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Winterfat Shrubland Boundary Dynamics
Under Different Grazing Histories
A.L.Hild
D. B. Wester
our hypotheses that winterfat plants in interior and peripheral community positions would respond differently to defoliation, and that these differences may also be altered by
grazing history. Research sites were on lands owned by t~e
National Rifle Association Whittington Center located In
Colfax County, approximately 10 km south of Raton, New
Mexico.
Three visually-recognizable winterfat communities were
studied. Each ofthese communities is bisected by fence lines
constructed prior to 1945. The Whittington side of this fence
line has been protected from domestic grazing since 1973.
The other side has been seasonally grazed by cattle from
May to October until and during the time of the study. Both
of these locations receive wildlife use, primarily by mule
deer, elk, bear, and many small mammals.
Within interior and peripheral positions of each of three
communities, six 4 x 2.5 m plots were fenced to exclude
ungulate herbivory. "Interior" locations within each community were defined and visibly recognizable as winterfatdominated areas that were surrounded by similarly dominated areas and were not adjacent to "peripheral" locations
(fig. 1). "Peripheral" locations were positioned on the boundary ofa winterfat community and adjacent vegetation dominated by grassland. Peripheral plots were aligned so that the
longest axis of the rectangular plot was perpendicular to the
community boundary. Study plots cover an 11-m 2 area and
were located in peripheral and interior locations.
Plots were randomly assigned to one of three defoliation
treatments. Plants in control plots were not clipped (treatment C = control). A second defoliation treatment involved
clipping all grass, forbs, and shrubs (treatment A =all plants
defoliated except yucca Yuccaglauca, cacti, Opuntia sp. and
Echinocereus sp., and broom snakeweed Gutierrezia
Ahstract-This study examined boundary areas between winterfat
(Ceratoides lanata) shrublands and adjacent blue gram a (Bouteloua
gracilis) grasslands in northern New Mexico. Vegetation response
to defoliation was compared for interior and peripheral winterfat
shrubland community positions on two locations having different
grazing histories. Whittington locations were removed from domestic grazing in 1973, while adjacent lands sustained season long
grazing. Seedlings and mature plants of winterfat and forbs were
affected by community position, depending on grazing history,
while basal cover of grasses was not. Defoliation may diminish the
importance of community positions. Winterfat recruitment was
affected by defoliation and community position.
Landscape boundaries have recently received much attention in ecology (Correll 1991; Holland and others 1991;
Risser 1993; Turner and others 1991; Wiens and others
1985). Boundaries are recognized by both composition and
structure of vegetation (van der Maarel 1976). Transition
zones, where one plant community begins to grade into
another, may be useful as indicators of vegetative change
(Holland and others 1991). These ideas are not new to
ecology. Many years ago Da ubenmire (1947) recognized that
plant distributions may reveal the limits to growth. As an
extension of these ideas, we suggest that plants in boundary
positions may be more sensitive to changes in biotic and/or
abiotic factors when compared to individuals of the same
species positioned interior to a community. Additionally,
given that winterfat (Ceratoides lanata) is a valued forage on
western range and the distribution of these shrublands is
known to be limited under grazing (Stevens and others
1977), we selected the boundary between winterfat
shrublands and adjacent blue grama (Bouteloua gracilis)
grasslands to study the importance of community position
for winterfat plants that may be constrained by past grazing
history.
Adjacent
Methods and Materials
Study Site and Plot Layout
Winterfat-dominated communities and surrounding grass
communities, dominated by blue grama, were used to test
In: Barrow, Jerry R.; McArthur, E. Durant; Sosebee, Ronald E.; Tausch,
Robin J., comps. 1996. Proceedings: shrubland ecosystem dynamics in a
changing environment; 1995 May 23-25; Las Cruces, NM. Gen. Tech. R:ep.
INT-GTR-338. Ogden, UT: U.S. Department of Agriculture, Forest SeI'Vlce,
Intermountain Research Station.
