• Grazing Intensity and Spatial Heterogeneity in Bare Soil in a Grazing-Resistant Grassland

      Augustine, David J.; Booth, D. Terrance; Cox, Samuel E.; Derner, Justin D. (Society for Range Management, 2012-01-01)
      We used very large scale aerial (VLSA) photography to quantify spatial patterns in bare soil in the northeastern Colorado shortgrass steppe. Using three pairs of pastures stocked at moderate (0.6 animal unit months [AUM] ha-1) versus veryheavy (1.2 AUM ha-1) rates, we detected greater bare soil under very heavy (mean=22.5%) versus moderate stocking (mean=13.5%; P=0.053) and a lower coefficient of variation across pastures under very heavy (0.48) versus moderate stocking (0.75; P=0.032). Bare soil exhibited significant positive spatial autocorrelation across distances of 60-120 m under moderate stocking (Moran’s I=0.14), while patchiness at this scale was eliminated under very heavy grazing (I=20.05). Across distances of 120-480 m, we observed no spatial autocorrelation with either stocking rate. Spatial autocorrelation was greatest at a separation distance of 2 m (I=0.48-0.58) but was unaffected by stocking rate at this scale. Thus, very heavy grazing did not increase spatial autocorrelation in bare soil across scales of 2-480 m. Means and variability in the distribution of bare soil were not influenced by ecological site. Bare soil increased primarily at the scale of individual plant clusters through both increases in the density of small (2-20 cm) bare patch intercepts and increases in the frequency of bare patch intercepts of 20-60 cm (rather than <20 cm). Our approach demonstrates the utility of VLSA for analyzing interactions between grazing and other landscape features and highlights the importance of spatially explicit sampling across broad scales (pastures) while testing for potential shifts in patchiness of bare soil at the scale of plant interspaces./Utilizamos fotografía de escalas aéreas muy altas (VLSA) para cuantificar patrones espaciales en suelo desnudo en un pastizal de pastos cortos del noreste de Colorado. Usando tres pares de pastizales con carga animal moderada (0.6 unidades animal por mes [AUM] ha-1) comparándolos contra pastizales con cargas animales altas (1.2 AUM ha-1), detectamos una mayor cantidad desuelo desnudo en lugares con cargas animales altas (media=22.5%) en comparación con las cargas animales moderadas (mean=13.5%; P=0.053), y un bajo coeficiente de variación a través de pastizales con cargas animales muy altas (0.48) en comparación con cargas animales moderadas (0.75; P=0.032). El suelo desnudo presentó auto-correlaciones significativas y positivas espaciales en las distancia de 60 a 120 m bajo una carga animal moderada (Moran’s I=0.14), mientras, la agregación a esta escala se eliminó dentro de áreas de carga animal alta (I=20.05). A la escala de 120–480 m, no observamos correlación alguna con ninguna carga animal. La auto correlación espacial fue la mayor en la separación de 2 m (I=0.48–0.58) pero no fue afectada por ningún nivel de carga animal en esta escala. De esta manera los lugares pastoreados fuertemente no incrementaron la auto correlación espacial en suelo desnudo en la escala de 2–480 m. Las medias y la variabilidad en la distribución de suelo desnudo no fueron influenciadas por los sitios ecológicos. El suelo desnudo se incrementó principalmente en las escalas de plantas individuales y agregados, ambas mediante el incremento en la densidad de pequeñas intercepciones de suelo desnudo (2–20 cm), e incrementos en la frecuencia de intercepciones de suelo desnudo de 20 a 60 m (en lugar de <20 cm). Nuestro enfoque demuestra la utilidad de VLSA para analizar interacciones entre el pastoreo y otras características del paisaje, y recalca la importancia del muestreo espacialmente explicito a través de amplias escalas (pastizales) mientras se evalúa el potencial de cambio de áreas a suelo desnudo a la escala de espacios entre plantas.
    • Influence of Fire on Black-Tailed Prairie Dog Colony Expansion in Shortgrass Steppe

