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Journal of Arid Environments (1996) 34: 101–114
Species interactions on the biome transition zone in
New Mexico: response of blue grama (Bouteloua
gracilis) and black grama (Bouteloua eripoda) to fire
and herbivory
Rusty J. Gosz & James R. Gosz
Biology Department, University of New Mexico, Albuquerque, New
Mexico 87131-1091, U.S.A.
(Received 16 May 1995, accepted 6 June 1995)
The desert/grassland biome transition zone in central New Mexico provides
an important region for testing species differences to changing environmental
conditions and various land management practices. Interactions of black
grama (Bouteloua eripoda) and blue grama (Bouteloua gracilis) significantly
affect the resultant plant community and its influence on system structure
and function. Black grama demonstrated higher productivity, especially after
wet years, and this species has increased its dominance during the 20-year
period since livestock grazing was removed. While black grama can alter the
previous pattern of overgrazing and desertification in this transition zone, our
experiments demonstrated that it was more strongly inhibited by treatments
of fire, simulated grazing and native herbivore activity than blue grama. Thus,
new environmental conditions of wet or drought, changed fire regimes caused
by increased grass fuel and altered patterns of herbivore activity have the
potential to reverse the current trends. Many interacting factors must be
considered in predicting future community conditions or developing management practices that avoid grassland degradation in this semi-arid region.
©1996 Academic Press Limited
Keywords: Bouteloua eripoda; Bouteloua gracilis; fire; grazing; biome
transition zone; New Mexico; Sevilleta; Long-Term Ecological Research
Introduction
Species interactions are influenced by many factors in natural and managed habitats
with fire and herbivory being two of the dominant forces in the south-western United
States (Buffington & Herbel, 1965; Hulbert, 1969). Fires normally require hot, dry
weather conditions with a sufficient source of fuel. For natural fires, these conditions
can occur during periods associated with dry lightning storms in the early summer.
The dead grass biomass produced during the previous summer/fall may be sufficient
for rapid and extensive burns. Prescribed burning is often performed during the same
period (Mueggler, 1976; Sando, 1978). Heavy grazing pressure by livestock can result
in a reduction of fuel sources, preventing fires from starting or spreading over large
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© 1996 Academic Press Limited
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R. J. GOSZ & J. R. GOSZ
areas or reducing the need/opportunity to use fire as a management tool (Wright,
1980). Removal of fire from the landscape because of intensive grazing may have
changed the competitive relationships among species as much as the often hypothesized pressure from livestock grazing itself. Research evaluating various combinations
of these factors is needed to identify the potential for species change. Combined
research efforts of the Sevilleta National Wildlife Refuge (SNWR) in central New
Mexico and the University of New Mexico’s Long-Term Ecological Research (LTER)
Program are dealing with such variables to determine the effects of wildfire and
herbivory on plant and animal community dynamics.
The SNWR is a special place for conducting this research because it is located in a
biome transition zone between Great Plains Grassland, Chihuahuan Desert, Colorado
Plateau Shrub-Steppe and Conifer Woodland (Fig. 1). Many species in this zone are
at their distributional limits and may be especially sensitive to factors that influence
competitive relationships. The current plant communities are mixtures of species from
normally widely separated areas (i.e. different biomes), although these individuals may
represent genotypes that are successful at the geographical limits for those species.
Plant interactions and responses to changing land-use patterns and climate dynamics
in the area may be particularly valuable additions to our accumulating knowledge of
global change influences. Knowledge gained in this region may forecast changes that
Conifer
Woodland
Conifer
Woodland
Great Plains
Grassland
Colorado Plateau
Shrub-Steppe
Conifer
Woodland
SNWR
Conifer
Woodland
N
W
Chihuahuan Desert
0
100
E
S
200
300 Kilometers
Figure 1. The state of New Mexico and location of the SNWR (Sevilleta National Wildlife
Refuge) in the transitional region between four major biomes: Great Plains Grassland,
Chihuahuan Desert, Colorado Plateau Shrub-Steppe and conifer forest/woodland. Elements of
communities from all four biomes are present within the boundary of the Sevilleta.
SPECIES INTERACTIONS IN NEW MEXICO
103
would take much longer to occur in other regions and help identify management
techniques that may be most effective.