A. L. Hild is Rangeland Scientist, Fort Keogh Livestock and Range
Research Laboratory, Miles City, MT 59301. D. B. Wester is Associate
Professor, Department of Range and Wildlife Management, Texas Tech
University, Lubbock, TX 79409. This is contribution T-9-740, College of
Agricultural Sciences and Natural Resources, Texas Tech University, Lubbock.
Figure 1-Community locations (Whittington and
adjacent) and positions (interior and peripheral).
Rectangles represent 11 sq. m areas.
51
sarothrae). Grasses and forbs were clipped in June 1992,
August 1992, June 1993, and June 1994, to a 2.5-cm stubble
height. At the same time, shrubs were clipped to remove one
half of all externally accessible new crown growth above 2.5
cm in height. A third defoliation treatment defoliated shrubs
in the same manner as in treatment A, except that herbaceous plants were not clipped, and defoliation of browse
plants was conducted in June 1992, September 1992, September 1993 and September 1994 (treatment B = browse).
This treatment was initiated in June 1992 to begin the study
but was defoliated in the fall 1992, 1993, and 1994.
conclusion ofthe study. Seedlings were identified and marked
so that on subsequent measuring dates the species of dead
seedlings was known.
Experimental Design and Data Analysis
The experimental design for the mature plant study was
a split split-plot arrangement of a randomized block design,
with three winterfat communities serving as blocks. Main
plots represented locations that differed in grazing history.
Subplots were interior or peripheral positions within a
community. Sub-sub plots were the three defoliation treatments assigned to two plots at each location and position in
each community. Sampling date was included as a repeated
measure. Biomass, cover, winterfat dimensions, and seedling data were subjected to an analysis of variance appropriate to the experimental design.
Because of the numerous species recorded in some of the
data collected, analysis was at times completed on groupings
of species such as all grasses, forbs, or shrubs. All data were
tested for conformance to assumptions of normality (Shapiro
and Wilk 1965) and sphericity (Mauchley 1940). When
violations of sphericity assumptions occurred, adjustment to
F -test degrees offreedom was completed using GreenhouseGeisser estimates of the degree to which sphericity was
violated by a particular data set (Geisser and Greenhouse
1958). When three-way and higher order interactions occurred, F -tests of interacting treatment factors within a
level of the third interacting factor were completed using
error terms specific to the test. Additionally, mean separation within interactions was accomplished with Least Significant Differences calculated using error terms specific to
the contrast. This approach to higher level interactions is
based on recommendations by Milliken and Johnson (1984).
Mature Plant Records
Basal cover of vegetation by species, bare ground, litter,
and intact root crowns were recorded from permanent line
transects in each plot. The plot width was traversed by four
permanent line transects across the width of the plot. Each
line was 2.5 m long resulting in a total of10 m ofline transect
per plot. Transects were read as continuous line transects to
a 0.5-cm resolution to create a linear map of basal cover.
Cover data were recorded twice between May and September for three consecutive summers. Clipping treatments
were not applied until after initial basal cover and winterfat
crowns were recorded. Vegetative biomass from clipping
treatments was removed from plots by species, dried and
weighed. Total crown growth of winterfat individuals within
plots was recorded for each plant. All individual winterfat
plants found in the plots were permanently marked and
measured for basal circumference, height, and two crown
diameters. Winterfat plants were monitored for crown volume in June 1992, 1993, and 1994. The 1992 data were
collected prior to application of defoliation treatments.
Canopy volume for winterfat shrubs was calculated as the
volume included under half of a spheroid, to represent the
natural shape of the canopy comparable to Ludwig and
others (1975) on broom snakeweed (Gutierrezia sarothrae).
In this computation, canopy volume was =4/3 1t r2 h; where
h is plant height and r is the average radius. The average
radius for these data was obtained by adding two measures
of the canopy diameter (the maximum diameter and the
diameter perpendicular to the maximum) and dividing their
sum by four. By numbering winterfat plants, we also recorded winterfat density.
