1a. Objectives (from AD-416):
1. Determine CO2 effects on grassland plant production, plant species composition, and soil C dynamics. 1A. Determine responses of leaf gas exchange (C assimilation, stomatal conductance), plant water status, and plant production of tallgrass prairie assemblages to a subambient to elevated gradient in atmospheric CO2 concentration. 1B. Determine responses of soil respiration and soil organic matter pools (soil C dynamics) of tallgrass prairie assemblages to a subambient to elevated gradient in atmospheric CO2 concentration. 1C. Determine the response of species composition of tallgrass prairie vegetation to a subambient to elevated gradient in atmospheric CO2 concentration. 1D. Determine responses of photosynthetic C assimilation, biomass production, and bioenergy-relevant tissue constituents of the native grass species Panicum virgatum (switchgrass) to a subambient to elevated gradient in atmospheric CO2 concentration. 1E. Determine whether CO2 enrichment from subambient to elevated concentrations increases the potential for invasion of tallgrass prairie assemblages by a non-native grass species. 2. Determine effects of inter-annual variability in precipitation on productivity of switchgrass monocultures and mixed-species plantings of tallgrass prairie species. 2A. Compare responses of aboveground net primary productivity (ANPP) of switchgrass monocultures and mixtures of tallgrass prairie species to inter-annual variability in precipitation. 2B. Determine whether the frequency and magnitude of water limitation to ANPP of switchgrass and mixed-species plantings of prairie vegetation differ between a mollisol and vertisol soil. 3. Validate plant growth and biogeochemistry models to enable simulations of the impact of CO2 enrichment and precipitation variability on grassland production. 3A. Parameterize and validate the ALMANAC model with data from the CO2 gradient experiment and field-scale plots of switchgrass and prairie species. 3B. Parameterize and validate a coupled soil-plant-atmosphere-biogeochemistry model with plant and soil data from the CO2 gradient experiment. 4. Test the efficacy of leaf beetles Diorhabda spp. for biological control of non-native saltcedar (Tamarix spp.) infestations of western rangelands, assess ecosystem recovery following biological control treatments, and initiate biological control of Russian olive (Elaeagnus angustifolia). 4A. Measure rates of increase, mortality, and dispersal of populations of the leaf beetle Diorhabda after release into saltcedar stands in western Texas and quantify the impact of beetles on saltcedar. 4B. Evaluate the impact of Diorhabda control of saltcedar on non-target plants and on recovery of native plant and bird communities. 4C. Evalute effects of integrating herbicidal and biological control methods on the growth and mortality of saltcedar trees and on native plant and bird populations. 4D. Discover and develop biological control agents for Russian olive.
1b. Approach (from AD-416):
Expose vegetated monoliths of three soil types to a continuous gradient in atmospheric carbon dioxide ranging from low levels of the pre-industrial period to elevated concentrations predicted within the century. We will measure leaf gas exchange (carbon assimilation, stomatal conductance), plant water status, plant production, and changes in the relative abundances of tallgrass prairie vegetation growing on each soil type. Soil carbon efflux and changes in soil organic carbon content will be measured in each soil as a function of carbon dioxide treatment. We will measure the responses of photosynthetic carbon assimilation and water use efficiency, biomass production, and bioenergy-relevant tissue constituents of the native grass species switchgrass to carbon dioxide and determine whether carbon dioxide enrichment increases the potential for invasion of tallgrass prairie vegetation by a non-native grass species. We also will compare responses of aboveground net primary productivity of field-scale plantings of switchgrass monocultures and mixtures of tallgrass prairie species to inter-annual variability in precipitation on upland and lowland soils. Two simulation models, the Agricultural Land Management Alternative with Numerical Assessment Criteria model and a coupled soil-plant-atmosphere biogeochemistry model, will be validated with data from the carbon dioxide experiment and field-scale plots of switchgrass and prairie species to simulate effects of changes in both atmospheric carbon dioxide concentration and precipitation patterns on grassland ecosystems. We also will measure rates of increase, mortality, and dispersal of populations of the leaf beetle Diorhabda after release into saltcedar stands in western TX and quantify the impact of beetles on saltcedar and on rates of recovery of native plant and bird communities. The efficacy of integrating biological control with herbicidal treatment of saltcedar will be studied at three sites in western TX. New or previously discovered insects from southern France, Israel, and Kazakhstan/China will be tested as potential biological control agents for the tree Russian olive.
