Mixed-grass Prairie Sites Research Articles

Consequences of Rainfall Manipulations for Invasive Annual Grasses Vary Across Grazed Northern Mixed-Grass Prairie Sites

Northern mixed-grass prairie rangelands are threatened by increasing drought severity and invasion by annual grasses. However, it is unclear whether climate change will amplify or dampen this invasion. We tested separate and combined effects of livestock grazing and experimental rainfall manipulation on invasion by annual brome grasses—cheatgrass (Bromus tectorum L.) and field brome (Bromus arvensis L.)—in two mixed-grass prairie sites (Montana and Wyoming, United States). To provide management-relevant results, we manipulated precipitation at five levels representing a gradient of precipitation reduction and implemented grazing strategies selected by stakeholders to represent realistic management choices: destock, stable, and heavy grazing scenarios. We measured soil moisture and three plant properties of invasive annual bromes (aboveground primary production, percent greenness, and percent cover) during two water manipulation yr (2019, 2020) and one recovery yr of natural rainfall (2021).Imposed precipitation reduction generally decreased absolute annual brome biomass and induced earlier senescence. However, during the recovery year, we observed prolonged time to senescence in the formerly droughted plots. In Wyoming, summer grazing had little appreciable effect on annual bromes, perhaps because annual bromes mature early in the growing season (mid-June) and may therefore be less affected by summer grazing. However, in the first year after ending water treatments during a natural drought in Montana, under heavy grazing, annual brome production marginally increased from 32.4 ± 10.6 kg · ha−1 to 130.8 ± 111.8 kg · ha−1 (mean ± standard error) with prior severe precipitation reduction. The magnitude of responses tended to be site dependent, which may be due to inherent vegetation differences between our sites, as well as site-scale differences in natural precipitation patterns. Together, these results suggest that annual brome abundance may increase in the context of drought combined with heavy grazing, a more likely scenario with continuing climate change.

Improvement of evapotranspiration estimates for grasslands in the southern Great Plains: Comparing a biophysical model (SWAT) and remote sensing (MODIS)

Study regionMixed- and tall-grass prairies in the southern Great Plains, USA Study focusEstimates of evapotranspiration (ET) are widely available from remote sensing and commonly used for water management. However, this approach is limited by prolonged satellite revisit periods and prominent algorithm-induced bias. We compared ET estimates from MODIS (Moderate Resolution Imaging Spectroradiometer) products and those based on the biophysically-based Soil and Water Assessment Tool (SWAT) to measurements using the eddy covariance (EC) technique for a subhumid, tall-grass prairie site near Stillwater, Oklahoma (OK) and a semiarid, mixed-grass prairie site near Clinton, OK in the southern Great Plains, USA. New hydrological insights for the regionSWAT and MODIS produced ET estimates closer to EC measurements for the tall-grass prairie (calibration site) than for the mixed-grass prairie (validation site), with a better performance from SWAT. For the tall-grass prairie, the R2 values were relatively high and comparable (0.77 and 0.87), and the biases were relatively small (−0.40% and 5.04%) for the SWAT and MODIS comparisons to EC. SWAT performed much better than MODIS for the mixed-grass prairie with R2 values of 0.68 vs. 0.13 and bias of − 1.87% vs. − 45.71%, respectively. The SWAT simulation also reproduced better estimates of aboveground net primary productivity than the MODIS products. This study suggests that site-specific SWAT simulations can produce better ET estimates than MODIS products, especially in the water-limited mixed-grass prairie.

Molecular evidence for impoverished mycorrhizal communities of Agropyron cristatum compared with nine other plant species in the Northern Great Plains

Invasive plants may alter arbuscular mycorrhizal fungi (AMF) communities on which resident plants depend. To determine if the invader crested wheatgrass (Agropyron cristatum [L.] Gaertn.) associates with different AMF than resident plants, we compared AMF communities of six A. cristatum−dominated sites to five sagebrush steppe and three mixed-grass prairie sites in the Northern Great Plains. Consistent with findings on some other invaders, roots of A. cristatum were without AMF and supported fewer AMF operational taxonomic units than seven native species and two nonnative species. Restoring natives to A. cristatum sites is notoriously difficult. Our findings suggest a lack of soil mutualists may contribute to the difficulty.