Objectives
The objectives of this research were to experimentally determine how factors such as
fire, herbivory and simulated livestock grazing (i.e. clipping) influence the growth and
resilience of the two dominant perennial grass species, blue grama (Bouteloua gracilis
(Kunth.) Lag. ex Griffiths) and black grama (Bouteloua eripoda (Torr.) Torr.).
Blue grama is a Great Plains species near its western distributional limits in this
region and black grama is a desert species of the Chihuahuan Desert near its northern
and eastern distributional limits. They exist in mixed communities on the Sevilleta.
Several questions were posed for this research: (1) Will forage production increase
following fire and herbivory? (2) How will different species respond to these different
types of disturbances? (3) What effect do native herbivores have on livestock-free
grasslands following fire? (4) Can fire be used as a management tool to maintain
grasslands in the south-west U.S. for use by livestock and wildlife? (5) Will the return
of fire remove desert species from this biome transition zone and result in the shift of
the transition zone?
Study area
A variety of studies at different scales document the dynamics of the ‘tension zone’
represented by the Sevilleta National Wildlife Refuge and attempt to understand the
causes of biological change. Such biome transitions have been shown to be dynamic,
demonstrate marked changes in life-form and function (i.e. biome boundaries), and
may be especially sensitive to changing environmental factors (Gosz et al., 1992).
These areas are prime research sites for understanding the effects of factors such as
climate change and land-use change.
Many species are involved in the transition between biomes. Often the distributional
limits of plant species with markedly different morphologies are used to mark the ‘edge
area’ of the biome; however, many species are involved and have demonstrated that
they respond individually to the gradients of factors occurring through the biome
transition (McLaughlin, 1986). Gosz (1993) showed that dominant species of two of
the biomes (Great Plains Grassland and Chihuahuan Desert) were able to invade (or
be eliminated) to different degrees along a transect in the Sevilleta LTER study area.
The dominants were creosote bush (Larrea tridentata (DC.) Cov.) and black grama
(Bouteloua eripoda), representatives of the Chihuahuan Desert, and blue grama
(Bouteloua gracilis) of the Great Plains Grassland. Each species demonstrated a
decreasing patch size as it approached the local limits of distribution along the transect;
however black grama extended further north than creosote bush. The patch sizes of
blue grama decreased to the south along the transect. The pattern of decreasing patch
sizes along the transect is hypothesized to be a response to increased sensitivity to
microsite conditions at the distributional limits of a species. It also reflects the
individualistic nature of species movement; i.e. germination and establishment
patterns, which are influenced by the interaction of environmental conditions
(including land-use influences) with the microsite environments. The net result of the
patch patterns for these species is increased patch heterogeneity and a fine-grained
mosaic pattern in the transition zone (Neilson, 1991). Individual plots (10 m2) have
varying combinations of dominant species along the transect. The ratio of species such
as blue and black grama influence total plant cover and the magnitude of change under
different environmental conditions. For example, black grama can be very responsive
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R. J. GOSZ & J. R. GOSZ
to summer moisture and increase its plant cover dramatically relative to blue grama.
Thus, plots with different combinations of blue and black grama respond differently to
the environmental conditions during a given year (Gosz, 1993).
Heavy grazing pressure in this region of New Mexico can reduce surface biomass to
the point where natural fire is rare (Reynolds & Bohning, 1956; Buffington & Herbel,
1965). After the creation of the Sevilleta NWR in 1973, vegetation biomass increased
and there has been a dramatic increase in the frequency of natural fires (typically
lightning-caused) in recent years. The dominant species in this transition zone are
expected to be affected differently by burning, and plant communities with varying
compositions of Desert and Great Plains grassland species may respond differently.
