. Unfenced reference areas were
compared with non-treated units inside exclosures to evaluate the effects of grazer
exclusion. Parameters measured included density, percent foliar/litter cover, mean basal
area/plant, productivity, and species richness and composition.
Analysis of covariance (ANCOVA) was used to account for pre-treatment
patterns that existed among treatment plots. Pre-treatment and first-year response data
indicated large amounts of variation between and within exclosures for all parameters
measured which may reflect the inherent variability present in vegetative communities at
Bandelier National Monument. Few significant responses were detected following a
VI
single year's treatment application and two unseasonably warm winter/spring periods
may have confounded or obscured treatment effects.
Responses differed between habitats and among treatments applied. In PJ habitat,
heavily clipped plots had more plants and less litter, but lightly trampled units had more
foliar cover, specifically forb cover, than moderately trampled units. In PG habitat, non-
trampled units had lower plant density, specifically forb density, but lightly trampled
units had more litter cover than heavily or non-trampled areas. Plots clipped at the
highest frequency also had more grass plants, but less litter when coupled with moderate
trampling. In MC habitat, moderate intensities of clipping and trampling resulted in
higher grass cover. Non-clipped units had higher litter cover. Light or moderate
intensities of trampling or a combination of moderate clipping/trampling frequencies
increased species richness. Frequent, heavy clipping also resulted in lower total
productivity in MC sites. Significant year effects were detected for basal measurements
in most cases, but these could not be attributed to treatments. Shifts in species
composition were dependent on habitat, but showed potential trends in response to
clipping or trampling. No significant effects were detected due to grazer exclusion.
Trampling more consistently impacted parameters and may stimulate plant
productivity at an intermediate intensity, especially in terms of forb response. Seasonal
progression and phenological development of individual plants may confound effects of
defoliation frequency and intensity. Longer time periods may be needed to detect
vegetative responses to changes in grazing pressure especially in ecosystems that have
developed with a history of grazing pressure.
vn
LIST OF TABLES
4.1 Adjusted mean density by habitat for different intensities of clipping and trampling treatment combinations in 2000 43
4.2 Adjusted mean foliar cover by vegetative type for different intensities of clipping and trampling treatment combinations in PJ habitat 44
4.3 Mean litter cover by habitat for different intensities of clipping and trampling treatment combinations in 2000 46
4.4 Relative composition of common species in 1999 vs. 2000 by habitat for different intensities of clipping and trampling treatment combinations 47
4.5 Adjusted mean density by vegetative type for different intensities of clipping and trampling treatment combinations in PG habitat 48
4.6 Mean litter cover by habitat for different frequencies of clipping and trampling treatment combinations in 2000 51
4.7 Adjusted mean density by vegetative type for different frequencies of clipping and trampling treatment combinations in PJ habitat 52
4.8 Average initial productivity following first clipping treatment and subsequent regrowth for second and third clipping treatment 53
4.9 Relative composition of common species in 1999 vs. 2000 by habitat for different frequencies of clipping and trampling treatment combinations 54
4.10 Adjusted mean density by vegetative type for different frequencies of clipping and trampling treatment combinations in PG habitat 55
4.11 Adjusted arcsine transformed means for foliar cover by vegetative type for different frequencies of clipping and trampling treatment combinations in MC habitat 57
4.12 Mean densities inside versus outside exclosures 60
4.13 Mean foliar cover inside versus outside exclosures 61
4.14 Mean litter cover inside versus outside exclosures 62
B.l True means for total density by habitat and treatment 93
vni
B.2 Tme means for foliar cover by habitat and treatment 94
B.3. Tme means for total litter cover by habitat and treatment 95
B.4. Tme means for total richness by habitat and treatment 96
CI Tme means for total density by habitat and treatment 98
C.2 Tme means for foliar cover by habitat and treatment 99
C.3 Tme means for total litter cover by habitat and treatment 100
C.4 Tme means for total richness by habitat and treatment 101
IX
LIST OF FIGURES
2.1 Bandelier National Monument, New Mexico 16
2.2 Region bumed by the 1977 La Mesa Fire in and near Bandelier National Monument, NM 17
2.3 Region bumed by the 1996 Dome Fire in and near Bandelier National Monument, NM 18
2.4 Monthly temperature for the 1998-1999 and 1999-2000 field seasons compared to the monthly average in Los Alamos, New Mexico 19
2.5 Monthly precipitation for the 1998-1999 and 1999-2000 field seasons compared to the average temperature at Los Alamos, New Mexico 20
3.1 Exclosure locations at Bandeher National Monument, New Mexico 27
3.2 Exclosure layout 28
3.3 Designation of clipping X trampling treatment combinations inside exclosures. 29
4.1 Adjusted mean foliar cover by habitat in response to trampling intensity 42
4.2 Mean litter cover by habitat in response to clipping intensity 45
4.3 Mean foliar cover for grasses for different intensities of clipping X trampling treatment combinations in mixed conifer exclosures 49
4.4 Mean litter cover by habitat in response to clipping frequency 50
4.5 Mean litter cover for different frequencies of clipping X trampling treatment combinations in PG exclosures 56
4.6 Species richness in response to different frequencies of clipping X trampling treatment combinations in MC exclosures 58
4.7 Mean primary productivity in response to clipping frequency in MC habitat 59
A.l Experimental layout for exclosure/reference area 'PJ-l ' 79
A.2 Experimental layout for exclosure/reference area 'PJ-2' 80
A.3 Experimental layout for exclosure/reference area 'PJ-3' 81
X
A.4 Experimenta
A.5 Experimenta
A. 6 Experimenta
A. 7 Experimenta
A. 8 Experimenta
A.9 Experimenta
A. 10 Experimenta
A. 11 Experimenta
A. 12 Experimenta
A. 13 Experimenta
layout for exclosure/reference area 'PJ-4' 82
layout for exclosure/reference area 'PJ-5' 83
layout for exclosure/reference area 'PG-2' 84
layout for exclosure/reference area 'PG-3' 85
layout for exclosure/reference area 'PG-4' 86
layout for exclosure/reference area 'MC-l' 87
layout for exclosure/reference area 'MC-2' 88
layout for exclosure/reference area 'MC-3' 89
layout for exclosure/reference area 'MC-4' 90
layout for exclosure/reference area 'MC-5' 91
XI
CHAPTER I
INTRODUCTION AND LITERATURE REVIEW
Grazing has the potential for altering botanical composition, cover, and soil
physical properties through herbivory and associated trampling (Warren et al. 1986).
Kind of animal, season and intensity of use, soil characteristics, individual plant and
community characteristics (e.g., density, morphology, physiology), and temporal and
spatial variation in environmental conditions influence the type and degree of impact
(Weigel et al.l990, Bastrenta et al. 1995). Direct impacts of grazing may include plant
defoliation, nutrient removal and redistribution through excreta, and mechanical
manipulation of the soil and plant material through associated trampling. Indirect
impacts can include changes in infiltration rates, soil erosion and sediment production,
and plant productivity.
A substantial body of information exists conceming the effects of grazing by
domestic ungulates (Olson and Richards 1988, Ralphs et al. 1990, Biondini et al. 1998),
but the majority of these studies were conducted by manipulating grazing systems.
Grazing systems are partially defined by the intensity (amount of plant material removed)
and frequency (the number of times a plant is defoliated) of use which, in tum, affects
both the quality and quantity of forage produced (Motazedian and Sharrow 1990). Few
studies have attempted to isolate the effects of intensity or frequency of use (Gillen et al.
1990) and it is often unclear whether impacts are caused by the removal of vegetation or
associated trampling. Studies that have isolated impacts of grazing (Oesterheld 1992) or
trampling intensity (Warren et al. 1986, Abdel-Magid et al. 1987, ToUner et al. 1990)
have ignored possible interactive effects.
It may not be possible to control the intensity of defoliation on individual plants
in conventional grazing systems since management controls only the frequency of
grazing events through periodic rest/rotation grazing regimes to maintain desired
species composition (Ralphs et al. 1990). Demer et al. (1994) confirmed that rotational
grazing allows greater control of defoliation frequency, but not defoliation intensity.
They found similar heights of grazed plants within continuous and rotational grazing
systems and determined that grazed height was influenced primarily by stocking rate.
Frequency and composition of forages may decline as stocking rates increase (Ralphs et
al. 1990). Controlling grazing regimes and stocking rates is often not possible where
wild ungulates are the primary source of impacts and other management altematives
must be evaluated.
Vegetative impacts caused by Rocky Mountain elk (Cervus elaphus nelsoni)
grazing and trampling have been a growing concem for natural resource managers
(Gmell 1973, Houston 1982, Despain 1989, Kay 1997), but few studies have described
impacts caused by excessive elk grazing. McCulloch (1955) described over-browsing
by elk in the Selway-Bitterroot Wildemess Area, Idaho. He found that with a decrease
in browse species, there was an increase in less desirable species such as cheatgrass
(Bromus tectorum) and goatweed (Hypericum perforatum). Conifers in winter range
developed a browse line (bark entirely browsed to the height ungulates could reach) and
seedling emergence was poor (McCulloch 1955). Similarly, Hoskins and Dalke (1955)
found high erosion and an increase in less desirable plant species on the Pocatello Big
Game Range in Southeastem Idaho due to a combination of livestock and elk grazing.