Results and Discussion
Mature Plants
Winterfat Density-Density of mature winterfat individuals was not different between Whittington and adjacent
locations or defoliation treatments. Density of mature
winterfat plants was greater in interior than in peripheral
positions (2.01 and 0.45 plants per m 2, respectively). Additionally, the mean density ofwinterfat plants decreased over
the three years of the study from mean of 1.26 plants per m 2
in 1992 to 1.20 plants per m 2 in 1994. Winterfat density on
Whittington plots was slightly greater than that of adjacent
plots, with 1.58 and 0.89 plants per m 2 respectively.
Seedling Records
A seedling study was conducted to assess establishment of
species under each treatment in comparable locations and
positions. Adjacent to each end of the mature plant study
plots, two seedling areas 0.2 m x 2.5 m (0.5 m 2 area) were
marked resulting in a total of 144, 0.5 m 2 seedling areas.
Within the seedling areas, seedlings were marked by species
as they emerged, and their progress was followed for the
duration of the study or until the seedling's death. Seedling
plots were clipped identically to the mature plant plot to
which they were attached. Seedling emergence and survival
were recorded for three years, at least twice between May
and September, of each growing season. We recorded total
number of seedlings by species that emerged, and the survival of those seedlings on each measuring date until the
Winterfat Canopy Volume-Winterfat canopy volume
per plant differed between defoliation treatments and between community positions, and these differences depended
on sampling date. At the beginning of the study, canopy
volume of winterfat plants was similar in all three defoliation treatments (fig. 2). Canopy volume did not change in
control or browse-only defoliated plots. However, canopy
volume did decrease when both herbaceous and browse
plants were defoliated, and this effect was apparent by the
second year of the study. By 1994, winterfat canopy volume
was lower in defoliated plots, (regardless of the type of
defoliation) than in control plots.
52
--I.~==_A_Ll_ _ _-_- -_O_- -_BR_O_WS_E_ _ _~-_--_--_-~~~~~~-J
:r~~~
aA
•
+aA
~ 40 f.?=====-""""'---IaB
~3OsA
grass basal cover. Galleta grass (Hilariajamesii), western
wheatgrass (Pascopyron smithii), and buffalo grass (Buchloe
dactyloides) each totaled 0.1% of the total basal cover and
1.1%,0.9%, and 0.6 % of the total grass cover, respectively .
Other grasses found on line transects were in trace amounts
of less than one percent of total grass cover. In order of
decreasing basal cover, these incl uded squirrel tail (Sitanion
hystrix), three-awns CAristida sp.), ringmuhly (Muhlenbergia
torreyi), Foxtail barley (Hordeum jubatum), vine mesquite
(Panicum obtusum), wolf tail (Lycurus phleoides), alkali
sacaton (Sporobolus airoides), and sideoats grama (Bouteloua
curtipendula). Because species other than blue gram a and
its root crown contributed so little to basal cover of grasses,
results presented here are basal cover of all grass species
together.
Basal cover results revealed no effects of grazing history
or position on grasses. Basal cover of grasses increased on
plots under defoliation treatment A by 1993 (fig. 4). Although basal cover of all grasses was not different between
aB
~
0
bB
_ _ -----.
'0
g20t
bB
I
1:~1_________~_________
1993
1992
1994
Year
Figure 2-Winterfat canopy volume by defoliation and
year. Means within a defoliation treatment with the same
lower case letter do not differ. Means within a year with
the same upper case letters do not differ (p>O.05, LSD).
Despite the foregoing defoliation effects, defoliation treatments did not interact with grazing history or community
position effects. Thus, regardless of defoliation treatment
effects, canopy volume of winterfat plants was greater in
interior than in peripheral positions at the beginning of the
study. However, while canopy volume remained stable in
peripheral positions, canopy volume of winterfat plants on
the interior positions decreased between 1992 and 1994
(fig. 3).
a.
200
175
150
125
~ 100
dA
75
50
Winterfat Basal Area-Winterfat basal area differed
between grazing histories and between years. Basal area
increased between 1992 and 1993. Also, basal area of
winterfat plants at Whittington locations was less than
basal area of plants at adjacent locations. There were no
effects of defoliation treatment or position on winterfat
basal area.
