Location:2011 Annual Report
1a. Objectives (from AD-416)
The semi-arid grasslands of the western Great Plains, mixed-grass prairie and shortgrass steppe, provide a tremendous array of ecosystem services, including livestock forage, a diversity of native plants and animals, resistance to biological invasion, and carbon storage. Global change is expected to dramatically change grasslands and associated ecosystem services, but the nature of its impacts, and the mechanisms underlying those impacts, remain difficult to predict. In water-limited ecosystems, elevated CO2 and warming can have particularly strong and complex effects because, in addition to their direct effects, they alter water availability. Two main objectives will drive our research program over the next five years to understand how these changes might impact the ecosystem services of western rangelands. The first objective is to assess effects of predicted global changes on ecosystem services in a northern mixed-grass prairie. This will be accomplished by determining the effects of temperature, CO2 and precipitation on plant productivity, plant diversity, forage quality, community composition, weed invasion and the ability of native plant communities to recover from disturbance. The biogeochemistry underlying these responses will be studied to improve our understanding of ecosystem responses and to improve algorithms in biogeochemical models like Daycent. We will also evaluate whether and how responses of invasive species differ from those of native species. Our second objective is to develop knowledge and tools that allow rangeland managers to minimize greenhouse gas emissions. We will determine how temperature, CO2 and precipitation influence land-atmosphere exchanges of trace gases and soil carbon (C) storage, and evaluate the relative importance of water, nitrogen (N) and C limitation in regulating C storage. We will use this information plus additional soil C and CO2 flux data from long-term grazing experiments to determine the potential to mitigate greenhouse gas emissions through grazing management, and assess tradeoffs between mitigation and rangeland productivity.
1b. Approach (from AD-416)
To address our first objective concerning the responses of rangelands to global changes, we will use a well-replicated Free Air CO2 Enrichment (FACE) and warming experiment to determine how global change influences the northern mixed-grass prairie. We will examine responses of plant production and quality, composition of native plant communities, carbon and nitrogen cycling, and plant invasion. To understand the mechanisms underlying these responses, we will make extensive use of gas exchange, stable isotope, soil water and nitrogen monitoring, and computer simulation methods. We will use additional treatments to learn how seasonality of precipitation influences the northern mixed-grass prairie, and how the magnitude of those effects compares to effects of CO2 and warming. To address our second objective concerning greenhouse gas mitigation tools, we will measure soil respiration and fluxes of nitrous oxide (N2O) and methane (CH4) using static chambers, and net ecosystem CO2 exchange (NEE) using dynamic chambers within plots of the FACE, warming and irrigation manipulative experiment. Results from the static and dynamic chambers will allow us to quantify CO2-enrichment and warming effects on soil trace gas fluxes and ecosystem level CO2 fluxes, and how these fluxes are related to soil moisture and other environmental factors. We will also take advantage of three ongoing NP215 long-term grazing studies to assess the effects of grazing management strategies (stocking rate and season of use) on the size and dynamics of soil C and N pools, and the potential of these strategies to mitigate greenhouse gas emissions in NMP and SGS. We will use natural variation in precipitation to determine the relative influence of above- and below-average years of precipitation on C and N pool changes. The insights provided by these experiments will help scientists and land managers adapt management practices to sustain ecosystem services in the face of global change, and provide critical information for policy makers.