Gaffney (1941) reported similar effects of winter elk browsing on the Flathead
River, Montana, but found the greatest damage to grass occurred through early grazing
and trampling in the spring. Conifers developed a well-defined browse line in highly
populated areas. However, winter ovemse was not related to erosion in Montana.
Other studies have reported that intensive grazing results in an increase of less palatable
plant species and inhibited growth of willows (Salix spp.), shmbs, and aspen (Populus
tremuloides) shoots (Hoskins and Dalke 1955, McCulloch 1955, Kay 1994, Kay 1995,
Kay 1997).
Croft and Ellison (1960) and Gmell (1973) evaluated elk impacts on the Big
Game Ridge, Wyoming. Croft and Ellison (1960) concluded soil erosion, vegetative
destmction, aspen barking, and increased mnoff were the result of excessive use by elk.
In contrast, Gmell (1973) did not find evidence to support these conclusions. Croft and
Ellison (1960) recommended further studies of elk grazing and relative amounts of
trampling.
The trampling of ranges associated with intensive grazing may be of value for two
reasons (Balph and Malecheck 1985). First, the action of the hooves of an animal in
breaking the soil surface and in mixing and breaking up litter may aid plants in obtaining
water and nutrients. Second, trampling of caespitose grasses may break down the
standing dead material within ungrazed plants, which is known to inhibit grazing.
Modifications of soil stmcture and/or microtopography by animal trampling can also
influence the number of microsites available for seed germination (Savory and Parsons
1980, Weigel et al. 1990, Winkel and Roundy 1991). Trampling, in conjunction with
plant competition, soil characteristics, and moisture regime, may affect plant species
differently enhancing some while reducing the abundance of others (Weigel et al. 1990).
Livestock trampling also affects infiltration rates and bulk density of soils
(Tollner et al. 1990). However, productivity (e.g., root biomass, forage growth) may
not be significantly reduced. Warren et al. (1986) found that infiltration rates were
consistently higher and sediment production lower before trampling than after
regardless of soil moisture at the time of trampling. Impacts on infiltration increased
with increased stocking rates (Warren et al. 1986). In many semi-arid and arid regions
where vegetation is sparse, recovery from trampling effects may be slow.
Sun and Liddle (1991) emphasized the importance of plant resistance (ability to
withstand impact), survival (the probability of surviving following impact), and recovery
(the growth rate following damage) in regulating ecosystem stability. Similar concepts
were discussed by Cole (1988, 1995a,b) in which he measured plant "deterioration" and
"species persistence" following human trampling of ecosystems. Relative cover varied
significantly with both trampling intensity and vegetation type. In general, community
response varies with habitat type and species (Olson et al. 1985, Campa et al. 1992, Cole
1995a,b).
Identifying causative factors for vegetative change is a complex problem.
Fluctuations in vegetative communities may relate more to short-term climatic
conditions rather than to the effects of ungulates. Houston (1982) evaluated impacts of
the northern Yellowstone elk herd prior to the 1988 Yellowstone fire and determined
that evidence does not support interpretations from early reports of widespread or
accelerated erosion in the area. Data reflect fluctuations in plant communities in
response to short-term climatic conditions rather than long-term directional changes
caused by elk (Houston 1982). He showed that persistent heavy winter and variable
spring grazing, with associated trampling, did not cause extensive plant mortality or
progressive changes in density or basal area on his study area in Yellowstone National
Park. Periods of major elk reductions did not result in areas with different species
abundance, basal area, or composition compared to heavily grazed areas.
Though not much can be found in the literature about vegetative responses to
grazing pressure in conjunction with climatic factors, environmental variation can play
an important role. Winkel and Roundy (1991) found seedling emergence in response to
cattle trampling differed among years and treatments relative to precipitation pattems
and periods of available water. Olson et al. (1985) indicated each species reacts to
precipitation regimes and grazing pressure in a unique manner. They concluded that
changes in stocking rates need to be anticipated and planned to coincide with available
forage because of large fluctuations in cover due to varying precipitation. It is
important to determine the effects of grazing and environmental variation on the rates
of plant growth and to determine how these may interact to regulate the dynamics of
plant communities (Bastrenta et al. 1995).
Exclosures are a standard method used to observe changes in vegetative
communities when grazing animals are excluded from an area. In southwestem North
Dakota, Brand and Goetz (1986) found major differences in species composition between
exclosed and adjacent domestically grazed plots over the course of 3 years. However,
total yield and total below ground biomass were not significantly different in 3 of their 4
5
plots. Tollner et al. (1990) used exclosures in their study on trampling effects of
domestic stock on soil properties in Georgia. They found trampling altered surface soil
stmcture, but productivity (root biomass, forage growth) was not significantly reduced.
In Colorado, Tucker and Leininger (1990) reported that total vascular vegetation, shmb,
and graminoid canopy cover was greater in riparian exclosures than in grazed areas
whereas forb canopy cover was similar between treatments. In addition, exclosures had
nearly 2 times more litter cover whereas grazed areas had 4 times more bare ground.
Exclosures have been used to determine grazing impacts by elk (Gmell 1973, Despain
1989) but results are inconsistent and ftirther studies are needed.
Grazing and trampling can have a wide range of effects from direct changes in
botanical composition and stmcture to changes in soil physical properties. The impacts
of domestic grazing and trampling are well-documented, but the need for wild ungulate
research on national parks has been iterated (Leopold 1963). Studies have used
exclosures to document impacts by elk, but results are inconsistent. In addition, long
time periods - periods in the range of 20 years - may be needed to detect vegetative
responses to changes in grazing pressure (Chong 1992) especially in ecosystems that
have developed with a history of grazing pressure. Further studies isolating the effects of
elk grazing and trampling are needed.
CHAPTER II
BACKGROUND INFORMATION
Study area
Bandelier National Monument (BAND) is located in the Jemez Mountains of
north central New Mexico (35:53:38N 106:17:02W) approximately 8 km south of Los
Alamos in an area called the Pajarito Plateau (Figure 2.1). The monument was
established as a National Park in 1916 by Presidential Proclamation (Lissoway 1996).
Although the area is rich in natural resources, the park was originally established to
preserve and protect the relicts of the Anasazi people who occupied the canyons and
mesas of the plateau from the 1 lOO's through the 1500's. It is estimated that there are
over 3,000 active archeological sites within the region (Foxx 1984). The threat to
archeological resources and naturally functioning ecosystems through accelerated soil
erosion, degrading plant communities, and unnatural vegetation change now rank as the
highest priorities in Bandelier's Resource Management Plan (Fettig 1997).
The Pajarito Plateau was formed by an ash flow of volcanic activity about 1.4
million years ago (Wilcox and Breshears 1994) resulting from the formation of a giant
caldera, the "Valle Grande", 30.4 km west of the Bandelier entrance station. Soils of the
area are derived from ryholite, tuff, ash, and other volcanic debris and can be classified
into three groups: (1) extremely rocky, shallow soils less than 51 cm deep on steep
canyon slopes, (2) moderately deep (51-91.5 cm) soils on mesa tops, and (3) sandy
alluvial soils on the canyon bottoms (Foxx 1984). Bandelier is bordered by Santa Fe
National Forest to the west, Los Alamos National Environmental Research Park
7
(LA/NERP) to the east, and private lands to the north and south. Native American
reservation lands are also found throughout the surrounding areas.
The monument's 13,290 hectares range in elevation from 1,680 m in the lower
canyons near the Rio Grande to about 3,240 m near the summit of Cerro Grande. The
area is transected by 3 main canyons - Frijoles, Capulin, and Alamo - and a system of
smaller canyons making the terrain rough and virtually inaccessible in some places.
Vegetative pattems are highly dependent on elevation and topography (Wilcox and
Breshears 1994).
Three distinct vegetation associations can be described (Conley et al. 1979).
Lower elevations (1,680-2,015 m) are characterized as piilon-juniper woodland. Species
composition in these areas includes moderate stands of pifion pine (Pinus edulis) and
one-seed juniper (Juniperus monosperma) with understory shmbs of wavy leaf oak
(Quercus undulata), Apache plume (Fallugia paradoxa), and mountain mahogany
(Cercocarpus montanus). Mid-elevational transitional areas (2,015-2,440 m) are
characterized by an overstory of ponderosa pine (Pinus ponderosa) and understory of
Gambel oak (Quercus gambelii), New Mexican locust (Robinia neomexicana), and
mountain mahogany. Typical grasses in the transitional zone include mutton grass (Poa
fendleriana), June grass (Koeleria cristata), and mountain muhly (Muhlenbergia
montana). Upper elevations (2,440-3,240 m) are classified as mixed-conifer with a
variety of overstory species that include Douglas fir (Pseudotsuga menziesii), white fir
(Abies concolor), blue spmce (Picea pungens), and quaking aspen. Gambel oak, rock
spirea (Holodiscus dumosus), and waxflower (Jamesia americana) are typical understory
shmbs and slender wheatgrass (Agropyron trachycaulum), Canada bluegrass (Poa
8
compressa), and Parry oatgrass (Danthonia parryi) are common grasses in the mixed-
conifer zone. Canyon bottoms have riparian communities that include narrowleaf
Cottonwood (Populus angustifolia) and boxelder (Acer negundo).