25
sA
abA
abA
bA
sA
0
Jun-92
dB
Jun-93
Aug-93
175
150
125
~ 100
aA
sA
75
60.
50
c:
40
~30r"L__-
Jun-93
Aug-93
bA
150
--------.
125
-----0------- ---0
aA
cdA
aB
Aug-94
abA
:::===----:bA
aA
cA
bA
bB
-~
bcA
cB
75
aA
Jun-94
aA
~ 100
'0
u20
Aug-92
175
t--------~
.!!l
:i
beB
C.
200
aA
beB
a~~
0
Jun-92
- - { ] - - Periphery
aA
sA
----~
abB
50
----------------,j
Interior
Aug-94
200
25
•
Jun-94
b.
Basal Cover of Grasses-Basal cover of all grasses
averaged across all dates and treatments was 16.1% of the
total ground cover. Within total grass cover, the most prevalent species was blue grama which accounted for 96.8% of
E
beB
I
Aug-92
dB
50
25
10
0
Jun-92
---~+-------------I
Aug-92
Jun-93
Aug-93
Jun-94
o~----------_+------
1992
1993
1994
Figure 4-Basal cover of (a) grasses, (b) bare ground, and
Year
(c) litter by defoliation and date. Means within a defoliation
treatment with the same lower case letters do not differ.
Means within a date with the same upper case letters do not
differ (P>O.05, LSD).
Figure 3-Winterfat canopy volume by position and
year. Means within a position with the same lower
case letter do not differ. Means within a year with the
same upper case letter do not differ (P>O.05, LSD).
53
Aug-94
browse-defoliated and control plots, these two defoliation
treatments differed from treatment A defoliation plots in
1993 and this difference was maintained throughout the
remainder of the study.
Perennial Forb Seedling Emergence-Emergence of
perennial forbs was affected by grazing history and depended on both position and date. Analysis of these data was
also completed on ranks. At interior positions, emergence of
perennial forbs differed between grazing history locations
on June 1992 and June 1994 (fig. 6a and 6b). On peripheral
positions, emergence was different for the two grazing histories in August 1992 and June 1994. For these dates, on both
the interior and periphery positions, there was greater
perennial forb emergence at adjacent locations than at
Whittington locations. Although an apparent reversal of
differences between grazing histories takes place on the
periphery in June 1993, grazing history mean perennial forb
emergence was not different. In both locations and positions,
August dates had lower emergence of perennial forbs than
did June dates. Defoliation treatments did not differ in
emergence of perennial forbs, nor did defoliation interact
with any other treatment.
Bare Ground and Litter Cover-By June 1993, bare
ground was also greater in plots with defoliation of both
browse and herbaceous plants than in plots with defoliation
of browse only or control plots, and this difference was
maintained throughout the remainder of the study (fig. 4).
Bare ground was not significantly affected by location or
position treatments.
Litter cover showed trends opposite to those shown by
grasses or bare ground (fig. 4) by decreasing immediately
following application of defoliation treatments. However,
both browse-defoliated plots and control plots recovered
litter cover in June 1993, while litter cover continued to
decrease in plots where both browse and herbaceous vegetation was clipped. The lower cover of litter on the treatment
A plots persisted for the remainder of the study.
Winterfat Seedling Emergence-On the initial sampling date (June 1992), winterfat seedling emergence occurred only at Whittington locations (fig. 7); most of these
Seedling Emergence
Annual Forb Seedling Emergence-Emergence of
annual forbs was affected by grazing history and date.