For example, Reynolds & Bohning (1956) reported that on the Santa Rita
Experimental Range near Tucson, Arizona, density and herbage production of all
perennial grasses were greatly reduced by burning but most recovered by the end of the
second to fourth growing season. Black grama, one of the most important forage
species of the semi-desert grassland type, was seriously damaged by burning, and did
not recover during the period of study. It is not expected that blue grama, with its
deeper root system (Riegel, 1940), will be affected as much by burning. With the
return of fire to this semi-arid grassland in the transition zone, community dominants
may change which would influence the mosaic patterns and apparent ‘edge’ of the
biome boundary. These two factors (fire and precipitation) may produce very different
results in the transition area. Summer precipitation tends to increase the dominance of
black grama (and vegetation biomass) while fire frequency, which will increase with
increased fuel, may reduce black grama dominance. The net outcome of these opposed
influences may vary through time with climate patterns of wet and dry intervals, such
as those associated with El Niño and La Niña periods. El Niño episodes occur when
warm surface waters move into the central and eastern tropical Pacific ocean and cause
more frequent storms passing from the Pacific to the U.S. south-west during the fall
and winter (Cayan & Webb, 1992). La Niña episodes result when unusually warm
tropical surface waters are restricted to areas west of the International Date Line
causing less frequent moisture-bearing storms to move into the U.S. south-west.
Although El Niño episodes recur on average at 3–4 year intervals (Rasmusson et al.,
1990), there is a strong biennial component that is due to the tendency for El Niño and
La Niña episodes to follow one another in sequential years. It is not yet clear how the
biome boundary, as influenced by plant dominants and climate dynamics, will change
through time.
The formation of the Sevilleta NWR also changed land-use dramatically. The
elimination of cattle grazing has resulted in a dramatic fence line contrast in vegetation
biomass with black grama demonstrating increased frequency and biomass. Grazing
pressure has been documented as particularly influential on black grama. This species
is highly palatable and nutritious both in summer and winter, making a valuable yearlong forage plant, especially for cattle (Nelson, 1934). Black grama ordinarily cures
well on the stalk and retains its nutritive value through the dry spring period when
other range forage is parched and harsh. Although the plant can withstand recurrent
grazing by livestock, too heavy utilization impairs its vigor. During drought periods
plants so weakened succumb, thereby greatly depleting the stand. The release from
cattle grazing, the increase of fire frequency and the differential response to climate
factors such as summer precipitation form a complex system of factors and provide an
excellent experimental system for studying the dynamics of these species in this biome
transition zone.
Methods
The experiment consisted of a randomized complete block design to test the factors of
SPECIES INTERACTIONS IN NEW MEXICO
105
Table 1. ANOVA table for the randomized, complete block design with
replication
Source
df.
Blocks (4)
Species (2)
Treatments (3)
Herbivory (3)
S×T
S×H
T×H
S×T×H
Error
Total
3
1
2
2
2
2
4
4
267
287
Description
(blue grama, black grama)
(burning, clipping, control)
(full cage, partial cage, control)
(4 replicate plants)
blocks, species (blue and black grama), simulated herbivory (clipping), natural
herbivory (exclosure, partial exclosure, open) and fire. The ANOVA table (Table 1)
identifies the treatments and interaction factors.
The high variation in percent cover for grasslands of this region cause highly variable
results for experiments that measure results on an area basis. Very large numbers of
replicate plots are required to quantify significant treatment effects. Our approach was
to make morphological measurements on equal numbers of individual plants
(replicates) to quantify the effects of our experimental treatments and increase the
power of the randomized complete block design. Other LTER studies relate plant
populations and plant mosaic structure to spatial estimates and landscape pattern and
process. Thus, this research complements many other studies and, at the same time,
is aided by these other studies. That is one of the fundamental benefits of the LTER
program. Many of the other research efforts and data sets from the LTER program can
be viewed on the Sevilleta Information Management System (SIMS) web homepage
(http://sevilleta.unm.edu/).
The McKenzie Flat region of the Sevilleta NWR was used in this study and
experiments were conducted in a previously established system of 300 m 3 300 m
plots for fire studies in the LTER research program. The plant community is
dominated by blue and black grama. The dominant soil texture is sandy loam to loamy
sand (75–80% sand) and there is minimum microtopographic relief. Four blocks were
identified, each containing two 300 m 3 300 m plots. One of the plots in each block
was randomly chosen for the burning experiment and the second was used as a control.