In 1977, approximately 6,180 ha of land in Santa Fe National Forest, BAND, and
LA/NERP bumed in the "La Mesa" fire (Figure 2.2). Sixty-nine percent, or 4,250 ha, of
the total area bumed was on BAND alone (Foxx 1984). The La Mesa fire converted
dense, monotypic ponderosa pine forests into a more productive and diverse mosaic of
grassland, shmbland, and forests. Severely bumed areas were denuded of herbaceous
cover but were revegetated with aerial seeding of native grasses (Conley et al. 1979).
The 1977 La Mesa fire is believed to have contributed to a population increase of the elk
herd (Allen 1996). Other more recent fires, such as the "Dome" fire in April 1996
(Figure 2.3) and a number of prescribed bums set by management at BAND have also
contributed to the ecology of this region, although direct effects on the elk population
have not been documented.
Average annual precipitation on the Pajarito Plateau is 330 to 460 mm (Davenport
et al. 1996, Wilcox et al. 1996) of which about 45% occurs in July, August, and
September. Average daytime temperatures range from 32.2°C in the summer (max. =
41.1°C) to -9.4°C in the winter (min. = -30.6°C). Over the two years of data collection
for this study, temperatures were generally higher than the 1920-1999 average with the
exception of April 1999 when an unseasonal cold front came through (Figure 2.4).
Annual precipitation during the 1998-1999 and 1999-2000 field seasons were also lower
than average (Figure 2.5).
Justification
Management of wildlife on National Park Service (NPS) land is a controversial
topic (Buono 1997). With the establishment of the first national park, Yellowstone, in
1872, Congress forbade the wanton destmction and market hunting offish and game in
the area. Forty-four years later, the Organic Act of 1916 charged the NPS with a mission
".. .to promote and regulate the use of all Federal areas known as national parks,
monuments, and reservations.. .which purpose is to conserve the scenery and the natural
and historic objects and the wild life therein and to provide for the enjoyment of the same
in such manner and by such means as will leave then unimpaired for the enjoyment of
future generations" (Buono 1997, p. 23). Despite such regulations, there have been
considerable disputes over federal versus state ownership of wildlife (e.g., Geer vs.
Connecticut 1896, Hunt vs. United States 1928, United States vs. Moore 1986; Buono
1997).
New Mexico and Bandelier National Monument are not strangers to the legal
constraints surrounding the management of wildlife on federal lands. In 1968 (New
Mexico vs. Udall), New Mexico challenged the NPS in regard to obtaining permits from
the state to sacrifice deer (Odocoileus hemionus) in Carlsbad National Caverns Park for
research purposes (Buono 1997). New Mexico stated that destmction of the animals did
not serve to protect federal property. After winning their case in District Court, New
Mexico was defeated before the U.S. Tenth Circuit Court of Appeals which stated that
the research was necessary to ascertain the number of deer the area would support based
on dietary analysis (Buono 1997). A second major battle arose after Congress enacted
the Wild Free-Roaming Horses and Burros Act (1971). Again, the state challenged the
10
regulatory authority of the federal government over management of wildlife on
unreserved public lands (Kleppe vs. New Mexico 1976; Buono 1997). The court system
eventually over-mled New Mexico stating "the complete power that Congress has over
public lands necessarily includes the power to regulate and protect the wildlife living
there" (Buono 1997, p. 21) and citing jurisdiction under the Property Clause of the U.S.
Constitution. In the early 1980's, BAND was sued to prevent the removal of wild burros
from monument property, but the judicial system mled in favor of BAND and allowed
removal of the non-native burro population. Today the scene is set for a major political
and litigious battle conceming elk management in northem New Mexico that hauntingly
parallels some of these earlier battles.
Rocky Mountain elk are considered native to the Jemez Mountain region of north
central New Mexico, but it is thought they were extirpated from the area by 1906 (Allen
1996). In 1948, the New Mexico Department of Game and Fish (NMGF) released 21
cows/calves and seven bulls imported from Yellowstone National Park into the Jemez
Mountain region (Allen 1996). By 1961, the estimated population was 200 animals, all
descendents of the original 28 founders (Allen 1996). Since then, it is believed that the
population has exhibited an exponential increase, partially in response to the 1977 La
Mesa fire when thousands of hectares of wintering range was created (Wolters 1996).
Current estimates of the population in and around Bandelier National Monument are
6,000 to 8,000 individuals with an annual growth rate of 13% and a doubling time of 5.7
years (Fettig 1997). Allen (1996) estimated an annual growth rate of 21.3% and doubling
time of 3.6 years between 1978 and 1992 on Bandelier alone. Other studies have also
reported annual growth rates of elk exceeding 20% (McCorquodale et al. 1988).
11
Some impacts of elk on the vegetation and soils of BAND are apparent (Fettig
1997). Intensive browsing may be largely eliminating aspen suckers from upland
portions of the La Mesa fire and the headwaters of the Frijoles watershed (Allen 1996).
Mature aspen trees are heavily barked in many areas. Heavy browsing is apparent on a
variety of shmbs including New Mexico locust, oaks (Quercus spp.), and small
buckbmsh {Ceanothus fendleri) throughout the elk wintering range on BAND. Meadows
appear to be kept in the early stages of succession by excessive elk use (Wolters 1996).
Severe erosion, potentially the result of a combination of historical cattle {Bos
spp.)(through the early 1900's) and elk grazing, exists in Bandelier's piii on-juniper
woodlands (Wilcox et al. 1996). Erosion is estimated at an unsustainable rate of 0.53
cm/year (Fettig 1997). About 70% of Bandelier's cultural resources are being damaged
by erosion (Allen 1996) and there is concem that elk impacts to the area may accelerate
erosion due to excessive trampling and loss of herbaceous cover resulting from grazing
(Fettig 1997). Similar impacts of domestic livestock grazing are found throughout the
scientific literature (Warren et al. 1986, Tollner et al. 1990), but information on the
impacts of wild ungulate grazing is limited.
The New Mexico Department of Game and Fish is developing an elk management
plan for the state. However, National Park Service and NMGF interests often conflict
with each other and neither agency has a confident estimate of the actual population size
is in or around BAND. Though NMGF is aware that the elk populations are expanding,
their main objective is to provide recreational opportunities for area sportsmen and other
interested individuals (NMGF 1997). They are concemed that expanding populations
throughout the state are responsible, in part, for increased depredation complaints from
12
private landowners and land resource agencies (NMGF 1997). Furthermore, constituents
are concemed that increasing elk numbers are directly related to declining deer
populations, although this has not been documented. NMGF (1997, p. 4) states, "Elk
populations, in some areas, may have exceeded social/political carrying capacities."
The increasing elk populations have resulted in other impacts to the Jemez
Mountain region. Local residents of Los Alamos and White Rock report an increased
number of elk/vehicle accidents on roads along the monument boundaries (Parker 1997).
To complicate matters, nearby San Ildefonso Pueblo has issued a proposal that would
allow them to hunt in BAND using archaic methods and equipment for traditional and
ceremonial purposes (Fettig 1997). Divergent management objectives of the state of
New Mexico, LA/NERP, BAND, tribal communities, and the large, once privately owned
"Baca Ranch" hampers effective elk management (Allen 1996).
Hunting on BAND is expressly prohibited by Code of Federal Regulations (CFR),
Title 36, Part 2, Sections 2.1-2.4 (Fettig 1997); any direct reduction of the elk herd would
require a legislative act of Congress. Before any actions can be taken, however, there is a
need for quantitative, scientific information on the potential impacts of the Bandelier elk
herd. Population dynamics must be evaluated and impacts to vegetative and soil
communities, if any, must be quantified.
The need for ungulate research on national parks has been iterated. In 1963, an
Advisory Board was requested by the Secretary of the Interior to consider the procedure
of removing excess ungulates from national parks (Leopold et al. 1963). The Board
evaluated management methods and concluded that active management within national
13
parks should be aimed at restoration of natural communities. The application of proper
research methodology to address management concems is a necessity.
Objectives
There is no question that elk numbers are increasing in and around BAND.
Instead, the question becomes how to manage the elk herd. The National Park Service
and the state of New Mexico need quantitative research to document elk impacts on
vegetation so informed management decisions can be made. Though inferences can be
drawn from the existing literature on the effects of domestic grazing, many questions still
need to be answered on a site-specific basis for wild ungulates.