Grasshopper populations respond similarly to multiple moderate intensity livestock grazing treatments

Livestock grazing frequently affects grasshopper populations, but no prior studies have simultaneously examined a wide range of moderate intensity livestock grazing treatments in the Northern Great Plains. Grasshopper densities varied significantly between years, but five moderate grazing treatments, including both rotational and continuous grazing treatments, did not differentially affect grasshopper densities or species composition. Grasshopper populations appear resilient to different types of moderate grazing at this Northern Great Plains mixed-grass prairie site.

Access mats partially mitigate direct traffic impacts on soil microbial communities in temperate grasslands

Significant areas of the mixedgrass prairie (MGP) in Alberta have experienced heavy equipment traffic due to industrial infrastructure development, which may change edaphic conditions that influence soil microbial communities. Direct wheeled traffic effects may be alleviated by using access mats that spread traffic impacts over a larger area; however, no scientific assessments of access mat efficacy have been conducted on soil microbial responses. Our objectives were to assess soil properties and the diversity, community structure, and size of soil microbial communities, as impacted by wheeled traffic occurring with and without access mat use on grassland. Two representative sites with differing soil texture (sandy versus loamy) were selected from a set of MGP sites established at the University of Alberta Mattheis Research Ranch in SE Alberta, Canada. Treatments at each site included non-disturbed vegetation controls, areas affected by direct traffic, and grassland with access mats placed for 12 weeks with traffic applied. Soil was sampled in July 2015 (immediately after access mat/traffic treatments ended) and in 2016 (12 months after access mat/traffic treatments) from both sites. Soil DNA was extracted and used to target N-cycling genes via qPCR and for next-generation sequencing (Illumina Miseq) of bacterial, archaeal, and fungal communities. Direct wheeled traffic without access matting increased the size of the N2O-reducing community in the sandy soil and conversely deceased the size of this community in the loamy soil. Non-metric multidimensional scaling, perMANOVA, and indicator species analyses indicated a large effect of traffic without access mat use on fungal community structure, as well as distinct bacterial and fungal indicator OTUs associated with disturbed soils compared to control soils. Fungal community structure remained affected by traffic impacts 12 months later, highlighting the extended impacts of traffic on fungal communities. Access matting mitigated some of the impacts direct traffic had on fungal community structure, but in the loamy site increased soil N and decreased the size of the N2O-reducing bacterial community. The latter indicates that ecosystem functioning of these grasslands may not only be impacted by direct wheeled traffic but also by using matting as a mitigation technique.

Are SSURGO organic matter estimates reliable for cold arid steppes? Implications for rangeland soil health

The SSURGO (Soil Survey Geographic Database)-based Web Soil Survey (WSS) is a geospatial tool for managers to assess soil properties and make management decisions, presumably without field sampling. To determine whether WSS percent organic matter (%OM) estimates were reliable in cold arid rangelands, we sampled soils at a northern mixed-grass prairie site and a sagebrush-steppe site in Wyoming, USA. Loss on ignition testing was used to determine soil %OM. Field samples contained higher %OM than WSS %OM estimates and did not covary with WSS %OM estimates. Baseline sampling is necessary prior to management in order to detect %OM changes.

Effects of shallow natural gas well structures and associated roads on grassland songbird reproductive success in Alberta, Canada.