During the first week of June 1993 a total of 288 individuals (144 blue grama and 144
black grama) of similar size and apparent health were permanently marked with metal
pins (18 individuals of each species in each of eight plots). Grazing was simulated on
plants in the control plots with a single clipping (hand clippers) of foliage above the
root crown on June 8. Herbivory by native, rodent herbivores was evaluated using wire
mesh (0·60 cm openings) cylinder exclosures 0·5-m tall and 0·3-m diameter placed
over individual plants. Partial exclosures were constructed by cutting small openings
(2·5 cm 3 2·5 cm) at four locations around the base of the mesh cylinder to exclude
rabbits but allow entry to smaller rodents. Control plants were uncaged. Replicate
plants were measured for: maximum height (longest leaves) (19 July, 10 August, 15
October); maximum seed stalk height (15 October); above-ground plant biomass (16
October) (each plant was clipped, separated into live and dead tissue, and weighed
after oven-drying at 60°C).
Growth height has been demonstrated to be a useful index of relative vigor and
volume production of range grasses in different years (Nelson, 1934). Height
106
R. J. GOSZ & J. R. GOSZ
measurement allowed non-destructive analyses of plant response during the growing
season. Final above-ground biomass measurements allowed an accurate measure of
forage production.
Burning was conducted on 28 June by the U.S. Fish and Wildlife Service with help
from student assistants of the Sevilleta LTER research program. Drip torches were
used to initiate fires under the fire prescriptions set by the U.S. Fish and Wildlife
Service. Each individual plant in the burning treatment was completely burned back to
the root crown, typical of a moderate fire intensity. Surface litter was also consumed
by burning.
Precipitation was measured using two gauges on the east and west sides of the study
area as well as with a nearby weather station operated for the LTER research program
(Deep Well Station). The weather station continuously records and downloads data
via radio to the Sevilleta computer.
The LTER research makes monthly censuses of rabbit populations on the
McKensie Flat region. The census provide a general status of the long-term cycles of
this major herbivore in the region. The partial and full exclosures on individual plants
prevented rabbit herbivory; however, control plants were subject to foraging by rabbits.
There are also early summer (mid-May through June) and mid-to-late summer (July–
August) samplings of small rodents using Sherman-type live traps arranged in a
‘trapping web’ design. The use of ‘webs’, rather than the more traditional trapping
grids, permits the direct estimation of rodent densities (numbers of animals per unit
area), and requires few assumptions regarding capture probabilities of the animals (R.
Parmenter, pers. comm.). Density estimates from the field data were performed using
Program DISTANCE.
Statistical analyses from SAS were performed using analysis of variance (ANOVA),
multiple mean tests, and repeated measures analysis of variance. Statistical significance
is reported for differences with p less than 0·05.
Results
By the first measurement period in July, there were significant differences between the
foliage growth (plant leaf length) rates for the two species (blue grama greater than
black grama) and among the three treatments of burning, clipping and control. There
were no significant interaction effects between species and treatments. Burning
inhibited growth to the greatest degree for both species, with clipping causing an
intermediate response (Fig. 2). A number of individuals had died during the period,
primarily black grama on burned plots; however, the ANOVA results using only living
plants did not vary from the results with all data. Results in Fig. 2 are for living plants
only.
There were no significant differences caused by the cage or partial cage enclosures;
therefore the results of Fig. 2 use only the control (i.e. no enclosures) data. Data for
the three herbivory treatments were not pooled.
By 10 August, plant height increased for both species; however, there remained
significant differences between the plant growth response of blue and black grama
(Fig. 2). In all cases blue grama growth exceeded that of black grama. There were
significant differences between all three treatments with lowest growth rates on the
burned plots. There were still no significant interaction terms, meaning each species
was responding similarly to treatments; however, blue grama growth was consistently
greater than that of black grama.
In October, at the end of the growing season and at peak biomass, blue grama plant
height was significantly greater than that of black grama. For black grama, the burning
and clipping treatments were not significantly different; however, heights of both were
significantly less than heights of the control (Fig. 2). For blue grama, control plants
SPECIES INTERACTIONS IN NEW MEXICO
107
16
Mean plant height (cm)
14
12
10
8
6
4
2
0
July
August October
July
Burn
August October
Clipped
July
August October
Control
Figure 2. Mean height (cm) and 95% confidence limits for blue grama (h) and black grama (j)
individual plants measured in July, August and October on plots subject to burning, clipping or
no treatment (control). Results were calculated only from living plants without herbivore
exclosures.
were slightly taller than burned plants (but statistically significant). There were no
significant differences between control and clipped or between clipped and burned
plants.