Analysis of these data was completed on ranked data; mean
separation was completed on ranked means, and means
presented in figures are means from the original data. For
both grazing histories, emergence of annuals differed significantly between June and August sampling dates for each
year (fig. 5). There was greater emergence of annual forbs at
grazed adjacent land locations than on locations recently
removed from grazing (Whittington) in August 1992 and in
both June and August 1994. In all summers, early emergence
of annual forbs prior to June was greater than emergence
from June to August. In general, there was greater emergence
of annual forbs at adjacent locations than at Whittington
locations. Mean emergence of annual forbs was 6.0 and 4.11 m 2
for adjacent and Whittington plots, respectively.
a.
c=~_~-n~di I~~~_ ~~ Wh~ Inter I
aA
9
8
aA
7
6
3
as
cA
bA
0
Jun-92
----------
[- - - - - - Adjacent
~--------
-------~-~
---[}----
Jun-94
18
16
12
14
~
10
aA
\
\
as
""
~
6
6
cA
Jun-93
Aug-93
Jun-94
"~~
~
0 -j-----------'1F'-------··_-+-------"P'----+1- - -
'0
cS ---t-------t~------+ ____ ~ ___ _
\
\A
4
2
2
\
12
0- 10
~
8
8
Aug-92
Aug-94
aA
20
aA
14
o
Aug-93
b.
--
------.-----~--~--~
Jun-92
Jun-93
Whlttingt;~l
16
g-
Aug-92
Jun-92
bS
bA'
Aug-92
Jun-93
Aug-93
Jun-94
Aug-94
Figure 6-Perennial forb seedling emergence on
(a) interior and (b) peripheral positions by location
and date. Means within a location and position with
the same lower case letters do not differ. Means
within a date and position with the same upper case
letters do not differ (p>O.05. LSD).
Figure 5-Annual forb seedling emergence by
location and date. Means within a location with the
same lower case letters do not differ. Means within
a date with the same upper case letters do not differ
(P>O.05, LSD).
54
bA
Aug-94
---1.1--- Adjacent
----0---
VVh~
---1.1--- Adjacent
aA
3
----Q---
VVhittington
I
2.5
aA
2.5
2
aA
2
aA
2
i
~ 1.5
0-
J!!
-
1.5
-
1
abB
0.5
0.5
0+---~~+-------+-------~----~~------4
bB
Jun-92
o.-----~~~----~------~------~----~
Jun-92
Aug-92
Jun-93
Aug-93
Jun-94
Aug-92
Jun-93
Aug-93
Jun-94
Aug-94
Aug-94
Figure 8--Emergence of other shrub seedlings by
location and date. Means within a location with the
same lower case letters do not differ. Means within a
date with the same upper case letters do not differ
Figure 7-Winterfat seedling emergence by location
and date. Means within a location with the same lower
case letters do not differ. Means within a date with the
same upper case letters do not differ (p>O.05, LSD).
(p>O.05, LSD).
Seedling survivors were divided into three groups: perennial forbs, winterfat, and shrubs other than winterfat. These
three groups were analyzed for their respective contributions to the total number of perennial survivors. (For example, the contribution of other shrubs to perennial survivors equals the number of other shrub survivors/number of
total perennial survivors.)
seedlings emerged in interior positions. Winterfat seedling
emergence was similar between locations on all other sampling dates. At Whittington locations, more seedlings emerged
in June 1992 and June 1994 than on any other sampling
date; at adjacent locations, emergence of winterfat seedlings
was higher in June 1994 than at any other sampling date
(fig. 7).
Location and position interacted in their effects on winterfat
seedling emergence. At Whittington locations, emergence
was greater in interior positions than in peripheral positions; emergence at these two positions did not differ at
adjacent locations. Additionally, emergence was similar
between locations at interior and peripheral positions. Defoliation treatments also affected winterfat seedling emergence. Winterfat seedling emergence was greater in control
plots than in defoliated plots.
Perennial Forb Contribution to Seedling Survivors-Perennial forbs were more important in peripheral positions
than in interior positions, irrespective oflocation or defoliation treatments. Mean contribution of perennial forbs to
total numbers of seedling survivors was 44.8% on the interior and 84.7% on peripheral positions (fig. 9).