Grassland songbird populations across North America have experienced dramatic population declines due to habitat loss and degradation. In Canada, energy development continues to fragment and disturb prairie habitat, but effects of oil and gas development on reproductive success of songbirds in North American mixed-grass prairies remains largely unknown. From 2010–2012, in southeastern Alberta, Canada, we monitored 257 nests of two ground-nesting grassland songbird species, Savannah sparrow (Passerculus sandwichensis) and chestnut-collared longspur (Calcarius ornatus). Nest locations varied with proximity to and density of conventional shallow gas well structures and associated roads in forty-two 258-ha mixed-grass prairie sites. We estimated the probabilities of nest success and clutch size relative to gas well structures and roads. There was little effect of distance to or density of gas well structure on nest success; however, Savannah sparrow experienced lower nest success near roads. Clutch sizes were lower near gas well structures and cattle water sources. Minimizing habitat disturbance surrounding gas well structures, and reducing abundance of roads and trails, would help minimize impacts on reproductive success for some grassland songbirds.

Nesting ecology of grassland birds following a wildfire in the southern Great Plains

We studied the response of nesting grassland birds occupying short-grass and mixed-grass prairie sites 2 and 3 y following two, large-scale wildfires that burned ≥360,000 ha in the Texas Panhandle in March 2006. Nest success was greater on burned plots compared to unburned plots, though this varied by species and year. Woody vegetation cover was greater around nests on unburned plots compared to burned plots for Cassin's sparrow (Peucaea cassinii) and lark sparrow (Chondestes grammacus). Cassin's sparrows and lark sparrows nested in more-woody vegetation than did grasshopper sparrows (Ammodramus savannarum), and woody vegetation was reduced following the wildfires. The wildfires appear to have had few if any negative influences on the avian community 3 years postfire. This may be due to grassland breeding birds being adapted to landscapes in which, historically, periodic disturbance (e.g., wildfire, intensive grazing by bison [Bison bison]) resulted in vegetation heterogeneity.

Invasion as a Function of Species Diversity: A Case Study of Two Restored North Dakota Grasslands

Invasive species are common occurrences on many landscapes and pose serious threats to biodiversity. This study investigated the relationships among plant diversity, nutrient addition, and susceptibility to invasion by smooth bromegrass ( Bromus inermis ) and crested wheatgrass ( Agropyron cristatum ) at two different locations in North Dakota, one representing the tallgrass prairie ecoregion and one representing the mixed-grass prairie ecoregion. Results indicate ecoregional differences in invasion by the two species. At the tallgrass prairie site, smooth bromegrass was the major invasive species and was inversely related to species diversity. At the mixed-grass prairie site, crested wheatgrass was the major invasive species and was also inversely related to species diversity. Nitrogen fertilization significantly increased smooth bromegrass biomass at the tallgrass prairie site while phosphorus fertilization had no effect. At the mixed-grass prairie site, there was a significant increase in smooth bromegrass biomass only when crested wheatgrass was used as a covariate, indicating that managers must identify which specific invasive species poses the greatest threat to grassland communities in the ecoregion. The results of this study indicate that increasing species richness can be a useful management tool to reduce invasive species biomass in grassland restorations.

Drought and Grazing Effects on Oklahoma Phlox (Polemoniaceae, <i>Phlox oklahomensis</i>)

Oklahoma phlox (Phlox oklahomensis Wherry) is endemic to Butler, Chautauqua, Comanche, Cowley, and Elk Counties of Kansas and Woods and Woodward Counties of Oklahoma. The species comprises populations of a few scattered individuals to several hundred in mixed-grass prairie sites in Oklahoma where cow-calf production is the common agricultural enterprise. It has successfully withstood periods of short-term drought (1 to 4 years) under light to moderate continuous stocking rates (41 to 52 animal unit days per hectare). Under heavy continuous stocking rates and/or prolonged drought, populations of P. oklahomensis tend to decrease in size and number and may disappear in some localities. Prolonged heavy stocking rates and drought will disrupt the reproductive cycle, severely limiting seed production and recruitment of new individuals to populations. During drought periods, livestock managers should use lighter stocking rates or deferred grazing to ensure rapid recovery of all prairie plants, including P. oklahomensis, once the drought ends. This recommendation would likely sustain prairie sites during long-term (longer than 10 years) droughts as well.