Our objectives were to:
• assess changes in density, percent foliar/litter cover, basal area, species
richness, and composition through the application of different intensities
and/or frequencies of clipping and trampling within elk exclosures;
• develop models that relate plant community response to intensity and
frequency of use in three habitat types (i.e., piilon-juniper, ponderosa pine,
mixed-conifer) at Bandelier National Monument;
• determine the effects of ungulate exclusion on the vegetative parameters
mentioned above through the use of exclosures.
Chong (1992) indicated that long time periods - longer than the scope of this study - are
needed to detect vegetative responses to changes in grazing pressure. Change due to
grazing is especially difficult to detect in ecosystems that have developed with a history
of grazing pressure. The greatest changes may come from the release of grazing pressure
14
inside ungulate exclosures. The null hypothesis states that there will be no change in
density, percent foliar/litter cover, basal area, species richness, or composition in
response to clipping/trampling treatments. I predict that measured vegetative parameters
will only respond to extreme levels of clipping and trampling because moderate levels
will mimic what is actually occurring in the system and changes in vegetative stmcture
will not be evident during the scope of this study. In addition, changes in species
composition and richness will be most evident in non-clipped plots inside exclosures
when compared to reference plots outside exclosures due to the release from grazing
pressure.
15
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Fig. 2.2. Region bumed by the 1977 La Mesa Fire in and near Bandelier National Monument, NM.
17
/ ^ F o r e s t Roads 289 and 142 A/Highways I I Bandelier National Monument / \ / Major Creeks and Streams 1 ^ I^nic Fire Region
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Fig. 2.3. Region bumed by the 1996 Dome Fire in and near Bandelier National Monument, NM.
18
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20
CHAPTER III
MATERIALS AND METHODS
Experimental Design
During summer of 1998, 15 60m x 60m ungulate exclosures (5 exclosures in each
of 3 different habitats - MC, PG, and PJ) were erected on BAND (Figure 3.1). Each
exclosure contained an unfenced reference area of the same size. In October of 1998,
two sets of plots (designated 'A' and 'B') were established in each exclosure (Figure 3.2).
In the PJ and PG sites, plot 'A' consisted of 12 experimental units 1-m^ in size whereas
plot 'B ' consisted of 16 1-m units. Because of vegetative complexity and diversity,
experimental units in MC sites were 0.25m . Sixteen 1-m experimental units were also
placed in paired reference areas. Two of these units were randomly selected to serve as
non-treatment controls to the non-treated units inside the exclosures in an effort to
determine the effects of ungulate exclusion.
Experimental units were delineated using a 30-cm aluminum stake in one comer
and a 20-cm spiral adobe nail on the diagonal comer. A 1 -m buffer strip was maintained
between experimental units to reduce confounding effects between treatments or from
researcher impacts. Detailed plot layouts for each exclosure can be found in Appendix A.
A two-factor factorial, randomized block design (Steel and Torrie 1980) was used
to evaluate the impacts of clipping, trampling, and their interaction. Treatment plot 'A'
was established to determine impacts of treatment intensity at 3 clipping intensities [none
(0%), moderate (40-60%), or heavy (100%) standing crop removal] and four trampling
intensities [none (0 footfalls/m^), light (5 footfallsW), moderate (25 footfalls/m^), or
21
heavy (100 footfalls/m^)] after a single treatment application. Treatment plot 'B' was
established to determine impacts of treatment frequency. Clipping treatments in plot 'B '
were applied at 100% standing crop removal at none (0 times), light (1 time), moderate (2
times), or heavy (3 times) frequencies whereas trampling was applied at 0, 5, 25, or 100
footfalls/m at none (0 times), light (1 time), moderate (2 times), or heavy (3 times)
frequencies, respectively (Figure 3.3). Clipping intervals in plot 'B ' were 2-3 weeks. A
split-plot arrangement with time as a subplot factor allowed for analysis over the two
years of the study. Treatments (clipping and/or trampling) were randomly assigned to
experimental units (Im /0.25m plots) within each block (exclosure).
Clipping was used to simulate grazing. All vegetation within an experimental unit
was clipped by uniformly removing portions of each plant within the unit at the
prescribed defoliation level. Clipping treatments were applied prior to trampling
treatments.
Trampling was simulated using an artificial hoof cast from dental acrylic (Pro
Orthodontic Services - Racine, Wisconsin) molded from the front hoof print of an elk
(Acorn Naturalists® - Tustin, California). Two of these artificial hooves were securely
bolted to the bottom of a pair of sandals which could then be strapped on the feet of the
investigator. The average front hoof load of an elk is approximately 685 g/cm^ (Telfer
and Kelsall 1984). For purposes of this study, applied hoof load was calculated to be in
the range of 673 - 818 g/cm^. This design attempted to emulate the rocking or churning
effect caused by a hoof when the animal is walking which has been overlooked in other
simulated studies where mechanical devices have been used (Warren et al. 1986). The
action of hooves may be important in rupturing the soil surface and breaking up litter
22
which aid plants in obtaining water and nutrients (Balph and Malecheck 1985).
Trampling in the mixed conifer plots were applied to a fiill Im area even though effects
were measured only on 0.25m^.
Pre-treatment data were collected and treatments were applied in January through
May 1999 when elk normally would use each habitat type. Post-treatment data were
collected and treatments were applied again during the same time periods in 2000. Post-
treatment data for the second year of treatment applications are not included in this study.
Parameters measured included density, foliar/litter cover, basal area, productivity, species
richness, and composition. Plants were identified to the species level. Unknown plants
were collected in the field and numbered for later identification. If no specimens outside
plots could be located, individuals were marked inside experimental units for later
identification once the plant matured.
Parameters Measured
Density. Complete counts were made on a species-by-species basis to determine
the density of each species for the total area sampled (Brown 1954). For small, numerous
species (e.g., white clover [Trifolium repensj) total numbers were counted up to 100
individuals and approximated to the nearest ±25 individuals thereafter. Species
connected by rhizomes or stolons (e.g., wild strawberries [Fragaria americana]) were
counted as individuals if plants did not appear to be obviously connected.
Foliar/litter cover. Cover estimates were determined using a 10cm x 10cm grid
cell system (Daubenmire 1959). Foliar cover was determined by estimating the number
of grid cells covered on a species-by-species basis. Litter was not measured in 1999 as a
23
species category and is analyzed for 2000 only. We did not attempt to separate litter
from overstory sources (e.g., ponderosa needles, aspen leaves) or other vegetative
material that could have originated outside our plots. Piilon-juniper sites had more litter
than normal because a "cut-and-slash" treatment was applied by management at BAND
inside exclosures and adjacent reference areas as part of a watershed restoration study.
This study was aimed at evaluating vegetative responses to slash as a soil stabilizer and
precipitation interception technique but was not a component of our research. Though
slash was removed from our plots prior to initiation of our study, much litter fi-om those
trees remained in our plots.
Basal area. Calipers were used to measure basal area of individual plants (Brown
1954). During spring 1999, maximum width, perpendicular width to the maximum,
transect width, and distance from the edge of the quadrat were recorded for >5 plants on
>2 line transects spaced at 10-cm intervals across the quadrat. We re-measured the same
plants in 2000. Basal area was calculated using the standard equation for an ellipse
(ab*7i) where a = major axis length and b = minor axis length. Three plants from each
experimental unit were randomly selected and an average basal area/plant (cm ) was
calculated.
Species composition and richness. Composition was analyzed using descriptive
techniques. Relative percent composition was calculated for each species within a given
clipping/trampling treatment regime, ranked in order of dominance, and graphed pre- and
post-treatment. Species in which relative composition was <1% of the total in either year
were not considered in these analyses.
24
Species richness was analyzed using the logarithmic relationship discussed by
Shmida (1984) where species richness is inversely related to the log of the sample area:
# Species/log (area).
Standing crop. Complete removal of standing crop in lightly, moderately, and
heavily clipped experimental units in plot 'B' allowed for analysis of productivity among
these treatments. Subsequent visits also provided an opportunity to evaluate re-growth in
these units. Following clipping, vegetation was dried for 48 hours at 60°C and weighed.
Clipped weights from our initial visit in spring 2000 were subjected to analysis to
determine what effects, if any, clipping/trampling treatments from 1999 had on total
standing crop (i.e., productivity).
Analysis of variance (ANOVA) and mean separation were used to determine
treatment effects within and across habitats for total density and foliar cover. Habitat
effects were tested using the Type III mean square for block (habitat) as an error term.
Potential block (exclosure) X treatment interactions were tested using Tukey's test for
non-additivity. When sub-divided by grass and forb growth classes, density data and
foliar percentages that were not normally distributed were transformed using square root
or arcsine transformations, respectively (Sokal and Rohlf 1995). Experimental units
which lacked a grass or forb class were treated as sampled units and zeroes were added to
the dataset. Basal area, species richness, and litter cover were only analyzed within
habitats. For all analyses, significance was delineated at a = 0.10. We chose this
significance level in order to reduce the chances of committing a Type II error (failing to
reject the null hypothesis when it is false) which we felt was more probable after only a
25
single year post-treatment than committing a Type I error (rejecting the null hypothesis
when it is true).