A repeated measure analysis of variance was performed for the three collections of
plant height (Table 2). As anticipated, there was a significant time effect and a
significant time 3 treatment interaction effect. No other time 3 variable interactions
were significant. Thus, while plants in all treatments increased in height with time,
there were significant differences in the magnitude of plant height response among
Table 2. Repeated measures analysis of variance for plant height during July,
August and October 1993
Manova test criteria and exact F-statistics for the hypothesis of no TIME effect
Statistic
Wilks’ Lambda
Pillai’s Trace
Hotelling-Lawley
Roy’s Greatest Root
F-value
df.
p>F
101·122
101·122
101·122
101·122
2
2
2
2
0·0001
0·0001
0·0001
0·0001
Manova test criteria and F-approximations for the hypothesis of no TIME ×
TREATMENT effect
Wilks’ Lambda
Pillai’s Trace
Hotelling-Lawley
Roy’s Greatest Root
29·838
26·660
33·059
66·611
4
4
4
2
0·0001
0·0001
0·0001
0·0001
F-statistic for Roy’s Greatest Root is an upper bound. F-statistic for Wilks’ Lambda is exact.
108
R. J. GOSZ & J. R. GOSZ
Mean dry plant biomass (g)
treatments. The burning treatment inhibited plant growth of both species the most,
control conditions inhibited growth the least.
Plant biomass showed significant species and treatment effects and was the only
measurement to show a significant treatment 3 species interaction. This was a result
of control black grama plants having much higher biomass than blue grama plants as
well as higher biomass than the clipped and burned black grama plants. Although
black grama shoots (or tillers) were not longer (see Fig. 2), the increased number of
shoots resulted in greater biomass for that species (higher biomass/height ratio). Blue
grama plants were not significantly different in biomass between treatments in October
(Fig. 3). As with the previous results, performing the statistical analyses with all data
or with data from live plants only made no difference in the significance tests.
Plant mortality is a complementary measure of the treatment effects although it was
not tested for statistical significance. Greatest mortality occurred for black grama; 17
plants. vs. seven for blue grama. These were concentrated in the burned plots where
over 31% of the black grama plants (17 individuals) died compared with 8% of blue
grama plants (four individuals).
Seed stalk heights were not significantly different between species or among
treatments for blue grama, principally because of the high variance in these
measurements. When all data were used (incorporating zeros for plants that died), the
control plants of both species had significantly taller stalks than those subject to
clipping or burning (Fig. 4(a,b)). This was primarily a result of the high mortality in
black grama plants on burned plots. For black grama, the small herbivore exclosures
(Cage) and partial exclosures (P-Cage) caused a significant increase in mean seed stalk
height (Fig. 4(b)). The differences between partial exclosures and full exclosures were
not significant. This implicates a role for granivores that is more significant than
herbivores in this experiment. The results of rodent trapping for this study site
indicated that certain species exhibited significantly greater densities in the spring of
1993 compared to densities observed in some previous years. Rodent genera having
5
4
3
2
1
0
Burn
Clipped
Treatment
Control
Figure 3. Mean biomass (oven-dry weight) and 95% confidence limits for individual plants of
black grama (j) and blue grama (h) subject to burning, clipping or no treatment (control).
SPECIES INTERACTIONS IN NEW MEXICO
109
greater densities included Peromyscus and Neotoma, and these results were consistent
across all habitat types (grasslands, desert shrublands, and Piñon-juniper woodlands)
in the region (Parmenter et al., 1993).