      Augustine, David J.; Cully, Jack F.; Johnson, Tammi L. (Society for Range Management, 2007-09-01)
      Factors influencing the distribution and abundance of black-tailed prairie dog (Cynomys ludovicianus) colonies are of interest to rangeland managers because of the significant influence prairie dogs can exert on both livestock and biodiversity. We examined the influence of 4 prescribed burns and one wildfire on the rate and direction of prairie dog colony expansion in shortgrass steppe of southeastern Colorado. Our study was conducted during 2 years with below-average precipitation, when prairie dog colonies were expanding throughout the study area. Under these dry conditions, the rate of black-tailed prairie dog colony expansion into burned grassland (X ̄ 5 2.6 ha 100-m perimeter-1 y-1; range = 0.8-5.9 ha 100-m perimeter-1 y-1; N = 5 colonies) was marginally greater than the expansion rate into unburned grassland (X ̄ 5 1.3 ha 100-m perimeter-1 y-1; range = 0.2-4.9 ha 100-m perimeter-1 y-1; N = 23 colonies; P = 0.066). For 3 colonies that were burned on only a portion of their perimeter, we documented consistently high rates of expansion into the adjacent burned grassland (38%-42% of available burned habitat colonized) but variable expansion rates into the adjacent unburned grassland (2%-39% of available unburned habitat colonized). While our results provide evidence that burning can increase colony expansion rate even under conditions of low vegetative structure, this effect was minor at the scale of the overall colony complex because some unburned colonies were also able to expand at high rates. This result highlights the need to evaluate effects of fire on colony expansion during above- average rainfall years, when expansion into unburned grassland may be considerably lower. 
    • Livestock as Ecosystem Engineers for Grassland Bird Habitat in the Western Great Plains of North America

      Derner, Justin D.; Lauenroth, William K.; Stapp, Paul; Augustine, David J. (Society for Range Management, 2009-03-01)
      Domestic livestock have the potential to function as ecosystem engineers in semiarid rangelands, but prevailing management practices largely emphasize livestock production and uniform use of vegetation. As a result, variation in vegetation structure might not occur at appropriate spatial and temporal scales to achieve some contemporary conservation objectives. Here, we introduce the utility of livestock as ecosystem engineers and address potential benefits and consequences associated with heterogeneity-based management practices for conservation grazing in the semiarid rangelands of the western North American Great Plains. To illustrate the potential value of this approach, we provide specific examples where engineering effects of livestock could alter vegetation heterogeneity at within-pasture (< 100 ha) and among-pasture (<100 ha to thousands of hectares) scales to improve habitat for declining native grassland birds. Experimental evaluations of the efficacy of livestock to achieve desired modifications to vegetation structure are needed, along with the economic aspects associated with implementing heterogeneity-based management practices. Using livestock as ecosystem engineers to alter vegetation structure for grassland bird habitat is feasible in terms of application by land managers within the context of current livestock operations, and provides land managers important tools to achieve desired contemporary objectives and outcomes in semiarid rangelands of the western North American Great Plains. 
    • Opportunities for Increasing Utility of Models for Rangeland Management