Grassland Bird Communtiy Response To Large Wildfires

We studied breeding season communities of grassland birds on short-grass and mixed-grass prairie sites during the second and third breeding seasons following two large wildfires in March 2006 in the Texas panhandle, USA. There was an apparent temporary shift in avian community composition following the fires due to species-specific shifts associated with life-history traits and vegetation preferences. Species that prefer sparse vegetation and bare ground on short-grass sites, such as Horned Lark (Eremophila alpestris), benefited from wildfires, while others, such as Western Meadowlark (Sturnella neglecta), that prefer more dense vegetation, were negatively impacted. Mixed-grass sites had species-specific shifts in 2007, two breeding seasons after the fires; grassland bird communities on burned plots were similar by 2008 to those on unburned plots. Avian communities appeared to return to pre-burn levels within 3 years following wildfires. Many of the responses in our study of wildfire were similar to those reported following prescribed fires elsewhere. Prescribed fires appear to have similar effects on the avian community despite differences in intensity and environmental conditions during wildfires.

Pyric–Herbivory and Cattle Performance in Grassland Ecosystems

Achieving economically optimum livestock production on rangelands can conflict with conservation strategies that require lower stocking rate to maintain wildlife habitat. Combining the spatial and temporal interaction of fire and grazing (pyric–herbivory) is a conservation-based approach to management that increases rangeland biodiversity by creating heterogeneous vegetation structure and composition. However, livestock production under pyric–herbivory has not been reported. In both mixed-grass prairie and tallgrass prairie, we compared livestock production in pastures with traditional fire and grazing management (continuous grazing, with periodic fire on tallgrass prairie and without fire on mixed-grass prairie) and conservation-based management (pyric–herbivory applied through patch burning) at a moderate stocking rate. Stocker cattle weight gain, calf weight gain, and cow body condition score did not differ ( P > 0.05) between traditional and conservation-based management at the tallgrass prairie site for the duration of the 8-yr study. At the mixed-grass prairie site, stocker cattle gain did not differ in the first 4 yr, but stocker cattle gained more ( P ≤ 0.05) on conservation-based management and remained 27% greater for the duration of the 11-yr study. Moreover, variation among years in cattle performance was less on pastures under conservation management . Traditional management in mixed-grass prairie did not include fire, the process that likely was associated with increased stocker cattle performance under conservation management. We conclude that pyric–herbivory is a conservation-based rangeland management strategy that returns fire to the landscape without reduced stocking rate, deferment, or rest. Lograr el óptimo económico en la producción ganadera en pastizales puede interferir con estrategias de conservación que requieren baja carga animal para mantener el hábitat para la fauna silvestre. Combinando de manera espacial y temporal la interacción de fuego y pastoreo (fuego-herbivorismo) es un concepto con fundamente en la conservación para manejar el aumento de la biodiversidad del pastizal creando estructuras de vegetación y composición heterogéneas. Sin embargo, la producción de ganado bajo fuego-herbivorismo no se ha reportado. En ambas, praderas de pastos medianos y largos comparamos la producción ganadera en potreros con manejo tradicional de fuego y manejo del pastoreo (pastoreo continuo con periodos de fuego en praderas de pastos largos y manejo basado en la conservación (fuego-herbivorismo aplicado a través del quemas en parches) con carga animal moderada. Ganancia de peso en novillos en repasto, ganancia de peso en becerros y el grado de condición corporal de la vaca no fueron diferentes ( P > 0.05) entre el manejo tradicional y el conservador en el sitio de pasto largos en ocho años de duración del estudio. En el sitio de pastos mixtos la ganancia de peso de novillos no fue diferente en los primeros cuatro años pero, los novillos en repasto ganaron más ( P ≤ 0.05) en el manejo basado en la conservación y se mantuvieron con un 27% mayor en los once años de duración del estudio. Por otra parte, la variación entre años en el desempeño del ganado fue menor en los potreros bajo el manejo de conservación. El manejo tradicional en praderas mixtas no incluyeron el fuego, el proceso probablemente estuvo asociado al incremento en el desempeño de los novillos bajo el manejo de conservación. Concluimos que el fuego-herbivorismo es una estrategia de manejo basado en la conservación del pastizal que reincorpora al fuego en el paisaje sin reducir la capacidad de carga, diferimiento o descanso.