Comparison of various meteorological data from 1989 to 1993 demonstrated that
June through October precipitation for 1993 was near ‘normal’; 4 of the 5 years ranged
from 123·3 cm to 128·4 cm for the 5-month interval. The summer of 1991 was
45
(a)
Mean seed stalk height (cm)
40
35
30
25
20
15
10
5
0
Open Cage P-Cage Open Cage P-Cage Open Cage P-Cage
Undisturbed
Clipped
Burned
Treatment
45
(b)
Mean seed stalk height (cm)
40
35
30
25
20
15
10
5
0
Open Cage P-Cage Open Cage P-Cage Open Cage P-Cage
Undisturbed
Clipped
Burned
Treatment
Figure 4. (a) Mean seed stalk height (cm) and 95% confidence limits for individuals of blue
grama in October 1993, subject to burning, clipping or no treatment (control). (b) Mean seed
stalk height (cm) and 95% confidence limits for individuals of black grama in October 1993,
subject to burning, clipping or no treatment (control). Cage refers to the herbivore exclosure and
P-Cage refers to exclosures with small holes in the mesh to allow entry to small rodents.
110
R. J. GOSZ & J. R. GOSZ
exceptionally moist (214·9 cm) as a result of very high precipitation in June (74·4 cm).
The winter of 1991–92 was also very moist as a result of the strong El Niño that
developed during that period. Precipitation during November 1991 through May 1992
was 196·7 cm compared to only 65·2 cm during that period in 1992–93. Other
meteorological variables suggest the summer of 1993 was fairly typical.
The longer-term pattern of plant response to environmental conditions can be
estimated from the 1-km long transects that are censused during the spring and fall of
each year on McKensie Flat. The percent cover of both blue and black grama
increased from 1989–1993 with black grama demonstrating the largest percent
increase (Table 3). Black grama demonstrated a consistent pattern of increase from
spring to summer while blue grama showed both increases and decreases during
different years. The wet winter of 1991–92 was associated with a large decrease in
percent plant cover followed by a large increase in black grama during the summer of
1992 which continued in 1993. These data suggest that the area is continuing to
recover (by increasing percent plant cover) following the release from grazing when the
NWR was established in 1973. The species composition of the area as identified on the
transects, and percent cover changes over time for all species, litter and soil can be
viewed on the SIMS homepage (http://sevilleta.unm.edu/plant/transect/transecthome.html).
Rabbit populations (blacktail jackrabbits, desert cottontails) are demonstrating a
general increase as evidenced by the density estimates from the road surveys carried
out by the LTER research program (R. Parmenter, unpublished data). Numbers have
increased during recent years with highest intra-annual densities during the summer
months of any particular year as a result of the production of young in the spring. The
past several years appear to be an interval of minimum population size in what is
expected to be a 10–11 cycle of rabbit population fluctuations.
Discussion
The questions posed in the Objectives section are the basis for the following
discussion.
Table 3. Percent plant cover along a 1 km line intercept transect on the
McKensie flat study area of the Sevilleta Long-Term Ecological Research
program. Measurements along the transect were made to a 1 cm resolution.
Spring censuses were made in late May and fall censuses in late September
Percent cover
Year
Season
Blue grama
Black grama
1989
spring
fall
spring
fall
spring
fall
spring
fall
spring
fall
7·4
5·5
8·6
8·2
8·6
8·8
7·9
9·1
9·0
9·2
12·0
13·7
14·7
16·2
16·1
18·0
11·5
16·7
19·2
19·5
1990
1991
1992
1993
SPECIES INTERACTIONS IN NEW MEXICO
111
Will forage production increase following fire and herbivory?
The data from the 1 km transect suggest the landscape, in general, continues to
demonstrate an increase in production and plant biomass following the release from
heavy grazing pressure in 1973. Inter-annual fluctuations can be dramatic in response
to large variations in precipitation (annual and seasonal) related to El Niño
phenomena, and different species contribute differently to these environmental
signals. Black grama appears to be the most dynamic of the perennial grasses in the
study area, especially in response to summer precipitation (Gosz, 1993). This agrees
with numerous studies in the U.S. south-west that demonstrate the importance of
summer moisture for this C4 species (Nelson, 1934; Paulsen & Ares, 1962). The
shallow root system and the coarse textured soils are important characteristics in the
ability of this species to take advantage of the intense precipitation that occurs during
the convective summer precipitation storms.
The experimental data demonstrate that blue grama can recover completely from
burning and clipping during the course of the summer relative to controls (Fig. 2).