      Derner, Justin D.; Augustine, David J.; Ascough, James C.; Ahuja, Lajpat R. (Society for Range Management, 2012-11-01)
      A large number of empirical and mechanistic simulation models and decision support tools have been produced for rangelands. Collectively, these models have considerably increased our fundamental knowledge and understanding of the dynamics of ecosystem functions, processes, and structure. We explore three areas where models for rangeland management are often challenging for land managers and enterprise-level decision making: 1) coping with spatiotemporal and climatic variability in implementing scenario forecasting, risk assessments, and adaptive management; 2) addressing outputs of multiple ecosystem goods and services and determining whether they are synergistic or competitive; and 3) integrating experimental and experiential knowledge and observations into decision making. Increasing the utility of models for rangeland management remains a key frontier and a major research need for the modeling community and will be achieved less by further technical advances and model complexity and more by the use of existing topoedaphic databases, the capacity to readily incorporate new experimental and experiential knowledge, and the use of frameworks that facilitate outcome-based, adaptive decision making at the enterprise level with associated economic considerations. Opportunities exist for increasing the utility of models for decision making and adaptive rangeland management through better matching of model complexity with enterprise-level, decision-making goals. This could be accomplished by incorporating a fundamental understanding of herbivory, fire, and spatiotemporal interactions with weather patterns that affect multiple ecosystem functions. Most important, effective models would allow land managers in a changing and variable climate to 1) evaluate trade offs in producing multiple goods and services, 2) optimize the application of conservation practices spatially (comparing costs and benefits accrued across different timescales), and 3) incorporate manager capacity, including experience, skills, and labor input./Se ha producido un gran número de mecanismos empíricos, modelos de simulación y herramientas para apoyar la toma de decisiones para los pastizales. En conjunto, estos modelos han incrementado considerablemente nuestro conocimiento fundamental y entendimiento de la dinámica de la función de los procesos y estructura de los ecosistemas. Exploramos tres áreas donde los modelos para el manejo de pastizales son regularmente un reto para los manejadores de pastizales y los niveles de toma de decisiones en las empresas: 1) en conjunto con espacio-tiempo y variabilidad climática en la predicción de escenarios, evaluación de riesgos y la implementación de manejo adaptativo, 2) enfocándose a las de salidas de múltiples bienes y servicios de los ecosistemas, y si estos son sinérgicos o compiten entre sí, e 3) integración del conocimiento experimental y experiencial y observaciones dentro de la toma de decisiones. Incrementar la untilidad de modelos para el manejo de pastizales permanece como una frontera clave y una necesidad e investigación muy importante para modelar la comunidad, y se logrará mediante nuevos avances técnicos y menos complejidad de los modelos y más aun mediante el uso de base de datos topoedáficos existentes, la capacidad para fácilmente incorporar nuevos conocimientos experimentales y experienciales, y el uso de marcos de referencia que faciliten los resultados, la toma de decisiones adaptativa en los niveles empresariales con las consideraciones económicas asociadas. Existen oportunidades para incrementar la utilidad de los modelos en la toma de decisiones y en el manejo adaptativo de los pastizales mediante un mejor ajuste de la complejidad del modelo con nivel empresarial y las metas en la toma de decisiones. Esto puede hacerse mediante la incorporación de un entendimiento fundamental de las actividades de los herbívoros, fuego e interacciones espacio- temporales con patrones climáticos para afectar las múltiples funciones del ecosistema. Mas importante aun, modelos efectivos podrían permitir a los manejadores de tierra en un cambiante y variable clima a 1) evaluar las ventajas y desventajas en la producción de múltiples bienes y servicios, 2) espacialmente optimizar la aplicación de prácticas de conservación (comparando los costos y beneficios acumulados atreves de diferentes escalas de tiempo), y 3) incorporar la capacidad de los administradores incluyendo experiencia, habilidades y mano de obra.
    • Prescribed Fire, Grazing, and Herbaceous Plant Production in Shortgrass Steppe

      Augustine, David J.; Derner, Justin D.; Milchunas, Daniel G. (Society for Range Management, 2010-05-01)
      We examined the independent and combined effects of prescribed fire and livestock grazing on herbaceous plant production in shortgrass steppe of northeastern Colorado in the North American Great Plains. Burning was implemented in March, before the onset of the growing season. During the first postburn growing season, burning had no influence on soil moisture, nor did it affect soil nitrogen (N) availability in spring (April-May), but it significantly enhanced soil N availability in summer (June-July). Burning had no influence on herbaceous plant production in the first postburn growing season but enhanced in vitro dry matter digestibility of blue grama (Bouteloua gracilis [Willd. ex Kunth] Lag. ex Griffiths) forage sampled in late May. For the second postburn growing season, we found no difference in herbaceous plant production between sites that were burned and grazed in the previous year versus sites that were burned and protected from grazing in the previous year. Our results provide further evidence that prescribed burns conducted in late winter in dormant vegetation can have neutral or positive consequences for livestock production because of a neutral effect on forage quantity and a short-term enhancement of forage quality. In addition, our results indicate that with conservative stocking rates, deferment of grazing during the first postburn growing season may not be necessary to sustain plant productivity. 
    • Spatial Redistribution of Nitrogen by Cattle in Semiarid Rangeland