26
Elk Exclosures Trails
^ / F o r e s t Roads 289 and 142 Highways Bandelier National Monument Major Creeks and Streams Bandelier Wildemess Dome Wildemess
LJ A/:
0
0
4 Kilometers
4 Miles
Figure 3.1. Exclosure locations at Bandelier National Monument, New Mexico. Exclosures (5 in each of 3 habitat types - MC, PG, and PJ) are 60m x 60m with a paired, unfenced reference area of the same size.
27
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29
CHAPTER IV
RESULTS
The following results are indicative of first-year post-treatment data. Initial
treatments were applied following preliminary data collection in spring 1999 and first-
year post-treatment data were collected prior to reapplication of treatments in spring
2000. Second-year post-treatment data will not be collected until spring 2001.
Intensity Studv
Analyses (ANOVA) of pre-treatment plot data revealed block (i.e., exclosure) X
treatment interactions (Fi,43 = 31.61, P<0.001) for density in the PJ habitat type as well as
for foliar cover in MC (Fi,43 = 4.09, P = 0.049) and PJ (Fi,43 = 22.46, P<0.001) habitats.
Initial block X treatment interactions were also detected in PG habitat, but removal of
exclosure PG-5 that had been bumed in 1998 and exclosure PG-1 that had been bumed in
1999 remedied this effect for both density (P = 0.334) and foliar cover (P = 0.226)
estimates. Because differences among habitats existed before treatments were applied,
pre-treatment densities or foliar cover estimates were used as covariates in analysis of
covariance (ANCOVA) for subsequent analyses. Litter and basal area were analyzed
using ANOVA. All pre-treatment data for total density, foliar cover, and species richness
were normally distributed except for foliar data in the mixed conifer zone. Litter data
were normally distributed in MC and PG habitats, but failed to meet normality in the PJ
zone. Attempts to normalize these data via transformation were unsuccessful and litter
30
resuhs for the PJ habitat type should be interpreted with caution. Summary statistics for
parameters by habitat can be found in Appendix 'B' .
Habitat Effects. A significant habitat effect (F2,io = 15.39, P = 0.001) was
detected for density data with more plants/m^ in MC than in either PG or PJ habitats.
There was a significant habitat by trampling interaction for foliar cover (F6,iio = 3.23, P =
0.006) indicating that trampling treatments were not having the same effect in each
habitat (Figure 4.1). Moderately trampled experimental units in PG sites had
significantly higher foliar cover ( y = 25.7%) regardless of clipping intensity. In contrast,
moderately trampled plots in the MC habitat had significantly less foliar cover ( y =
6.7%) than any other trampling treatment regardless of clipping intensity. No other
significant differences were detected in density or foliar cover for clipping, trampling, or
their interaction across all habitat types.
PJ Habitat. We detected a significant clipping effect (F2,43 = 3.55, P = 0.038)
regardless of trampling intensity for density measures. Mean separation of adjusted
means indicated that heavily clipped plots had higher plant densities than moderately
clipped or undipped plots (Table 4.1). There was a trampling effect (F3,43 = 2.33, P =
0.088) on foliar cover. Moderately ( y = 10.1%) trampled plots had less foliar cover than
lightly trampled units ( y = 12.9%, P = 0.031), but heavily ( y = 10.4%) and non-trampled
( y = 12.1%) areas were not significantly different from either one.
Analysis by vegetative type revealed a significant effect (F3,43 = 2.50, P = 0.072)
of trampling intensity on total forb cover but not on grass cover. Lightly trampled plots
had higher forb cover than heavily trampled and moderately trampled plots (Table 4.2).
31
Clipping had a significant effect (F2,44 = 3.46, P = 0.04) on litter cover in the PJ
habitat; however, data were not normally distributed and results should be interpreted
with caution. Mean separation revealed greater litter cover in non-clipped plots than in
heavily clipped areas (Figure 4.2, Table 4.3).
Relative species composition changed. Blue grama (Bouteloua gracilis) and
deervetch (Lotus wrightii) increased across all treatment combinations in 2000 whereas
mountain muhly and yellow ragweed (Bahia dissecta) decreased. However, this
difference was relatively consistent regardless of clipping/trampling treatment.
Snakeweed (Gutierriza sarrothrae), on the other hand, tended to increase as
clipping/trampling intensity increased. This may indicate a possible treatment intensity
effect (Table 4.4).
Mean basal area/plant was lower in 2000 than in 1999 (Fi,48 = 10.02, P = 0.003)
across all treatments probably due to abiotic (i.e., climate) factors. However, no effects
can yet be attributed to clipping, trampling, or their interaction.
PG Habitat. Trampling affected plant density (F3,2i = 4.14, P = 0.019) regardless
of clipping intensity. Non-trampled units had a significantly lower total density of plants
than lightly, moderately, or heavily trampled units (Table 4.1). Trampling intensity also
had a significant effect (F3,2i = 4.10, P = 0.020) on total density of forbs. Non-trampled
units had significantly fewer forbs than lightly, moderately, or heavily trampled units
(Table 4.5).
Trampling had a significant effect (F3,22 = 2.51, P = 0.085) on total litter cover.
Lightly trampled units had significantly more litter cover than non-trampled and heavily
trampled units but did not differ from moderately trampled plots (Table 4.3).
32
Species compositional shifts were evident with a few select species. Westem
wheatgrass (Agropyron smithii) proliferated in response to moderate levels of trampling
as it shifted from 13.1% of total community in 1999 to becoming the dominant species at
29.6% in spring of 2000. Trailing fleabane (Erigeron flagellaris) increased with
increasing trampling intensity and moderate levels of clipping. Heavy clipping and
trampling had a deleterious effect on Arizona three-awn (Aristida arizonica) as it
decreased in relative composition (-2.28%) compared to an increase (+7.96%) when no
treatments were applied. Goosefoot (Chenopodium spp.) disappeared from all
treatments. Golden aster (Chrysopsis villosa) decreased (-3.23%) in relative composition
at the highest levels of clipping/trampling. However, it appeared to be more sensitive to
trampling as decreases in its relative composition increased at greater trampling
intensities (Table 4.4).
Mean basal area/plant was significantly lower in 2000 than in 1999 (Fi,24= 3.65, P
= 0.068) as a probable result of abiotic factors, but no significant changes in mean basal
area/plant or species richness were detected for clipping, trampling, or their interaction.
MC Habitat. Clipping X trampling treatment combinations had a significant
effect (F6,22= 2.34, P = 0.048) on total foliar cover of grasses (Figure 4.3). There was an
increase in grass cover at moderate intensities of trampling coupled with moderate
intensities of clipping. For all other clipping intensities, grass cover decreased at
moderate levels of trampling. In addition, clipping had a significant effect (F2,44 = 2.52, P
= 0.092) on litter cover. Non-clipped plots had significantly more litter than heavily
clipped plots (Table 4.3).
33
Trampling had a significant effect (F3,43 = 2.25, P = 0.096) on the total number of
species. Lightly trampled plots had more species ( y = 10) than heavily trampled (y = 8,
P = 0.032) areas, but not than moderately trampled ( y = 9, P = 0.132) or non-trampled
( y = 10, P = 0.845) sites.
Shifts in species composition were pronounced. Wild strawberry decreased in
relative composition at intermediate levels of clipping and trampling, but increased at the
extremes. White clover increased (+4.8%) at the highest trampling intensity, but
decreased at all other levels. In addition, it increased 2.7% in non-clipped units, but
decreased about 2.5% when moderate or heavy clipping was applied. The relative
contribution of bluegrasses (Poa spp.) increased with increasing clipping intensity
(+1.0%, 1.2%, and 3.7% for none, moderate, and heavy clipping, respectively).
Moderate intensities of clipping and trampling increased the relative contribution of
sedges (Carex spp.). Pussytoes (Antennaria spp.) increased with increasing clipping
intensity (+1.0%, +1.2%, and +4.2% for none, moderate, and heavy clipping,
respectively). Heavy intensities of clipping increased the relative contribution of
fleabanes (Erigeron spp.){-^A.9%) and Westem yarrow (Achillea lanulosa) (+1.1%).
However, when combined with the heaviest intensity of trampling, Westem yarrow
increased 0.8% in relative composition compared to a 4.7% decrease when no clipping or
trampling was applied. For relative changes in dominant species, see Table 4.4.
Mean basal area/plant was significantly less in 2000 than in 1999 (Fi 48= 27.06,
P<0.001). No other significant effects for clipping, trampling, or their interaction were
detected for any other parameter.
34
Frequency Studv
Two experimental units in exclosure 'PG-3' that were not supposed to be clipped
were excluded from analyses because the wrong treatments were applied in 1999. Due to
the resulting unbalanced design, non-clipped plots were excluded across all habitats or
within the PG habitat during analysis.
