Location:2009 Annual Report
1a. Objectives (from AD-416)
Evaluate how management practices and disturbance processes interact to influence A) transitions/thresholds in ecological phases and states, B) plant community heterogeneity and nesting habitat for grassland birds, C) mechanisms and risk of weed invasion, and D) temporal dynamics of key ecological indicators of rangeland health. Subobjective A. Determine the influences of season and intensity of grazing, season and frequency of prescribed burning, and shifts in stocking rate on plant species composition, plant diversity, biomass production, animal gains and nesting habitat of a bird species of concern. (Augustine 0.2, Blumenthal 0.2, Derner 0.5). Subobjective B. Evaluate the influences of fire X grazing interactions (i.e., patch burning) and prairie dog disturbances on within-pasture cattle grazing distribution, consequences for plant community heterogeneity and nesting habitat for a bird species of concern (Augustine 0.5, Derner 0.2, Morgan 0.2). Subobjective C. Determine how disturbance interacts with enemy release (the loss of specialized herbivores and diseases in the exotic range of a plant species) to influence weed invasion and the success of biological control. (Blumenthal 0.4, Morgan 0.1). Subobjective D. Assess the temporal dynamics of key ecological indicators of rangeland health (plant cover and bare ground) for entire pastures in sagebrush and shortgrass steppe. (Booth 0.8).
1b. Approach (from AD-416)
The planned research is designed to integrate contemporary goals of both livestock production and conservation in semiarid rangelands. Research will be conducted in shortgrass steppe, northern mixed-grass prairie and sagebrush steppe. Two experiments are replicated across three ARS locations (Miles City, MT; Nunn, CO; Woodward, OK) to determine ecological consequences of fire seasonality, return interval and grazing interactions along a north-south gradient in the western Great Plains. Rangeland monitoring efforts at two ARS locations with contrasting vegetation (grass-dominated shortgrass steppe, Nunn, CO; shrub-dominated sagebrush steppe, DuBois, ID) will use newly-developed techniques involving very large-scale aerial photography to assess plant cover and bare ground, and incorporate this information into a recently developed index to assess landscape function. Understanding the mechanisms that control disturbance effects on plant communities and animal responses will contribute to the development of innovative management strategies that optimize livestock production and conservation goals. In addition, because state-and-transition models function as a means for organizing current understanding of the processes resulting in stability and change in ecological systems, findings from these experiments will be incorporated into revised state-and-transition models of plant community dynamics that more accurately accommodate multiple successional pathways and stable states.
3. Progress Report
Efforts continue to emphasize fundamental ecological understanding and management-relevant, applied research for private and public land managers in semi-arid rangelands of the western US. Research projects address contemporary issues at the interface of production and conservation concerns at multiple spatial and temporal scales. We are in the third year of a multi-ARS location (Miles City, MT–northern mixed-grass; Nunn, CO–shortgrass; and Woodward, OK–southern mixed-grass) grazing by fire interaction experiment. Assessments of soil moisture, plant growth, animal movement and distribution patterns, animal weight gains, plant productivity, composition, diversity, vegetation structure, grassland bird nesting patterns, and bare ground and plant cover continue. Newly added monitoring efforts in 2009 addressed nitrogen cycling, grasshopper abundance and composition, and use patterns by pronghorn antelope. We are also in the fourth year of a multi-ARS location (same as above) interactive disturbance experiment with fire frequency (annual vs. 3 year) and seasonality (spring vs. fall). Assessments of plant productivity, composition, diversity, cactus response, and bare ground and plant cover continue. Newly added monitoring efforts in 2009 have addressed soil moisture and temperature dynamics. Monitoring of animal movement and distribution patterns, animal weight gains, plant cover and bare ground response patterns following colonization by prairie dogs, and subsequent reduction of impact of these keystone species by plague, continue in a grazing gradient study in shortgrass steppe (no grazing, light summer grazing, moderate summer grazing, heavy summer grazing, very heavy summer grazing, very heavy summer grazing, moderate summer grazing with prairie dogs). Added in 2009 was the application of the same stocking rates (moderate level) to a large landscape (16,000 acres) with differences in pasture-level attributes of topography, plant community, production potential, soils, and prior management practices to determine the influence of these factors on animal weight gains. For northern mixed-grass prairie, effects of reducing stocking rates from previous heavy levels to light or no grazing are being assessed for animal weight gains, plant productivity, plant composition and diversity, bare ground and plant cover, and added in 2009, was small mammal abundance and diversity. Efforts to understand causes of weed invasion have focused on the compilation and analysis of data on hundreds of European plant species that have been introduced to the US. We have discovered that fast-growing plant species, adapted to moist, nitrogen-rich environments host a larger number of pathogen species in their native range, and consequently lose a larger number of pathogens in upon introduction, than do slow-growing plant species adapted to dry, nitrogen-poor environments. Unit scientists continue to be highly involved in literature assessments and monitoring efforts regarding the effectiveness of major conservation practices employed on private lands through the USDA-Natural Resources Conservation Service Rangeland Conservation Effects Assessment Program (CEAP).