3. Progress Report
Research under project 5409-110-005-00D is guided by two objectives: (1) Assess effects of predicted global changes on ecosystem services in northern mixed-grass prairie, and (2) Develop knowledge and tools that allow rangeland managers to minimize greenhouse gas emissions. Both objectives are centered on the Prairie Heating and CO2 Enrichment (PHACE) Experiment in which ambient CO2 concentration, temperature and soil water are all being manipulated to further our understanding of how semi-arid rangelands respond to multiple global change factors. The second objective also includes measurements (to begin in 2011) of soil carbon within a separate, long-term grazing experiment. A core group of scientists from ARS, the University of Wyoming, Colorado State University, and the Biometeorology Institute in Florence, Italy, plus several graduate students and post docs continue to collaborate on this unique project. The experimental plots are divided into two halves, with one side comprised of a native northern mixed-grass prairie, and the other side seeded under different disturbance regimes with various native and invasive plant species. This plot arrangement allows us to evaluate both the basic responses of this rangeland to climate change, and also to investigate how global changes interact with disturbance and plant invasion. Our results to date suggest that the effects of elevated CO2 and warmer temperatures depend to a large extent on the combined effects of these two factors on soil plant water relations, and that warm-season C4 grasses appear to prosper under these future conditions. Due to higher-than-expected water savings from elevated CO2, our results suggest that productivity in such semi-arid rangelands may be greater under climate warming than previously suspected. However, we are also learning how a number of other plant and soil attributes, particularly the cycling of soil/plant N may determine the ultimate responses of this rangeland to climate change through competition for soil resources. We completed experimental work on Dalmatian toadflax (Linaria dalmatica), learning that future environmental conditions lead to a 13-fold increase in its invasion of mixed-grass prairie. We used open space in the experiment to begin a study of cheatgrass (Bromus tectorum), which has a winter-annual life history, and may therefore be strongly influenced by warming. In summer, 2011, A DOE grant was funded by a collaborator that will incorporate our results into modeling exercises to evaluate the long-term effects of climate change on C cycling, and that will extend the PHACE experiment through 2013. Finally, the addition of an ARS-funded PostDoc to the project is initiating new studies on 1) the evaluation of leaf economic traits and their involvement in climate change responses of semi-arid rangelands, and 2) the role of hydrology in the sensitivity of semi-arid rangelands to climate change. In other research, we will return to a long-term grazing experiment where soil samples will be collected next year and assayed in order to evaluate the long-term effects of cattle grazing on soil C sequestration.
1. Carbon dioxide eliminates desiccation in warmed semi-arid rangelands. Climate change is expected to bring warmer, desiccating conditions to many world rangelands. However, many analyses have not considered the direct effect of rising CO2, which ARS scientists hypothesized would positively improve plant water use efficiency, thereby off-setting the negative effects of warming-induced desiccation. ARS scientists in Cheyenne, WY, Fort Collins, CO, and Maricopa, AZ, plus collaborators from the University of Wyoming created higher CO2 and slightly warmer temperature conditions expected to occur in the second half of this century. They discovered that combined elevated CO2 and warmer temperatures favored growth of warm-season, perennial grasses, and that CO2 completely reversed the desiccating effects of the warmer temperature in a typical native semi-arid prairie environment. These results are helping climate change scientists make better predictions about how rising CO2 will affect the responses of rangelands to climate change, and are also being used to develop adaptive management strategies for ranchers and public land managers.
2. Elevated atmospheric CO2 affects grazing tolerance of grasses. Evaluations of rangeland sensitivity to climate change rarely address potential interactions with the effects of grazing, even though livestock grazing is the primary land use in rangelands. ARS scientists in Cheyenne, WY and Fort Collins, CO and collaborators at Washington and Lee University studied two dominant perennial grasses from the western Great Plains to assess whether increases in atmospheric CO2 might influence short-term mechanisms that these species use to recover from grazing. Under current CO2, the warm-season grass, blue grama (Bouteloua gracilis), increased its allocation of photosynthesis-derived sugars to roots after plants were defoliated; whereas the cool-season grass, western wheatgrass (Pascopyrum smithii), reduced its post-defoliation allocation of assimilate to roots. However, these responses were not altered for plants grown under elevated atmospheric CO2. One clear interactive effect of shoot removal and elevated CO2 was a substantial increase in photosynthesis rate of defoliated versus non-defoliated western wheatgrass, but only under elevated CO2. These findings suggest that elevated CO2 may enhance grazing tolerance of western wheatgrass. Furthermore, simulated grazing of western wheatgrass increased leaf nitrogen to the same extent that elevated CO2 reduced it. These findings indicate that grazing management can potentially be used to counter the negative effects of CO2 on forage quality for this important forage species in the western Great Plains.
3. Plant responses to decreasing variation in precipitation. Since water is the limiting variable on most rangelands, responses of vegetation to altered precipitation regimes can have substantial effects on the productivity and function of these important ecosystems for goods and services desired by society. ARS scientists in Cheyenne, WY, Fort Collins, CO, and Temple, TX, in collaboration with a scientist from Oklahoma State University altered the precipitation regimes in a southern tallgrass prairie by removing variation among and within years to determine responses of plant diversity and biomass. Reducing precipitation variability had limited effects on total aboveground biomass, grass and forb biomass, and biomass of key species; and species richness, species diversity, species evenness, and functional group richness and diversity all were similar across the precipitation treatments. These findings indicate that the southern tallgrass prairie ecosystem is adaptable to changes in precipitation. This adaptability suggests that predicted increased variation in future annual precipitation patterns may not significantly alter the management of these lands.