Pre-treatment data had block X treatment interactions for density data in PJ (Fi,59
= 4.11, P = 0.047) and MC (Fi,59 = 44.44, P <0.001) habitats. Initial block X treatment
interactions were also detected for density in the PG habitat, but removal of exclosure
PG-5 that had been bumed in 1998 and exclosure PG-1 that had been bumed in 1999
remedied this effect for density estimates. Block X treatment interactions were also
discovered for MC (Fi,59 = 5.93, P = 0.018) and PG (Fi,2i = 6.10, P = 0.022) foliar cover
data. In addition, a clipping effect prior to any treatment application was detected in PJ
foliar cover data indicating randomly selected plots differed prior to any treatment
application. Therefore, analysis of covariance (ANCOVA) was used in subsequent
within-habitat analyses, with the exception of litter cover and basal area, to account for
initial differences among plots. Baseline density and foliar cover estimates were used as
covariates. With the exception of density data in the PJ habitat, all initial data were
normally distributed. Summary statistics for parameters by habitat are presented in
Appendix C.
Habitat Effects. Plant density differed among habitats (F2,io = 12.06, P = 0.002).
Mixed-conifer sites had more plantsW ( y = 402 plantsW) than PG ( y = 89 plants/m^)
or PJ ( y = 45 plant/m^) sites. Total foliar cover did not statistically differ (P > 0.1)
between habitats.
35
PJ Habitat. No significant changes in total density, foliar cover, mean basal
area/plant, or species richness were found for clipping, trampling, or their interaction at
any frequency. Mean basal area/plant was significantly lower in 2000 than in 1999 (Fi,64
= 5.60, P = 0.021), but this could not be attributed to any treatment combination. Litter
cover results were non-significant; however, data were non-normal and should be
interpreted with caution (Figure 4.4, Table 4.6). When subdivided into grasses and forbs,
no significant differences in densities were found (Table 4.7). Standing crop weights
were also non-significant (Table 4.8).
Changes in species composition were few. Blue grama decreased at the highest
frequency of trampling (-14.5%) compared to moderate (+0.2%), light (+2.7%), or no
(+0.1%) trampling. In conjunction with the decrease in blue grama, there were
substantial increases in purple three-awn (Aristida purpurea) across all treatments
regardless of frequency of treatment. Snakeweed was negatively affected by increased
frequency of clipping. Change in relative composition between years steadily increased
from -0.3% with no clipping to 4.0% at the heaviest frequency of clipping (Table 4.9).
PG Habitat. Total grass densities were affected by clipping (F2,2i = 2.63, P =
0.096). Units clipped 3 times had higher densities than those clipped twice. Units
clipped only once were not significantly different from those clipped 3 or 2 times (Table
4.10).
Clipping X trampling treatment combinations had a significant effect (F6,22 =
2.43, P = 0.059) on total litter cover (Figure 4.5). Litter cover was negatively correlated
with clipping frequency at moderate frequencies of trampling. In contrast, at high and
low frequencies of trampling litter cover was highest at moderate clipping frequencies
36
but declined at the extremes. When no trampling was applied, low fi-equencies of
clipping had more litter cover than moderate or high frequencies of clipping.
With the exception of a significant year effect (Fi,24 = 11.50, P = 0.002) for basal
area measurements, no other significant effects were detected. Mean basal area/plant was
significantly lower in 2000 than in 1999, but this was probably in response to abiotic
factors and could not be attributed to treatment applications.
The most notable change in species composition between 1999 and 2000 occurred
with the complete loss of goosefoot species across all treatments. Mountain muhly
increased (+9.5%) at the highest frequency of trampling as well as with the highest
clipping/trampling treatment combination (+13.5%). Sages (Artemisia spp.) showed the
greatest relative increases in composition between years at moderate frequencies of
clipping (+15.5%) or trampling (+14.6%), but tapered off at the extremes. Westem
wheatgrass showed the greatest increase in relative density at the highest frequency of
clipping (+5.8%) or trampling (+4.8%) with changes between years being comparative
among the other treatments. Finally, little bluestem (Schizachyrium scoparium)
decreased in non-trampled (-5.2%) sites or when only trampled once (-4.7%). At
moderate frequencies of trampling, composition of little bluestem did not change between
years (6.7% and 6.0%) in 1999 and 2000, respectively). However, at the highest
frequency of trampling, it increased in relative composition (+3.0%) in spring 2000 when
compared to spring 1999 (Table 4.9).
MC Habitat. No significant effects were detected for density, foliar cover, or
litter. However, total grass foliar cover was affected by clipping frequency (F3,59 = 2.83,
37
P - 0.046). Non-clipped areas had higher grass cover than heavily and lightly clipped
plots (Table 4.11).
Total species richness was affected by clipping X trampling treatment
combinations (F9,59 = 1.98, P = 0.057; Figure 4.6). At light and moderate frequencies of
trampling, species richness increased when coupled with moderate frequencies of
clipping, but decreased at light and heavy clipping frequencies. In contrast, no trampling
or heavy trampling frequencies showed the exact opposite effect with a decrease in
species richness at moderate clipping frequencies and an increase at light and heavy
levels. When no clipping was applied, treatments where no trampling was applied had
the highest number of species ( y = 10 species/0.25m^) followed by light ( y = 9
species/0.25m^), moderate ( y = 9 species/0.25m^), and heavy ( y = 8 species/0.25m^)
frequencies of trampling.
A significant clipping effect (F2,43 = 2.44, P = 0.099) was also found for total
Standing crop. Standing crop was lower for heavily clipped plots ( y = 7.93g/0.25m , P =
0.050) than lightly clipped ( y = 10.59g/0.25m ) areas. Moderately clipped sites had
higher productivity ( y = 10.30g/0.25m , P = 0.083) than heavily clipped areas, but they
were not different from lightly clipped areas (P = 0.828; Figure 4.7). The only other
significant effect detected indicated that mean basal area/plant was significantly lower in
2000 than in 1999 (Fi,64 = 52.18, P<0.001), but this could not be attributed to any
treatment and was probably the result of abiotic influences.
Increased frequency of clipping had deleterious effects on bluegrasses {Poa spp.).
They decreased from +21.5%, +2.0%, -2.5%, -2.3% for no, light, moderate, and heavy
clipping, respectively. The opposite occurred with trampling as they increased by +0.3%,
38
+1.5%, +9.7%), and +8.0% for no, light, moderate, and heavy fi-equencies of trampling,
respectively. In contrast, rushes (Juncus spp.) were negatively affected by the highest
trampling fi-equency. They decreased by -7.2% compared to -0.6%) and +0.5%) for
moderate and no trampling, respectively. Junegrass increased (+3.7%)) at the highest
frequency of clipping whereas white clover showed the greatest increase between years
(+4.7%) at light levels of trampling (Table 4.9).
Reference Area Analysis
Despite removing 2 exclosures that had been bumed during the course of the
study (i.e., PG-1 and PG-5), analyses of pre-treatment data revealed block X treatment
interactions (P = 0.034) for density data in the PG habitat type. Analysis of covariance
(ANCOVA) was used in all subsequent analyses, with the exception of litter cover and
basal area, to account for initial differences among plots. Pre-treatment density and foliar
cover data were used as covariates. All initial data for total density, foliar cover, litter,
and species richness were normally distributed.
Habitats were different from one another in terms of density (F2,io = 8.48, P =
0.007), litter cover (F2,io = 40.89, P<0.001), and richness (F2,io = 30.95, P<0.001).
However, no habitat X treatment interactions were significant for any parameter
indicating that treatments were having the same relative effect regardless of habitat. As a
result, treatment effects are reported by parameter and not individually by habitat.
Total densities among habitats were significantly different from one another (F2,io
= 8.48, P = 0.007), but no habitat X treatment interactions were detected (F2,io = 0.86, P
= 0.453). There were more plantsW in MC ( y = 429) than in PG ( y = 77) or PJ (y =
39
45) areas. No differences in total densities were distinguished (P = 0.564) inside versus
outside exclosures (Table 4.12).
Surprisingly, no significant habitat effect was detected for total foliar cover (F2,io
= 0.74, P = 0.500) indicating there were no statistical differences in total foliar cover
among habitats. This was likely due to fewer, large bunchgrasses in MC sites compared
to PG and PJ habitat types. In contrast, MC sites had more, small forbs and numerous
grass blades due to the presence of sod-forming grasses such as Poa species. Total foliar
cover was not significantly different inside versus outside exclosures (Table 4.13). In
contrast, litter cover did exhibit a significant habitat (F2,io = 40.89, P<0.001) effect with
MC sites having more ( y = 68.6%) litter than PG (y = 23.2%) or PJ (y = 7.1%) sites.
Differences in percent litter cover were not significant (Fi,io = 0.23, P = 0.640) inside
versus outside exclosures (Table 4.14).
Without surprise, a significant year effect was detected for basal area in MC (Fi.io
= 15.68, P = 0.004) and PJ (Fi,io = 6.01, P = 0.040) sites, but year did not interact
significantly with treatment in any habitat. Mean basal area/plant was not different inside
versus outside exclosures for any habitat.
A significant habitat effect was detected (F2,io = 30.95, P<0.001) for total species
richness indicating absolute numbers of species differed among habitats. However, no
habitat X treatment interaction was identified and plots inside exclosures did not differ
from reference areas in terms of total species numbers.