3. Progress Report:
During the past year, we made substantial progress in addressing each of the three objectives and related sub-objectives of our project, all of which relate to Objectives identified in National Programs 212 and 215. In order to address sub-objectives of Objective 1, we have been analyzing 5 years of data from an experiment in which assemblages of grassland plants on three soil types were exposed to a CO2 gradient spanning pre-Industrial to elevated concentrations. Our goal is to better understand the mechanisms responsible for determining how plant productivity and species abundances respond to CO2. We found that increasing CO2 from pre-Industrial to elevated levels increased the grass fraction of aboveground productivity in assemblages of perennial forbs and grasses. The pre-Industrial to elevated increase in CO2 also led to a shift in relative abundances of established species that is similar in magnitude to differences observed between mid-grass and tallgrass prairies along a precipitation gradient in the central U.S., partly by reducing the frequency with which soil water content fell below levels that caused significant plant stress. With university collaborators, we demonstrated that both CO2 and soil type strongly influence the functioning of the soil microbes responsible for the decomposition of organic matter and cycling of nitrogen and phosphorus. With ARS collaborators at Lincoln, NE, we are evaluating CO2 effects on productivity and bioenergy-relevant tissue constituents in switchgrass (Panicum virgatum). Preliminary trends in tissue constituents were inconclusive. CO2 did not affect productivity of switchgrass grown under well-watered conditions but increased tiller mass by reducing tiller number. Significant progress also was made in better understanding the response of grassland productivity to variability in precipitation, Objective 2. We found limited effects of experimentally reducing between-year and within-year variability in precipitation on productivity of tallgrass prairie. Production of dominant perennial grasses was remarkably stable in the face of differences in the timing of precipitation. This finding simplifies the task of predicting prairie responses to the changes in precipitation regimes expected to result from climate change. With university collaborators, we also began to develop and test models describing CO2 effects on grasslands, Objective 3. Progress is being made to test the Agricultural Land Management Alternatives with Numerical Assessment Criteria (ALMANAC) model's capacity to simulate CO2 effects on soil evaporation and plant transpiration.
1. Grassland productivity – soil type mediates effects of atmospheric carbon dioxide enrichment. The continuing rise in atmospheric carbon dioxide (CO2) concentration is anticipated to increase productivity of water-limited grasslands by reducing plant transpiration rates and increasing plant growth per unit of water transpired. This direct and immediate benefit of CO2 likely will depend on soil properties that regulate water availability to plants, but longer-term [and largely unexplored] effects also may depend on whether changes in water availability favor species that are more or less productive at elevated CO2 than current dominants. ARS scientists at Temple, Texas, together with university collaborators, found that grassland productivity increased more on light-textured silty clay and sandy loam soils than on a heavy clay soil because CO2 increased soil water content and increased the relative abundance of a highly productive grass on the light-textured soils. CO2 enrichment favored the more productive grass by preferentially increasing carbon uptake per unit of transpiration and intensifying plant-plant competition for light or reducing the frequency with which soil water dipped below threshold levels. We demonstrated that CO2 enrichment increased productivity most strongly on soils on which CO2 benefits were expressed both directly and indirectly, the latter via an increase in soil water content and shift to a productive species. Our results are directly relevant to scientists and land management agencies responsible for forecasting future plant productivity and its variation across the landscape to guide grassland management.
Fay, P.A., Blair, J.M., Smith, M.D., Nippert, J.B., Carlisle, J.D., Knapp, A.K. 2011. Relative effects of precipitation variability and warming on tallgrass prairie ecosystem function. Biogeosciences. 8:3053-3068.