4. Elevated CO2 and warming effects on methane uptake. Methane is a ‘greenhouse gas’ that tends to be emitted by agricultural land ecosystems in wet climates and taken up in dry climates. There is a considerable amount of uncertainty about how variations in weather and management affect the exchange of methane between rangelands and the atmosphere. ARS scientists in Cheyenne, WY and Ft. Collins, CO, plus a collaborator from Colorado State University conducted a field experiment in which ambient CO2 was doubled and temperatures were slightly increased above ambient conditions. These conditions are expected to occur in the second half of this century. They found that under dry conditions typically found in semiarid rangelands, warming reduced soil methane uptake and CO2 promoted it. These responses tended to be opposite to those observed in wetter ecosystems, and add to a growing knowledge base that the effects of climate change and rising CO2 on methane fluxes may differ substantially between wet and dry ecosystems. This information is being utilized by climate change scientists to improve their ability to predict how future climate change might be modified by feed-backs of methane fluxes from terrestrial ecosystems.
5. Global climate change affects soil organic matter dynamics in temperate rangelands. There is considerable uncertainty about the effects of rising CO2 and climate change on soil organic matter, and how possible feed-backs in the terrestrial soil carbon cycle might alter climate change through future releases of greenhouse gases from the soil. A team of scientists from the University of Wyoming and ARS scientists in Cheyenne, WY and Ft. Collins, CO assessed how combined warming and rising CO2 affect the cycling of carbon through plants and soils in a native mixed-grass prairie. They demonstrated that the simulated climate change affected both resistant and easily-decomposed organic matter, and that plant activity modulated these soil climate change responses. The results add to a growing body of literature which suggests plants play an important role in regulating the cycling of soil organic carbon by soil microbes. Unfortunately, these results suggest that predicting how the carbon cycle will be affected by climate change and possibly feed-back on climate change is extremely complicated. The practical implication is that predictions of terrestrial greenhouse gas emissions from terrestrial ecosystems remain uncertain.
6. Synthesis: Climate change impacts on U.S. agriculture and rangelands. A large and bewildering literature faces those who wish to learn about climate change and its impacts on agriculture. Thousands of papers have been published on the topic, and the complex nature of climate change challenges a quick study. ARS scientists in Cheyenne, WY, Fort Collins, CO, Temple, TX, and Ames, IA; plus collaborators from the University of Maryland published two synthesis articles to: 1) interpret the latest scientific literature on climate change, and 2) develop a framework to better understand apparent conflicting and often confusing results from climate change experiments. The articles present potential implications of climate change on agricultural and rangeland ecosystems, and include management options and future challenges for U.S. agriculture. This information is being disseminated in companion government reports to governmental entities, including web access to the US Global Change Research Program, where interested ranchers, public land managers and others may download the documents (http://www.usda.gov/oce/climate_change/reports.htm) and learn more about the implications of climate change for US agriculture.
7. Precipitation has greater impact on water losses due to evaporation and plant transpiration, and plant productivity than livestock stocking rate. As climate change develops there is an increasing need for ranchers and public land managers to consider the implications of land management practices on the goods and service society obtains from rangelands. ARS scientists in Cheyenne, WY and Ft. Collins, CO and colleagues from Colorado State University and the University of Colorado implemented a livestock grazing experiment on the short-grass prairie to evaluate how different stocking rates affect the utilization of water and plant productivity of this native rangeland. They determined that while the removal of vegetation via animal grazing has the potential to significantly reduce the rate at which water is lost from this ecosystem via evaporation and from plant transpiration, environmental conditions are generally so dry that stocking rate has little significant effect on these water losses. Year-to-year variation in precipitation has far more important effects on hydrology and plant productivity than stocking rate. The results help scientists and rangeland mangers determine the extent and conditions under which stocking rate might be used as a management tool to manage soil water content as well as plant productivity.
8. Light use efficiency varies among rangeland types. Accurate assessments of plant productivity are needed to understand how management and climate effect the basic functioning of terrestrial ecosystems. A common method for measuring productivity is to estimate the amount of light an ecosystem intercepts by its leaves to develop a parameter that converts light energy into plant biomass. ARS scientists in Cheyenne, WY, Ft. Collins, CO, Temple, TX, Mandan, ND, and Woodward, OK evaluated light use efficiency of a northern mixed-grass prairie, a shortgrass prairie, and a southern mixed-grass prairie. They determined that ecosystem light use efficiency varies among different rangeland types in the Great Plains, and also that it can vary substantially in the same rangeland in different years. These results suggest that greater attention needs to be given to techniques, which use light use efficiency estimates to estimate plant productivity, and that models and remote sensing techniques need to improve these parameters estimates for accurate estimates of plant productivity.
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