Black grama had less ability to recover during the course of a single summer and may
need several years of favorable moisture (without disturbance) to recover completely
(Nelson, 1934; Reynolds & Bohning, 1956). Native herbivores were not a significant
factor affecting forage production in this experiment. The overall experiment suggests
that forage production following fire may approach that of non-burned areas, but only
for certain species. This will increase the patchy nature of forage production at both
small scales, reflecting plant to plant patterns, and larger scales caused by one species
being dominant in a given patch in the mosaic structure. We do not have data on the
quality of the forage produced following fire; however, the data from the plants on
burned plots without enclosures do not indicate that the burned individuals were more
attractive than control plants. At larger scales, the removal of standing dead biomass
during large fires followed by the ‘green flush’ of new growth may make burned areas
attractive, visible spots in the landscape for herbivores. Other studies on the Sevilleta
indicate that pronghorn antelope preferentially visit the large burned areas demonstrating some recognition of these areas at this larger scale.
How will different species respond to these different types of disturbances?
The data agree with previous studies that black grama is sensitive to disturbances such
as burning and herbivory (i.e. clipping) (Canfield, 1939; Wright, 1980). These past
studies (see below) were performed in areas where only one of the two species was
present. The unique feature of the Sevilleta NWR area is that both species are present
and effectively competing with each other. It is a valuable area in which to test the
abilities of two species with markedly different life histories and morphologies
(Lauenroth et al., 1994) to respond to the same factors on the same site. The data
suggest that the increased dominance of black grama (Table 3) reflects the absence of
burning and grazing on this NWR. While livestock grazing can be assured to be
removed as a factor on this refuge, increased fire occurrence may affect black grama
dominance negatively in the future. The interval between fires for a particular site will
be an important factor in the ability of black grama to recover and continue to
dominate the site. Reynolds & Bohning (1956) reported that black grama, one of the
most important forage species of the semi-desert grassland type, was seriously
damaged by burning and did not recover during the 5-year period of study.
What effect do native herbivores have on livestock-free grasslands following fire?
Our data from small exclosures does not show significant foliage consumption on
individual plants used in the experiment. Several factors could have contributed to
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R. J. GOSZ & J. R. GOSZ
these results. Individual plants are small targets in the landscape and the fact that
important native herbivores (i.e. rabbits) are at a low point in their population density
cycles may explain the low foliage consumption rates, even on control plants that were
not enclosed. If the densities of these herbivores continue to increase, the same
experiment in future years may demonstrate very different results with partial and full
exclosures experiencing significantly less herbivory than controls. One significant effect
caused by exclosures was their influence on seed stalks, especially for black grama (Fig.
4(b)). Both full and partial exclosures demonstrated protection relative to controls that
were open to granivores. Although we cannot rule out the possibility that shade or
cooler temperatures under the exclosures played some role, the exclosure results may
reflect the dominant role that granivores play in this system. There are many species
of small rodents and birds that rely on seed production in this grassland and their
abundances were very high in the spring of 1993. The LTER program routinely
monitors densities in this area and different species show dramatic inter-annual
fluctuations. The rodents Peromyscus truei (Schufeldt), Peromyscus difficilis (J.A. Allen)
and Neotoma albigula Hartley demonstrated the most dramatic density increases
between 1992 and 1993 (Parmenter et al., 1993).
The experiment also demonstrated the sensitivity of black grama to the treatments
in terms of seed stalk height. Plants protected by exclosures on clipped and burned
sites had significantly shorter seed stalks than even the open (uncaged) plants of the
control sites. Thus, treatments markedly affected the ability of plants to produce
fruiting structures. We do not have data on seed production and assume that seed stalk
height is correlated with good seed production (Nelson, 1934).
Can fire be used as a management tool to maintain grasslands in the south-western U.S.
for use by livestock and wildlife? Will the return of fire remove desert species from this
biome transition zone resulting in the shift of the transition zone?