Influence of a Large Late Summer Precipitation Event on Food Limitation and Grasshopper Population Dynamics in a Northern Great Plains Grassland

The complex interplay between grasshoppers, weather conditions, and plants that cause fluctuations in grasshopper populations remains poorly understood, and little is known about the ecological processes that generate grasshopper outbreaks. Grasshopper populations respond to interacting extrinsic and intrinsic factors, with yearly and decadal weather patterns and the timing of precipitation all potentially important. The effects of initial and increasing grasshopper densities on grasshopper survival and reproductive correlates were examined at a northern mixed-grass prairie site through manipulations of grasshopper densities inside 10-m2 cages. High-quality grass growth occurred after a 9.1-cm mid-August rain. Reduced proportional survival was apparent in the two higher density treatments before the rain, indicative of food-limited density-dependent mortality. However, the large late summer rainfall event mediated the effects of exploitative competition on demographic characteristics because of the high-quality vegetation growth. This led to weaker effects of food limitation on survival and reproduction at the end of the experiment. The results indicate a direct link between weather variation, resource quality and grasshopper population dynamics led to a severe grasshopper outbreak and show that infrequent large precipitation events can have significant effects on population dynamics. Additional research is needed to examine the importance of infrequent large precipitation events on grasshopper population dynamics in grassland ecosystems.

Estimation of Carbon Sequestration by Combining Remote Sensing and Net Ecosystem Exchange Data for Northern Mixed-Grass Prairie and Sagebrush–Steppe Ecosystems

Carbon sequestration was estimated a northern mixed-grass prairie site and a sagebrush–steppe site in southeastern Wyoming using an approach that integrates remote sensing, CO2 flux measurements, and meteorological data. Net ecosystem exchange (NEE) of CO2 was measured using aircraft and ground flux techniques and was linearly related to absorbed photosynthetically active radiation (APAR). The slope of this relationship is the radiation use efficiency (ɛ = 0.51 g C/MJ APAR); there were no significant differences in the regression coefficients between the two sites. Furthermore, ecosystem chamber measurements of total respiration in 1998 and 1999 were used to develop a functional relationship with daily average temperature; the Q10 of the relationship was 2.2. Using the Advanced Very High Resolution radiometer. Normalized Difference Vegetation Index and meteorological data, annual gross primary production and respiration were calculated from 1995 to 1999 for the two sites. Overall, the sagebrush–steppe site was a net carbon sink, whereas the northern mixed-grass prairie site was in carbon balance. There was no significant relationship between NEE and APAR for a coniferous forest site, indicating this method for scaling up CO2 flux data may be only applicable to rangeland ecosystems. The combination of remote sensing with data from CO2 flux networks can be used to estimate carbon sequestration regionally in rangeland ecosystems.

Effects of ungulates and prairie dogs on seed banks and vegetation in a North American mixed-grass prairie

The relationship between vegetation cover and soil seed banks was studied in five different ungulate herbivore-prairie dog treatment combinations at three northern mixed-grass prairie sites in Badlands National Park, South Dakota. There were distinct differences in both the seed bank composition and the aboveground vegetation between the off-prairie dog colony treatments and the on-colony treatments. The three on-colony treatments were similar to each other at all three sites with vegetation dominated by the forbs Dyssodia papposa, Hedeoma spp., Sphaeralcea coccinea, Conyza canadensis, and Plantago patagonica and seed banks dominated by the forbs Verbena bracteata and Dyssodia papposa. The two off-colony treatments were also similar to each other at all three sites. Vegetation at these sites was dominated by the grasses Pascopyrum smithii, Bromus tectorum and Bouteloua gracilis and the seed banks were dominated by several grasses including Bromus tectorum, Monroa squarrosa, Panicum capillare, Sporobolus cryptandra and Stipa viridula. A total of 146 seedlings representing 21 species germinated and emerged from off-colony treatments while 3069 seedlings comprising 33 species germinated from on-colony treatments. Fifteen of the forty species found in soil seed banks were not present in the vegetation, and 57 of the 82 species represented in the vegetation were not found in the seed banks. Few dominant species typical of mixed-grass prairie vegetation germinated and emerged from seed banks collected from prairie dog colony treatments suggesting that removal of prairie dogs will not result in the rapid reestablishment of representative mixed-grass prairie unless steps are taken to restore the soil seed bank.