Relative species composition showed some dramatic results. In PJ habitat, blue
grama decreased by >30% between years outside exclosures compared to a >7% increase
inside exclosures. Some species [e.g.. galleta (Hilariajamesii), minute muhly
40
(Muhlenbergia minutissima), and sand dropseed (Sporobolus cryptandrus)] appeared in
reference areas in relatively high proportions after not being recorded there the previous
year. Similarly, Indian paintbrush (Castilleja Integra) was the only species that appeared
inside exclosures. The only noticeable changes in species composition in PG habitat
included the disappearance of goosefoot (-21.6%) and a relative increase (+5.4%) in
Arizona three-awn inside exclosures (neither species was recorded in large numbers in
either year outside exclosures). In MC sites, bluegrasses appeared to receive the greatest
benefit when protected fi-om grazers. It showed a relative increase of 15.5% between
years inside exclosures compared to only an 8.5% increase outside exclosures. In
contrast, sedges appeared to be inhibited when excluded from grazers. They showed a
6.0% decrease in relative composition inside exclosures versus a 0.5% increase outside
exclosures. Pussytoes and strawberries may also benefit from grazer exclusion.
41
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59
Table 4.12. Mean densities (# plantsW ± s.e.) inside versus outside exclosures (1999 2000) - Bandelier National Monument, New Mexico.
Exclosure*
MCI
MC2
MC3
MC4
MC5
PG2
PG3
PG4
PJl
PJ2
PJ3
PJ4
PJ5
Treatment
inside
outside
inside
outside
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outside
inside
outside
inside
outside
inside
outside
inside
outside
inside
outside
inside
outside
inside
outside
inside
outside
inside
outside
inside
outside
Mean Density (1999)
108 ±17
208 ± 14
466 ± 32.5
262 ± 8.5
310 ±50.5
294 ± 0.5
1092 ±141
838 ± 60.5
274 ± 0.5
334 ±32.5
114±65
38 ±0
155 ±25.5
156.5 ±11.5
115±10
68 ±11
38 ±12
29.5 ±5.5
51.5 ±0.5
17.5 ±5.5
50.5 ±11.5
17.5 ±0.5
92.5 ±41.5
86 ± 9
28.5 ±18.5
30.5 ± 4.5
Mean Density (2000)
136 ±13
376 ±14
466 ±21.5
348 ±17
272 ±1
242 ± 4.5
1090 ±127.5
634 ±11.5
452 ± 46
274 ±25.5
69.5 ± 35.5
66.5 ± 4.5
18±12
127.5 ±5.5
86.5 ±18.5
93 ±11
36.5 ± 15.5
40.5 ± 8.5
53.5 ±1.5
20 ± 2
76.5 ± 11.5
30.5 ±1.5
53 ±18
60.5 ±12.5
35.5 ± 13.5
42 ± 8
Because MC sites were sampled on a 0.25m2 area, densities are multiplied by a factor of 4.
60
Table 4.13. Mean fohar cover (%/m ± s.e.) inside versus outside exclosures (1999 -2000) - Bandelier National Monument, New Mexico.
Exclosure*
MCI
MC2
MC3
MC4
MC5
PG2
PG3
PG4
PJl
PJ2
PJ3
PJ4
PJ5
Treatment
inside
outside
inside
outside
inside
outside
inside
outside
inside
outside
inside
outside
inside
outside
inside
outside
inside
outside
inside
outside
inside
outside
inside
outside
inside
outside
Mean Foliar Cover (1999)
6.202 ± 0.35
3.02 ± 0.003
14.81 ±1.80
4.002 ± 0.55
2.804 ± 0.00
5.708 ±0.12
9.202 ± 0.95
14.612 ±0.75
4.716 ±0.13
8.916 ±0.13
8.155 ±1.35
8.695 ± 0.79
2.016 ±1.30
8.5295 ± 2.27
10.025 ±0.95
4.9605 ± 0.71
8.085 ±0.12
6.3 ±3.9
8.8 ± 0.00
7.975 ±4.88
9.575 ± 0.52
7.785 ±1.39
7.175 ± 1.66
4.475 ± 0.45
4.465 ± 3.32
5.63 ± 0.78
Mean Foliar Cover (2000)
8.018 ±0.40
5.726 ± 0.43
15.21 ±0.05
6.61 ± 0.50
6.712 ±0.48
6.01 ± 0.20
28.61 ±5.05
14.416 ±0.001
6.42 ± 0.75
8.118 ±0.52
24.378 ±13.68
24.93 ± 3.03
5.154 ±0.72
11.1335±1.13
21.680 ±3.57
8.8545 ±1.20
16.35 ±3.75
17.201 ±8.80
11.878 ±3.47
11.252 ±0.75
17.976 ±2.38
17.1755 ±3.22
9.177 ±0.98
9.0755 ± 0.77
7.0255 ± 2.92
9.577 ±0.77
Because MC sites were sampled on a 0.25m2 area, % cover is multiplied by a factor of 4.
61
Table 4.14. Mean litter cover (%W ± s.e.) inside versus outside exclosures (1999 2000) - Bandelier National Monument, New Mexico.
Exclosure Treatment Litter Cover (2000)
MCI inside 92 ±1.0
outside 88 ±1.0
MC2 inside 64 ± 6.0
outside 56 ± 8.0
MC3 inside 44 ±5.0
outside 91 ± 1.75
MC4 inside 72 ± 5.5
outside 24 ±1.0
MC5 inside 86 ± 3.0
outside 69 ± 4.75
PG2 inside 18.875 ±2.38
outside 28.625 ± 10.88
PG3 inside 38.75 ± 19.25
outside 25 ±1.5
PG4 inside 12.2 ±2.55
outside 15.5 ±3.5
PJl inside 11.8 ±5.9
outside 4.2 ± 0.7
PJ2 inside 11.75 ±7.25
outside 1.8505 ±0.55
PJ3 inside 8.5 ±1.3
outside 11.875 ±0.38
PJ4 inside 3.8255 ±1.92
outside 5.625 ± 1.63
PJ5 inside 7.4 ±0.55
outside 4.075 ± 0.78
Because MC sites were sampled on a 0.25m2 area, % litter is multiplied by a factor of 4.
62
CHAPTER V
DISCUSSION AND MANAGEMENT IMPLICATIONS
During the course of this study, BAND experienced two unseasonably warm
years. Plant communities are influenced as much, if not more, by abiotic variables (e.g.,
inter-annual differences in growing season precipitation) than by ungulate populations
(Peterson et al. 1997). Winkel and Roundy (1991) found seedling emergence in response
to cattle trampling differed among years and treatments relative to precipitation pattems
and periods of available water. Olson et al. (1985) indicated that each species reacts to
precipitation regimes and grazing pressure in a unique manner. Control of vegetation
dynamics by precipitation is much more likely in arid and semiarid regions (Petersen et
al. 1997). Grazing and concomitant trampling can directly push these regions across soil
erosion thresholds by concurrently reducing intercanopy vegetation cover and soil water
infiltration capacities (Davenport et al. 1998).
Though foliar cover and density data were analyzed across as well as within
habitats, the within habitat analyses are probably more meaningftil. Given the
migrational behavior of elk, elevational partitioning occurs in different seasons resuhing
in heavier use of certain habitats during some parts of the year. Impacts caused by
clipping and/or trampling may also be confounded when looking at individual plant
species or differing precipitation regimes (Olson et al. 1985, Cole 1995, Cole 1998) - all
of which vary from one elevational zone to another. This may be somewhat illustrated
by the significant habitat X trampling interaction we detected for total foliar cover.
Moderately trampled units in ponderosa habitats had 2.5 times more foliar cover than in
63
PJ, and nearly 4 times more than in MC sites. However, within habitat analysis in PG did
not reveal any trampling effects on foliar cover.
Plant densities were higher in heavily clipped plots in PJ habitat versus
moderately or non-clipped experimental units. It is possible that bunchgrasses in PJ sites
responded to heavy clipping pressure coupled with above average seasonal temperatures
and lower moisture with a breakup of the original plants which were then counted as
more individual plants the following year. Trampling applied to those plots exacerbated
this effect by pulverizing the dead or dying connective portions of bunchgrasses.
However, the dominant species in this habitat type was blue grama which is deemed to be
tolerant of grazing and trampling {Fire Effects Information System 1996).
Grass densities in PG habitat followed this same trend in response to clipping
frequency with heavily clipped sites having a higher density than moderately clipped
areas. Intensity of clipping did not have any detectable effect on overall plant densities in
PG habitat; however, clipping appeared to interact with trampling intensity in MC sites.
Moderately grazed and trampled units tended to have relatively higher grass cover than
any other treatment combination in these sites. A clear explanation for this is not
available, although other studies have advocated the use of grazing or trampling to
stimulate forage production (Savory and Parsons 1980, McNaughton et al. 1983).
Though not supported by results in other habitats, it is possible that moderate levels of
clipping and trampling complement each other providing optimum growing conditions
for grasses in MC habitats in this region.