These two questions are related for this region because of the effects of fire on species
composition and forage production. Frequent fire is expected to negatively affect black
grama in terms of increased mortality and lowered forage production. Black grama is
more productive in this climatic region on coarse textured soils in the absence of
grazing and fire. Infrequent fires or light grazing may allow black grama to maintain its
position in the plant community on these sandy loam or loamy sand soils (Nelson,
1934; Miller & Donart, 1979). If fire became more frequent and blue grama increased
in dominance, it essentially would mean that the transition zone had moved and this
grassland would be more like the Great Plains biome. Important characteristics of a
blue grama grassland are its drought resistance (Lauenroth et al., 1994), ability to
recover more rapidly after fire (Fig. 2) and resistance to livestock grazing (Lauenroth
et al., 1994). Thus, although fire might reduce a species that can be very productive
(i.e. black grama), the domination by a species more resilient to fire and grazing (i.e.
blue grama) may allow increased livestock use.
Lauenroth et al. (1994) provided an excellent model evaluating the recruitment
ability of blue grama. They found that recruitment events occur as frequently as every
30–50 years on silty clay, silty clay loam, and silty loam soils, but less than once in 5000
years on sandy soils. The differences in silt content and available water-holding
capacity accounted for the difference between soil textures in the probability of
occurrence of recruitment events. Annual precipitation explained a large fraction of the
temporal variability in recruitment. On average, recruitment occurred in years when
precipitation was above the mean. Infrequent recruitment for this species is sufficient
to maintain a population because the maximum age for individuals of this species, even
under heavy grazing, is 450 years (Coffin & Lauenroth, 1988). In contrast, black
grama individuals commonly do not last more than 10 years. The ability of blue grama
SPECIES INTERACTIONS IN NEW MEXICO
113
to expand its population under conditions when black grama is decreasing on the
sandy soils of the Sevilleta may be limited to very specific climatic conditions.
Summary
Black and blue grama grasses have significantly different responses to environmental
variation and disturbances such as fire and grazing. In this biome transition region
where these two species co-occur, these differential responses can result in changes in
community composition and the structure and function of the system. Black grama
productivity responds dramatically to changes in summer moisture resulting in high
inter-annual variation in percent plant cover and forage biomass. Blue grama
demonstrates less response to the same variations in moisture. Our experiments on
fire, simulated grazing and exclusion of certain native herbivores also demonstrated
that black grama was more significantly affected by these factors than was blue grama.
Black grama individuals were not able to recover (in terms of plant growth) as rapidly
as blue grama by the end of the growing season. While black grama has the ability to
grow more rapidly than blue grama, when subject to forces such as fire and grazing,
black grama cannot recover as rapidly. These fundamental differences between the two
species suggest several potential scenarios for the future community composition in
this region. The high inter-annual variation in precipitation, combined with the sandy
loam soils of the Sevilleta, should favor black grama dominance. This appears to be
happening since the refuge was established in 1973 and livestock grazing was removed.
If grazing were to be reintroduced, we suspect the black grama would lose its
dominance as has happened in other areas in the region (Canfield, 1939; Buffington &
Herbel, 1965). The increased frequency of fire because of increased fuel could also
reduce the dominance of black grama. The critical aspect of the fire factor would be
the return time for burning on a specific site. Frequent burning (e.g. every 3 or 4 years)
may negatively affect this species while longer intervals may not be a significant factor.
Several years are required for black grama to resume productivity and dominance.
More sophisticated management procedures could take advantage of the differential
responses of both these species, their ability to recover from disturbance and the
natural variability in environmental factors for this region. For example, with better
developed methods for predicting wet years (e.g. El Niño events), the beneficial effects
of burning could be obtained simply by performing prescribed burns before the
coming wet year. Grazing intensity should also vary in accordance with the ability of
the forage species to recover from natural and management manipulations.
This research was supported by the Research Experiences for Undergraduates program (REU)
at the Sevilleta Long-Term Ecological Research site managed by the University of New Mexico.
This program is supported by the National Science Foundation. Many thanks to the other
undergraduate researchers that helped with this project and enriched the overall experience. Bob
Parmenter was instrumental in helping with the design of the experiment and implementation
of the field work. James Brunt, Doug Moore and Bob Parmenter helped with statistical analyses,
data management, graphics and made important editorial improvements. Ted Stans, Sevilleta
National Wildlife Refuge manager, helped with field work, especially the burning experiments,
and provided enthusiastic encouragement through all phases of the effort. This is contribution
No. 71 to the Sevilleta LTER program.
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