Bison-prairie dog-plant interactions in a North American mixed-grass prairie

Both bison and prairie dogs have multiple and dramatic effects on grassland landscapes and both are considered by many to be keystone herbivores. Numerous studies have documented their independent or combined impact on grassland ecosystem processes, but there have been few attempts to simultaneously assess the individual and interactive effects of bison and prairie dogs where they co-occur. We began a long-term study in late 1994 in Badlands National Park, South Dakota, USA, to evaluate the ecological consequences of the presence or exclusion of prairie dogs, bison, or both, upon various aspects of plant community dynamics and N cycling. Five different treatments were established at three separate mixed-grass prairie sites in the park: (1) off the prairie dog colony with bison excluded, (2) off colony with continued bison utilization, (3) on colony with bison excluded but continued prairie dog use, (4) on colony with utilization by prairie dogs and bison, and (5) on colony with both excluded. There were few differences in aboveground biomass or plant species composition between the two off-colony treatments or among the three on-colony treatments, even after 3years of treatment imposition. However, aboveground biomass was >2times greater in off-colony sites than on-colony sites, primarily due to the near elimination of grasses on prairie dog colonies. Off-colony sites were dominated by a few grass species, resulting in lower plant species diversity, while on-colony sites were dominated by several forb species. Net N mineralization early in the growing season was 4times greater on prairie dog colonies than at off-colony sites, but all sites exhibited net immobilization by the latter half of the growing season. The results of this study indicate distinct differences in several ecosystem properties between on- and off-colony treatments. Whether these patterns represent relatively stable alternate states or whether distinct changes will emerge in the different herbivore treatments after several additional years is of considerable interest.

Carbon dioxide fluxes in a southern plains prairie

Atmospheric CO 2 concentrations have increased because of changes in land use and burning of fossil fuels. Grasslands are important terrestrial ecosystems that, along with other temperate and arid rangeland resources, comprise more than 50% of the world’s land area. Even though grasslands dominate the world’s landscape, their role in the global C budget has not been adequately documented. The objective of this study was to determine the relationship of vegetation structure and dynamics to CO 2 fluxes for a grass and a sagebrush-dominated Southern Plains mixed-grass prairie (latitude 36°36′N, longitude 99°35′W, elevation 630 m) and evaluate their potential for carbon sequestration. The CO 2 flux calculated at 20-min intervals was measured from mid-February to early May through mid to late December in 1995–1997 on both sites using Bowen ratio/energy balance instrumentation. Plant measurements included aboveground and belowground biomass, leaf area, and canopy height. Estimated annual net CO 2 fluxes into these systems were 257 and 23 g m −2 yr −1, respectively, for the grass and sagebrush-dominated sites. Average daily flux during the sampling period and estimated annual rate of daily CO 2 flux for the grass-dominated prairie site were greater (1.54 and 0.70 g m −2 per day) than the sagebrush site (0.01 and 0.06 g m −2 per day). Water-use was similar for these two mixed-grass prairie sites. This study indicates that these Southern Plains mixed-grass prairie communities have the potential to sequester C.