1. Using Livestock as Ecosystem Engineers in rangelands. Management practices for rangelands have largely emphasized livestock production and the uniform use of vegetation, but emerging ecosystem services desired by society and contemporary conservation objectives from these lands may not benefit from these same prior management practices. As a result, the ARS Rangeland Resources Research Unit, in cooperation with the University of Wyoming and Shortgrass Steppe Long-Term Ecological Research Project, have introduced the use of livestock at ecosystem engineers (tools) to alter structure and function of rangelands, and provided examples where livestock can be used to improve habitat for decline native grassland birds in the western Great Plains. The use of livestock as ecosystem engineers to alter vegetation structure and function for ecosystem services is feasible for application by private and public land managers within the context of current livestock operations with some modifications, and provides these land managers with important tools to achieve desired contemporary conservation objectives.
2. Vegetation responses to prescribed fire in shortgrass steppe. Prescribed burning programs have recently been implemented on five National Grasslands encompassing >1 million acres of public rangelands in the western Great Plains to enhance wildlife habitat, control unpalatable plant species, and restore historic disturbance regimes, but livestock producers have been concerned about potential negative consequences for forage production. ARS scientists at Cheyenne, WY, and Fort Collins, CO, studied effects of prescribed burns conducted during late winter on forage production, forage nitrogen content, and plant species composition of shortgrass steppe in northeastern Colorado. Burns conducted under a wide range of precipitation conditions during 1997–2001 did not negatively affect forage 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. Prescribed burning 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. Findings show 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.
3. Fluxes of carbon dioxide from North American Rangelands. Interest in quantifying fluxes of greenhouse gases like carbon dioxide is increasing due to a growing concern that atmospheric build-up of these gases in Earth’s atmosphere is causing major climatic shifts around the world. This large-scale collaborative experiment used Bowen ratio micrometeorological tower methodology to quantify fluxes of carbon dioxide in four rangelands of the Great Plains, two of the desert Southwest, and two Northwest sagebrush steppe sites. Both sagebrush steppe sites operated as sinks for C (they assimilated C from the atmosphere and deposited it into plants and soils), three of four Great Plains grasslands were sinks, but the two Southwest hot desert sites were sources of C on an annual basis, releasing C into the atmosphere. The results demonstrated that native rangelands, which comprise about half of the Earth’s terrestrial landscape, may be important sinks for atmospheric CO2, and also that periods of significant C uptake during any one year are brief, often lasting less than three months. These results are important for understanding the role of rangelands in the global C cycle, and also provide useful information on how to manage these lands to optimize C sequestration.