One result that appeared to be consistent regardless of habitat was the effect of
clipping intensity on total litter cover. Significantly less litter cover was found in heavily
64
clipped plots in PJ and MC exclosures. A negative correlation of grazing intensity to
litter cover has been documented elsewhere (Biondini et al. 1998). This trend was also
apparent with clipping frequencies in PG, but not in PJ or MC habitats.
Measures of basal area did not change with this first year's treatments. Species
richness was only affected by treatments in MC sites where heavy intensities of trampling
or a combination of the extreme frequencies (i.e., 0 or 3 times) of clipping or trampling
coupled with moderate levels of the opposite treatment resulted in fewer species. Such
inconsistencies or lack of response can be explained in two ways. First, only a single
year has elapsed between initial treatment applications and the resultant data collection
reported here. Much longer time periods - periods in the range of 20 years - may be
needed to detect vegetative responses to changes in grazing pressure (Chong 1992)
especially in ecosystems that have developed with a history of grazing pressure. Short
duration studies are seldom sufficient to isolate the effects of treatment from short-term
climatic responses. Secondly, all vegetative parameters I measured had high intrinsic
variability. Though trends may be developing (as in the case of trampling), this
variability probably conceals most treatment effects. For instance, changes in basal cover
for individual plant species may be highly susceptible to precipitation (Olson et al. 1985)
indicating that dominant species and their response to precipitation fluctuation need to be
identified over the long-term/?nor to evaluating grazing/trampling effects.
At the lower elevations blue grama increased with a subsequent decrease in
mountain muhly. Blue grama is known to be tolerant of grazing (Santos and Trlica 1978,
Bock and Bock 1986) and may even increase in overgrazed range {Fire Effects
Information System 1996). In contrast, an Arizona study using long-term exclosures
65
indicated that mountain muhly showed the greatest increase in ungrazed quadrats,
occurred in "patches" when moderately grazed, and disappeared when overgrazed
(Arnold 1950). Similarly, in Colorado cover increased as grazing decreased (Johnson
1956) comprising 20% of the composition in ponderosa habitat on heavily grazed areas
compared to 45% on those not grazed. Results of our "intensity" study support these
pattems. However, the opposite effect was seen with regard to trampling frequency; blue
grama decreased with increasing trampling frequency whereas mountain muhly
increased.
Seasonal progression of environmental variables and phenological development
of individual plants may confound effects of defoliation frequency and intensity (Briske
and Richards 1995). Climate may confound interpretation since blue grama is more
commonly known to be drought-tolerant (U.S. Department of Agriculture 1940, Menke
and Trlica 1981, Fire Effects Information System 1996) - a condition which appears to be
prevalent the last couple of years in this region of the Jemez Mountains. Boryslawski and
Bentley (1985) reported a significant interaction between temperature regime and
clipping treatment with blue grama being less sensitive to clipping at higher temperatures
than Westem wheatgrass.
At mid-elevational ponderosa grassland sites, broom snakeweed and Westem
wheatgrass exhibited compositional shifts. Snakeweed, which was also found in PJ sites,
is an aggressive invader occupying sites where climax vegetation has depleted because of
fire, grazing, or drought (U.S. Department of Agriculture 1940), but is useless as forage.
Westem wheatgrass is also drought- and grazing-tolerant and is good winter forage for
elk and deer (U.S. Department of Agriculture 1940), but it is known to be less tolerant to
66
clipping at higher temperatures than blue grama (Boryslawski and Bentley 1985). Our
results do not support this observation since we observed an increase in composition of
wheatgrass at the highest frequencies of clipping and trampling and at moderate
intensities of trampling alone.
Other notable shifts in species composition in PG included a decrease in
goosefoot spp. numbers and an increase in trailing fleabane. Trailing fleabane is
relatively poor forage because of its ground-level growth form and is generally classified
as an increaser on grazed lands (U.S. Department of Agriculture 1940). The
disappearance of goosefoot spp. in our sites cannot be explained because of a lack of
information on responses to grazing and trampling in the literature. However, treatments
were applied at the earliest part of the growing season when these plants were seedlings
and were probably unable to tolerate such extreme conditions.
Arizona three-awn decreased in relative composition when heavy clipping or
trampling was applied, but increased on non-treated units and inside versus outside
exclosures. Little information exists in the literature regarding response to grazing
pressure, but it is generally thought to have poor forage value because of its prominent
awns (U.S. Department of Agriculture 1940).
In mixed conifer regions, noticeable shifts in relative composition occurred with
white-clover, bluegrasses, sedges, and pussytoes. White clover is considered excellent
forage for ungulates {Fire Effects Information System 1996) and withstands trampling
well (U.S. Department of Agriculture 1940), but it is not tolerant of drought conditions
(Gibson and Cope 1985). Our results support this observation with the greatest relative
increases in white clover occurring at the heaviest intensity of trampling and at low
67
frequencies of trampling. Though not evident with this study, plants can adapt to severe
defoliation by developing smaller leaves and more stolons (Ryle et al. 1989) and it has
been reported that this plant may grow stronger and bulkier (i.e., become more robust)
when grazed (U.S. Department of Agriculture 1940, Chapman et al. 1992, Hay et al.
1989). In addition, white clover is a species with a high degree of phenotypic plasticity
which can result in large fluctuations in size of individual plants (Hay et al. 1989).
Individual species of Carex, Poa, and Antennaria respond to grazing pressures
and climate differently (U.S. Department of Agriculture 1940) making it difficult to
interpret our results which are based on generic classifications. In general, Poa spp. are
known to be good to excellent forage and resistant to heavy trampling, grazing, and
drought conditions (U.S. Department of Agriculture 1940, Stubbendieck et al. 1985)
because growing points are belowground throughout the growing season for most species
(Ehrenreich et al. 1963). Poa spp. increased with increasing clipping intensity in our
study, but decreased as clipping frequency increased. Contradictorily, this genera also
increased when grazers were excluded through the use of exclosures. Intermediate
intensities of clipping and trampling increased the relative contribution oi Carex in our
study and it did not appear to benefit when protected from grazers. Antennaria spp.
increased with clipping intensity, but also appeared to increase when protected from
grazing (i.e., inside exclosures). One species, A. parvifolia, has been shown to decrease
slightly under light to moderate grazing, but increase in heavily grazed areas (Smith
1967). In ponderosa communities, it has been shown to survive heavy trampling (Smith
1967). Again, conflicts in our results are probably indicative of climatic and phenotypic
variability during the course of this study. In addition, the unseasonably warm weather
68
resulted in elk remaining at higher elevations. Therefore, the unfenced reference areas
did not receive the level of impact they normally would if elk were in the area. This
made interpretation of results inside versus outside exclosures difficult.
Trampling, in general, more consistently impacted parameters measured though
results were not statistically significant in all cases. In fact, in many of the ANOVAs and
ANCOVAs we ran, trampling effects showed the most significant response (i.e.,
approached a = 0.10) when compared to either clipping or trampling X clipping effects.
Specific responses to trampling were difficult to interpret. In PJ communities,
moderately trampled plots had less total foliar cover than lightly trampled units in our
"intensity" study. Further inspection indicated that lightly trampled treatments had
significantly higher forb cover in this community. In contrast, moderately trampled units
had a higher density of plants than non-trampled areas in PG, but densities were not
different from lightly or heavily trampled sites. Finally, MC sites had significantly more
species in lightly trampled locations compared to heavily treated areas, but not when
compared to moderately or non-trampled quadrats. In total, these observations suggest
that there may be an intermediate threshold at which trampling may stimulate plant
productivity, especially in terms of forb response.
Similar responses to trampling have been reported in the literature. Cole and
Spildie (1998) concluded forb-dominated sites were highly vulnerable to trampling
effects, but recovered rapidly. The ability of a vegetation type to tolerate recurrent
trampling disturbance may be more a function to recover from damage than its ability to
resist being damaged (Cole 1995). Cole (1995) ftirther stated that certain vegetation
types might exhibit thresholds of vulnerability in response to trampling that, when
69
exceeded, may result in even greater damage to the plant. Guthery and Bingham (1996)
developed a theoretical basis to predict the probability of domestic trampling loss,
influence of underlying assumptions, and development of altemative models for dealing
with nonrandom grazing which may be enlightening to the land manager.
There are three main explanations for the variable and inconsistent results we
found in this study. First, there may be a possibility that heavy winter and variable spring
grazing, with associated trampling, may not cause extensive plant mortality or
progressive changes in basal area (Houston 1982), especially during dormant phases of
plant life cycles. As was previously mentioned, much longer time periods - periods in
the range of 20 years - may be needed to detect vegetative responses to changes in
grazing pressure (Chong 1992) especially in ecosystems that have developed with a
history of grazing pressure. Finally, initial variability of pre-treatment data was
indicative of the amount of inherent variability present in the vegetative communities of
BAND. Standard error values for all parameters (see appendices) vacillated to the extent
that potential significant effects could be concealed by variability. These factors,
coupled with climatic variables, make interpretation of results volatile with only a single
year of post-treatment data.
70
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77
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