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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 0140–1963/96/010101 + 14 $18.00/0 © 1996 Academic Press Limited 102 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 104 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 112 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. References Buffington, L.C. & Herbel, C.H. (1965). Vegetational changes on a semidesert grassland range from 1858 to 1963. 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