4. Manipulating nitrogen reduces plant invasion in semi-arid rangeland. Diffuse knapweed and Dalmatian toadflax are among the most problematic invasive weeds in western rangeland, reducing forage production and biological diversity on millions of acres of public and private land. High levels of nitrogen in the soil, due to nitrogen deposition or disturbance, may contribute to their invasion. Reducing nitrogen has been suggested as a long-term solution. Little is known, however, about how nitrogen influences invasion in dry ecosystems where water, rather than nitrogen, often limits plant growth. We found that experimental reductions in nitrogen reduced invasion of diffuse knapweed by 95% and completely prevented invasion by Dalmatian toadflax. These results show that reducing nitrogen can help control invasive weeds even in relatively dry rangeland ecosystems. More broadly, they suggest that minimizing N deposition and disturbance will be required to stem the spread of invasive plants in western rangeland.
5. Gaps between plants and functioning of rangeland ecosystems. An important indicator of rangeland health is the proportion of the soil surface covered by gaps between plants as this influences erosion from wind and water resulting in potential off-site transport of soil and changes in water flow patterns, infiltration, and runoff. Livestock grazing management practices such as stocking rate can modify the structure and function of rangelands through effects on species composition of plant communities and morphology of plants. The ARS Rangeland Resources Research Unit, in cooperation with the University of Wyoming, using a long-term stocking rate (25 years) study in the northern mixed-grass prairie, demonstrated that heavy stocking rates altered the vegetation structure to a bunchgrass-dominated community that had a higher number of gaps but these gaps were smaller than those occurring in a rhizomatous-dominated community under no grazing. This resulted in less of the soil surface being occupied by gaps between plants in the heavy grazed area (68%) than the ungrazed area (87%), but this difference was offset by greater litter cover in the ungrazed area. The alteration of the size and distribution of gaps between plants by livestock grazing can be mitigated by management practices that increase litter cover in these gaps to reduce the potential of erosion from wind and water, and maintain proper functioning of this rangeland ecosystem which covers 75 million acres.
6. Detecting a difference in vegetation cover in shortgrass steppe. Ground-based methods of interpreting indicators of rangeland “health” are labor intensive, not well suited for vast expanses of rangelands, and are often a point of disagreement among livestock operators, land management agencies, and environmental activists. ARS scientists in Cheyenne, WY, tested nadir (vertical) aerial and ground digital photography as a sampling method for detecting stocking-rate differences across 320 acres of shortgrass prairie; the resolution of both aerial and ground images was sufficient for single grass blades (1 mm—0.039 inches--per pixel). Manual (SamplePoint) and automated (VegMeasure) image-analyses software programs were used with both aerial and ground images. Vegetative cover changes due to grazing were detectable within ±5%. Poor agreement was found between automated and manual image-analysis methods, but high agreement occurred between manual analyses of aerial and ground imagery. High-resolution digital-image rangeland monitoring can increase monitoring speed and accuracy while reducing costs and providing a permanent, difference-detectable record of resource conditions, even for relatively homogenous systems like the shortgrass prairie. Image-based monitoring can contribute to better management decisions and quantitative, rather than qualitative, discussions among rangeland stakeholders.
7. Removing the shadows from remotely-sensed images. Shadows in nadir (vertical) space, aerial and ground images have historically contributed to inaccurate or incomplete measurements from shadow-affected images. ARS scientists at Cheyenne, WY, created composite images using multiple, varied-exposures—called high-dynamic range or HDR images--to remove most of the shadow from a set of digital images. HDR was then compared with conventional images of the same grassland plots for measurements of ground cover and various measurements of image technical qualities. Because of reduced shadow, HDR images showed a 26% increase in the area of the images that could be studied and measurements taken. This is the first demonstration of the successful application of HDR imagery in remote sensing. The technique has potential application in all fields where measurements are made from space, aerial or ground images. Currently there are several problems to be addressed, including decreased image sharpness due to minor misalignment of images or moving vegetation, time required to create HDR images, and difficulty with acquiring HDR images from moving platforms.Blumenthal, D.M., Mitchell, C.E., Pysek, P., Jarosik, V. 2009. Synergy Between Pathogen Release and Resource Availability in Plant Invasion. Proceedings of the National Academy of Sciences 106(19):7899-7904.