Contribution of perennial forbs to seedling survivors at
adjacent locations did not differ between defoliation treatments (fig. 10). At Whittington locations, there was greater
proportion offorbs seedling survivors in plots under browseonly defoliation than in controls or plots under defoliation of
both browse and herbaceous plants. For browse-defoliated
plots, grazing histories did not differ in the contribution
made by perennial forbs. However, contribution of perennial
Other Shrub Seedling Emergence-Shrubs other than
winterfat emerged differently between the two grazing histories (fig. 8). Analysis of ranked data indicated that emergence of these species was greater on adjacent than at
Whittington locations only on the initial sampling date.
Greatest shrub emergence at adjacent locations occurred in
June of the first two field seasons and shrub emergence
tended to be lower on dates later in the summers.
100%
Seedling Survival
~
0
80%
.~
~
Winterfat Seedling Survival-The total number of
winterfat seedlings that survived differed between defoliation treatments. Survival of winterfat seedlings was greater
in controls than in either of the defoliated treatments.
:::l
Ul
!illI % Other shrubs
60%
(ij
·cc:
40%
'0
#
20%
~
Composition of Seedling Survivor Totals-There
were significantly more total seedling survivors on peripheral positions than on interior positions at the end of the
study. Mean numbers of survivors was 7.2/m2 and 15.9/m2
for interior and periphery, respectively. There were no other
treatment impacts on total number of seedling survivors.
0% Winterfat
e!
I •
0"/0
Interior
Periphery
Figure 9-Composition of seedling survivors by
position. Position means within each vegetation
type differ (p<O.05, LSD).
55
O/OPerennial forb I
90
aA
bA
Importance of Position
80
70
Differences in canopy volume of mature winterfat, emergence and survival of winterfat seedlings, and survival of
shrub and forb seedlings between interior and peripheral
community positions have been documented in this study.
Winterfat seedling emergence at Whittington locations was
primarily on interior positions. Canopy volume of interior
winterfat decreased from 1992 to 1994 while peripheral
canopy volume did not. Consequently, although initial
winterfat canopy volume was greater on interior positions,
it did not differ from peripheral positions in 1993 and 1994.
Growth of winterfat canopies on the periphery is more stable
over time and we suggest that this position effect may
express the presence of different competitive interactions in
the two positions. Winterfat and shrub seedling survival
was a greater portion of all survivors on interior positions
while forbs constituted a larger portion of survivors on
peripheral positions. These effects are independent oflocation and defoliation treatments.
560,
.~
I
:::ISO
I/)
'iij40
'0
~
30
20
10
o
ALL
BROWSE
defoliation
CONTROL
Figure 1O-Contribution of forbs to seedling survivors by
location and defoliation. Means within a location with the
same lower case letter do not differ. Means within a
defoliation treatment with the same upper case letter do
not differ (p>O.05, LSD).
forbs to seedlings survivors was greater at adjacent locations than at Whittington locations in plots under defoliation of both browse and herbaceous plants and control plots.
Importance of Grazing History
Winterfat Contribution to Seedling Survivors-Control plots had greater proportions of winterfat seedlings
than did browse-defoliated plots. Winterfat survivors in
plots with defoliation of both browse and herbaceous plants
were intermediate to and not different from either controls
or browse-defoliated plots. Additionally, winterfat survivors
contributed more to survivorship on interior positions than
on peripheral ones (mean contributions were 34.8% on
interior positions and 7.3% on peripheral positions). This
trend is reversed for perennial forb portions of seedling
survivors (fig. 9).
Grazing history treatments affected seedlings and
winterfat basal cover, while ground cover, basal cover of
grasses, and mature winterfat canopy volume did not reflect
differences in grazing history. Basal cover ofmature winterfat
plants was greater at adjacent locations than at locations
with twenty-year removal of cattle. Additionally, on dates
when the two locations differed, emergence of annual and
perennial forbs and shrubs other than winterfat was greater
at adjacent locations, while emergence of winterfat seedlings was greater at Whittington locations. Grazing history
had little impact on seedling survival once seedlings had
emerged (see interaction with defoliation effects above).
Contribution of Shrubs (exclusive of Winterfat) to
Seedling Survivors-Shrubs (exclusive of winterfat) also
made greater contributions to total number of seedling survivors on interior positions than on peripheral ones (fig. 9).