      Augustine, David J.; Milchunas, Daniel G.; Derner, Justin D. (Society for Range Management, 2013-01-01)
      Nitrogen (N) availability can strongly influence forage quality and the capacity for semiarid rangelands to respond to increasing atmospheric CO2. Although many pathways of nitrogen input and loss from rangelands have been carefully quantified, cattle-mediated N losses are often poorly understood. We used measurements of cattle N consumption rate, weight gains, and spatial distribution in shortgrass rangeland of northeastern Colorado to evaluate the influence of cattle on rangeland N balance. Specifically, we estimated annual rates of N loss via cattle weight gains and spatial redistribution of N into pasture corners and areas near water tanks, and used previous studies to calculate ammonia volatilization from urine patches. Using measurements of plant biomass and N content inside and outside grazing cages over 13 yr, we estimate that cattle stocked at 0.65 animal unit months (AUM) ha-1 consumed 3.34 kg N ha-1 yr-1. Using an independent animal-based method, we estimate that cattle consumed 3.58 kg N ha-1 yr-1 for the same stocking rate and years. A global positioning system tracking study revealed that cattle spent an average of 27% of their time in pasture corners or adjacent to water tanks, even though these areas represented only 2.5% of pasture area. Based on these measurements, we estimate that cattle stocked at 0.65 AUM ha-1 during the summer can remove 0.60 kg N ha-1 in cattle biomass gain and spatially redistribute 0.73 kg N ha-1 to areas near corners and watertanks. An additional 0.17 kg N ha-1 can be lost as NH3 volatilization from urine patches. Cumulatively, these cattle-mediated pathways (1.50 kg N ha-1) may explain the imbalance between current estimates of atmospheric inputs and trace gas losses. While NOx emission remains the largest pathway of N loss, spatial N redistribution by cattle and N removed in cattle biomass are the second and third largest losses, respectively. Management of cattle-mediated N fluxes should be recognized as one means to influence long-term sustainability of semiarid rangelands.
    • Vegetation Responses to Prescribed Burning of Grazed Shortgrass Steppe

      Augustine, David J.; Milchunas, Daniel G. (Society for Range Management, 2009-01-01)
      Over the past century, fire has been widely suppressed in the western Great Plains, in part because of the potential negative effects on forage production for livestock. More recently, interest in the use of prescribed fire in shortgrass steppe has increased because of the potential applications for wildlife management, control of unpalatable plant species, and restoration of historic disturbance regimes. We studied the effects of prescribed burns conducted during late winter on herbaceous production, forage nitrogen content, and plant species composition of shortgrass steppe on the Pawnee National Grassland in northeastern Colorado. Late-winter burns conducted in moderately grazed sites under a wide range of precipitation conditions during 1997-2001 did not negatively affect herbaceous production in either the first or the second postburn growing season. Burning followed by a severe drought in 2002 reduced production by 19% in the second postburn growing season of 2003. Burns temporarily suppressed the abundance of broom snakeweed (Gutierrezia sarothrae) and prickly pear cactus (Opuntia polyacantha) and enhanced forage nitrogen content during May and June of the first postburn growing season. These findings suggest that, except following severe drought, prescribed burns conducted during late winter in grazed shortgrass steppe for objectives unrelated to livestock production can also have neutral or positive consequences for livestock.