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 (6206-11220-005-00D), all of which relate to Objectives identified in National Programs 212 and 215. Objectives of this project are to: 1) Determine effects of atmospheric carbon dioxide (CO2) enrichment on grassland productivity, plant species composition, and soil carbon (SC) dynamics, 2) Determine effects of variability in precipitation on grassland plant productivity, and 3) Validate simulation models of the impact of CO2 enrichment and precipitation variability on grasslands. 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 below-present to elevated concentrations. Results from previous work indicate that the shape of the response curve of plant productivity to CO2 could differ among soils because of differing limitations on plant growth. Our goal is to better understand the mechanisms responsible for determining how productivity responds to CO2. We also are analyzing data collected in Temple, TX, to better understand how CO2 affects the abundances of grassland species. Previous work has shown that the types of species favored and mechanisms by which change is mediated by CO2 differ among ecosystems. With ARS collaborators at Lincoln, NE, we are evaluating CO2 effects on bioenergy-relevant tissue constituents in switchgrass (Panicum virgatum). Preliminary results were inconclusive, but additional samples are being collected and analyzed. We also made progress in identifying mechanisms that regulate CO2 effects on the nutritional value of forage for livestock. CO2 enrichment increased, rather than decreased (as often predicted), element concentrations in grass assemblages by favoring species with higher element concentrations. Significant progress was made in better understanding the response of grassland productivity to variability in precipitation, Objective 2. We used data collected from around the world to show that the grassland production per unit of rainfall generally increases as the percent silt content of soil and number of plant species in the plant community increase. With university collaborators, we also began to develop and test models describing CO2 effects on grasslands, Objective 3. Collaborators adapted an existing mathematical model of plant growth and water use by changing the way in which CO2 effects on leaf pores (stomata) are modeled. Stomatal responses were modeled so as to maximize the C capture via photosynthesis per unit of water lost from leaves. Collaborators added this approach to modeling water loss to a vegetation-atmosphere simulation model to demonstrate how differences in species composition, climatic conditions, and air flow rates affect performance of the novel controlled-environment chambers that ARS operates at Temple, TX, to experimentally impose CO2 treatments.
1. Grassland productivity – mechanisms responsible for variation across landscapes and through time. Accurate estimates of grassland productivity are required at local to global scales and over daily to annual timeframes in order to inform policy and guide livestock managers. Much remains to be learned about how productivity is affected by changes in environmental variables, such as precipitation and light availability, if we are to meet the challenge of optimizing livestock production to feed the world's expanding population. ARS scientists at Temple, TX, and other rangeland research groups in the western U.S., together with university collaborators, used measurements collected on grasslands located throughout the world to 1) determine the relationship of plant productivity to spatial variation in precipitation and 2) identify factors that regulate the sensitivity of productivity to light interception by plant leaves. We found that the increase in grassland productivity per unit of increase in precipitation declined when annual precipitation was high. Plant productivity generally increased as light interception increased, but the increase was greatest when humidity levels in air were high. We show that simple extrapolations of productivity based on precipitation or light interception may not be reliable. Our results are directly relevant to scientists, personnel in land management agencies, and others who are responsible for developing estimates of plant productivity to guide grassland management.
Polley, H.W., Phillips, B.L., Frank, A.B., Bradford, J.A., Sims, P.L., Morgan, J.A., Kiniry, J.R. 2011. Variability in light-use efficiency for gross primary productivity on Great Plains grasslands. Ecosystems. 14:15-27.