Mean shrub contribution was 20.3% of the total perennial
survivors on interior positions and 7.9% on the periphery.
Conclusions
------------------------------
Many of the results of this study were not surprising. Prior
studies have documented increase in basal cover of grasses
under clipping as well as decreased canopy volume of shrubs
with crown defoliation. Additionally, a history of past defoliation is known to increase emergence of forbs and nonpalatable shrubs.
Winterfat seedling results (initial emergence of winterfat
only at Whittington locations) may reflect past grazing
history. Increased litter and shade and decreased bare
ground have been identified as important to winterfat emergence and survival (Woodmansee and others, 1971). Additionally, smaller basal size ofwinterfat plants at Whittington
locations may indicate renewed winterfat regeneration within
the past 20 years of grazing removal.
One interesting result is the apparently greater stability
of peripheral winterfat canopies when compared to decreasing canopies on interior positions. Although it is not surprising that canopies decreased (2/3 of the plots in each position
were subjected to defoliation), it is interesting that peripheral plants were able to recover their canopy volume before
Summary
Importance of Defoliation
Defoliation of both browse and herbaceous plants (defoliation treatment A) decreased mature winterfat canopy volume, litter ground cover, and winterfat seedling emergence,
and increased grass basal cover and bare ground, relative to
controls. Browse-only defoliation had similar effects on
winterfat canopy and seedling emergence but did not effect
basal cover of grasses, bare ground and litter. These effects
were independent of grazing history and position. In addition, browse-only defoliation at Whittington locations had
greater portions of forbs within seedling survivors than did
control plots or plots under defoliation of both browse and
herbaceous plants.
56
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the next sampling date, while interior plants did not. This
suggests a very real impact of community position on mature
winterfat crown growth and this difference may partially
express competitive interactions.
An additional impact of position is apparent on the increased survival of shrub and winterfat seedlings on interior
positions. Interior positions are more favorable for survival
of shrubs than are periphery positions. Perennial forbs,
however, differed between positions and were more numerous on peripheries.
Finally it is note-worthy that differences in grazing history and in community position were not documented on
basal cover from line transects. Line transects documented
change in grass basal cover due to defoliation treatments.
Grass basal cover was not greatly changed by a twenty year
removal of grazing or by shifts from periphery to interior
positions within winterfat communities. This suggests that
either the effects of grazing history and community position
are not important to grass basal cover or that these impacts
are not expressed in grass bases. Either statement has great
ramifications for range management.
Many studies use exclosures to document the impacts of
grazing and its removal on grasslands. However, problems
with their use have also been noted. Painter and others
(1989) have found that grazing history can cause intraspecific populations to respond differently to defoliation and
competitive interactions. In our study, grazing history differences were seen in mature shrubs and in seedling dynamics. These data show that a short (20 year) removal of grazing
can alter populations of many species, while the same
removal may go unnoticed in grass basal cover. We suggest
that comparisons of protected and grazed ranges be monitored via many different parameters.
Acknowledgments _ _ _ _ _ __
This project was partially supported by National Rifle
Association Grants-In-Aid Contracts GIA #93-09 and GIA
#94-11. Additionally, we thank the N.R.A. Whittington
Center staff for providing study sites and other assistance.
We extend our deepest appreciation to B. Matthews and Z.
Salmon for their field expertise and endurance, to M. Benton,
K. Launchbaugh, and E. B. Fish for manuscript review and
creative insights, and to numerous volunteers who contributed their valuable time to this project. Without their help,
this study would not have been possible. Finally, we claim all
responsibility for any inaccuracies or omissions contained
within this paper.
References
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Correll, D. L. 1991. Human impact on the functioning oflandscape
boundaries. In: Holland, M. M.; P. G. Risser; R. J. Naiman. 1991.
Ecotones: The Role of Landscape Boundaries in the Management
and Restoration of Changing Environments. Chairman and Hall.
New York. 142 pp.
57