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 and with and without nitrogen fertilization. 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
This is the first report for project 6206-11220-005-00D initiated in November 2009. We made substantial progress in addressing our four objectives. 1) Determine effects of precipitation variability on grassland productivity. 2) Determine effects of atmospheric CO2 on grassland productivity and the nutritional value of grasses for livestock. 3) Validate simulation models of the impact of CO2 enrichment on grasslands. 4) Test the efficacy of leaf beetles Diorhabda spp. for biological control of saltcedar (Tamarix spp.). Progress includes: 1) Plant productivity is estimated by multiplying the amount of light absorbed by plant's canopy by light-use efficiency (Eg), the latter calculated using measurements of CO2 fluxes from plant canopies. We compiled CO2 flux data from three native grasslands to determine the contribution of precipitation variability to variation in Eg. 2a) Canopy light-use efficiency is a key parameter in simulation models to estimate plant productivity. We found that photosynthetic light responses varied as a function of growth CO2 concentration for plants of three grassland species. 2b) Rising CO2 may reduce the nutritional value of forage for livestock by reducing element concentrations in plants. We measured concentrations of 11 nutritionally important elements in grasses grown at different CO2 levels. 2c) Nitrogen (N) limitation may reduce benefits of higher CO2 for leaf photosynthesis and plant biomass. We found that the response of leaf N and carbon (C) concentration to growth CO2 differed among plant species and soil types. 2d) 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. 3) We are adapting a mathematical model of plant water use by changing the way in which CO2 effects on leaf pores (stomata) are modeled. Stomatal responses will be modeled so as to maximize the amount of C captured per unit of water loss. 4) Leaf beetles, Diorhabda sp., were introduced into Texas in 2003 to biologically control saltcedar. This year, we measured the field ecology of beetles and beetle defoliation of saltcedar and made 25 redistributive releases of beetles at 15 sites. Beetles established at 7 sites and defoliated as much as 50 acres. Beetle populations and defoliation increased from late March to October along Beals Creek near Big Spring, TX, with a peak in August-September. The distance along the creek with defoliated saltcedar increased from 15 miles in October 2009 to 40 miles in July 2010. Most trees experience > 95% defoliation, but re-grow 30% of pre-defoliation leaf area each spring. Three years of repeated defoliation kills about 25% of saltcedar trees. The number of plant species is 2- to 5-fold greater in native riparian vegetation than saltcedar stands. Re-growth of native vegetation covers 90-95% of the previously bare soil beneath saltcedar trees within 1-2 years of saltcedar defoliation. Bird populations were surveyed in saltcedar stands, native woodlands, and a stand with defoliated saltcedar at Big Spring.
1. Environmental effects on the carbon (C) balance of U.S. rangelands. The balance between C uptake from the atmosphere and C release into the atmosphere (C balance) varies among years on rangelands for reasons that are not completely understood. ARS researchers at Temple, TX; Tucson, AZ; Woodward, OK; Logan, UT; Burns, OR; Mandan, ND; Reno, NV; and Ft. Collins, CO, measured the C balance on eight native rangeland ecosystems in the western USA to determine how variation in environmental variables such as light and evaporative demand affected rangeland C. The C balance varied among years both because environmental factors varied and because the response of C balance to a given change in the environment differed among years. This latter component of variability, inter-annual differences in C balance-environment relationships, explained relatively more of the overall variability in the C balance on dry than wetter rangelands. Our results indicate that the standard practice of using environmental conditions to estimate rangeland C balance, and hence the capacity of rangelands to affect atmospheric C levels, is more reliable on relatively wet than drier rangelands.
2. Change in the carbon (C) balance of western rangelands derives more from change in photosynthesis than respiration. The net C balance of ecosystems represents the relatively small difference between plant photosynthesis (C uptake from the atmosphere) and respiration by plants and soil microorganisms (C release into the atmosphere). Rangeland C balance thus is sensitive to variation in both photosynthesis and respiration. ARS researchers at Temple, TX; Tucson, AZ; Woodward, OK; Logan, UT; Burns, OR; Mandan, ND; Reno, NV; and Ft. Collins, CO, determined the contribution of changes in photosynthesis and respiration to variation in the net C balance of eight native rangeland ecosystems in the western USA. Week-to-week and among-ecosystem differences in C balance were better correlated with variation in photosynthesis than respiration. Our results imply that we should focus on determining how photosynthesis is regulated on rangelands in order to estimate the C balance of these ecosystems.
3. Atmospheric CO2 enrichment increases root biomass of a grassland ecosystem. Most organic carbon (C) in grasslands resides in soil, having been derived from plant roots. This pool of C may increase and thereby slow the accumulation of carbon dioxide (CO2) in air if increasing CO2 stimulates root production without a compensating increase in the rate at which soil organisms decompose dead roots. ARS researchers at Temple, TX, together with collaborators from Duke University, measured root biomass in grassland communities exposed to a continuous gradient in atmospheric CO2 spanning pre-Industrial to elevated concentrations. Increasing CO2 caused a significant increase in root biomass of plant communities, with linear or curvilinear responses to CO2 depending on sampling date. Carbon dioxide enrichment reduced the lifespan of roots of the dominant perennial grass when neighboring roots were frequent, implying that CO2-enriched plants readily shed or turnover roots located in zones of dense root growth. Our results indicate that increasing CO2 concentration may increase soil C storage in grassland ecosystems by stimulating root production and turnover.
Polley, H.W., Emmerich, W., Bradford, J.A., Sims, P.L., Johnson, D.A., Saliendra, N.Z., Svejcar, T., Angell, R., Frank, A.B., Phillips, R.L., Snyder, K.A., Morgan, J.A. 2010. Physiological and environmental regulation of interannual variability in CO2 exchange on rangelands in the western United States. Global Change Biology. 16:990-1002.