Short-term effects of fire frequency on vegetation composition and biomass in mixed prairie in south-western Manitoba

The net effects of one, two, and three spring burns in consecutive yearson the aboveground biomass, species composition, and soil variables wereassessed in two different mixed-grass prairie sites in south-western Manitoba.Precipitation in the first year was greater than the 30-year average but lowerduring the next three years. The first site (Area 6) was characterized byBouteloua gracilis, Stipa spartea,Selaginella densa and lichens, while the second, somewhatdrier site (Area 10) was dominated by B. gracilis andCarex spp. Each burn treatment was applied to 15 plots inarandom block design. Vegetation and soil data collected following the thirdburnare presented. In general, the effects of repeated burning were more pronouncedin the drier Area 10, where litter was significantly reduced with eachadditional burn. In Area 6, litter was significantly reduced only after threeyears of burning. In both sites, the cumulative effect of fire had nosignificant effect on total standing crop, even after three consecutive yearsofburning. In the drier Area 10, however, the biomass of several componentsshifted such that B. gracilis biomass increasedsignificantly and forb biomass decreased significantly after three burns. Plantcover was also affected more in the drier Area 10. The cover of B.gracilis, the dominant C4 grass, significantly increasedand Carex spp. cover decreased after two burns in thedriersite. In Area 6, the most significant effect of fire was a reduction inSelaginella and lichen cover. Bare ground increased inbothsites as the number of burns increased. There was little change in soilnutrients with burning, but soil moisture somewhat decreased and surfacetemperature significantly increased after three burns.

Leafy Spurge (Euphorbia esula) Control, Forage Production, and Economic Return with Fall-Applied Herbicides

Leafy spurge control and forage production after various fall-applied herbicide treatment rotations were evaluated in a bluegrass pasture and a mixed grass prairie. Dicamba at 2 kg ae/ha or picloram at 0.6 kg ae/ha alone or combined with 2,4-D at 1 kg ae/ha applied annually for 3 yr provided consistent leafy spurge control of 85% or better, but forage production was greater at the bluegrass pasture site than the mixed grass prairie site. Glyphosate plus 2,4-D at 0.4 plus 0.7 kg ae/ha applied annually or rotated with picloram plus 2,4-D or dicamba provided 90% or better leafy spurge control after 3 yr at the bluegrass pasture. Control was 50% or less at the mixed grass prairie site where forage production was severely reduced by glyphosate plus 2,4-D. All treatments on the bluegrass pasture provided a positive economic return averaging $227/ha after 3 yr. Picloram or picloram plus 2,4-D at the mixed grass prairie site provided net returns averaging $285/ha, but treatment rotations that included glyphosate plus 2,4-D one or more times in 3 yr averaged a net loss of $32/ha due to severe grass injury.

Vegetational Responses of a Mixed-Grass Prairie Site following Exclusion of Prairie Dogs and Bison

Combined grazing by black-tailed prairie dogs (Cynomys ludovicianus) and bison (Bison) produces and maintains a series of changes in the vegetation of prairie dog colonies. However, because their grazing patterns differ in frequency and intensity through time, their individual impacts may be different. The objective of this study was to determine the individual and combined influences of these 2 herbivores in maintaining selected vegetation characteristics of a prairie dog colony in a mixed-grass prairie at Wind Cave National Park, S.D. This was assessed by monitoring plant responses during 2 years following exclusion from grazing by 1 or both species. In spite of their different grazing patterns, prairie dogs and bison had similar and independent (i.e., additive) effects in maintaining plant community structure. For example, total above-ground biomass increased 32-36% within 2 years of removal of each species, primarily as a result of increases in accumulation of graminoid biomass. Plant species diversity, equitability, and dominance concentration were similar in all treatments both years, although there were slight decreases in relative abundance of forbs and increases in relative abundance of grominoids in the second year after removal of grazers. Mean graminoid leaf nitrogen concentration (May to September) declined slightly but significantly after removal of prairie dogs (1.49 to 1.38%) in 1985, and after bison exclusion (1.64 to 1.50%) in 1986. We suggest that rate of vegetation change following removal of grazers depends upon weather conditions, plant species composition, and prior intensity and duration of grazing.