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United States Department of Agriculture

Agricultural Research Service

Research Project: IMPACTS OF GLOBAL CHANGES AND BIOLOGICAL CONTROL OF INVASIVE WEEDS ON WESTERN RANGELANDS

Location: Grassland, Soil and Water Research Laboratory

2007 Annual Report


1a.Objectives (from AD-416)
Determine how two manifestations of global change, atmospheric carbon dioxide (CO2) enrichment and reduced precipitation during summer, interact with regionally important differences in soil type to affect plant production and other components of the carbon (C) cycle on tallgrass prairie. Determine how history of cultivation and density and biomass of invasive woody plants affects the vertical distribution and sizes of pools of organic C in mesic grasslands. Determine whether climate change (temperature, precipitation) effects on net ecosystem exchange of C (NEE) from western rangelands may creditably be predicted from the response of NEE to seasonal and inter-annual variation in temperature and precipitation. Develop classical biological control agents for non-native weeds that have invaded western rangelands as directed by NPS. Continue research on saltcedar (Tamarix spp.) to develop the leaf beetle, Diorhabda elongata from the Mediterranean area, to control effectively saltcedars in the U.S. south of the 37th parallel, to include release methodologies, reducing mortality from biotic and abiotic factors, determining rate of spread and degree of control obtained in different ecosystems, and the need for and testing of additional agents from the Old World, and the improvement of native plant and wildlife communities and water supplies. Begin discovery, testing and release of natural enemies from the Old World for control of Russian olive (Elaeagnus angustifolia), giant reed (Arundo donax), African rue (Peganum harmala), camelthorn (Alhagi), and other invasive weeds as directed by NPS.


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. Measure plant carbon and changes in soil organic carbon content on never-plowed tallgrass prairie and on four previously cultivated grassland sites following 15 years with different densities of the shrub honey mesquite. Use continuous measurements of carbon dioxide fluxes at each of eight rangeland sites in the western U.S. to quantify relationships between net ecosystem exchange of carbon and precipitation and temperature at seasonal and inter-annual scales.


4.Accomplishments
Plant species composition regulates grassland response to carbon dioxide enrichment: The concentration of carbon dioxide (CO2) gas in air is increasing as the result of land use change and fossil fuel combustion. Plants usually respond to higher CO2 by increasing the rate at which leaves convert CO2 to plant carbon mass (carbon uptake) and by reducing the rate at which leaves expend water (water loss), but these initial responses may not be sustained if plants adjust their physiology or acclimate to higher CO2 levels. Scientists at the Grassland, Soil & Water Research Laboratory in Temple, Texas, measured effects of prior CO2 exposure on the response of grassland carbon uptake and water loss to CO2 change. Following CO2 change, CO2 uptake was 11% greater and water loss was 12% smaller, on average, at high than low CO2. Effects of short-term change in CO2 did not depend on prior CO2 exposure, but the amount by which C uptake was increased by higher CO2 increased as the proportion of broadleaf herbaceous plants (forbs) in the plant canopy (forb plus grass) increased. Our results imply that sensitivity of grasslands to the continuing rise in atmospheric CO2 concentration will depend more on how management and other factors influence plant composition than on the duration of exposure to elevated CO2 levels alone. (NP 204; Component III, Agricultural Ecosystem Impacts; Objective 3, Grazinglands (Range and Pastures))

Implications of extreme precipitation events for grassland carbon balance: Climate change driven by increasing atmospheric CO2 concentrations is causing measurable changes in precipitation patterns. Most climate change scenarios forecast continuing increases in extreme precipitation patterns for North American terrestrial ecosystems, manifest as larger precipitation events separated by longer dry periods. Changes in the size of precipitation events may differentially affect the processes controlling uptake and release of carbon (C) from terrestrial ecosystems, and therefore could alter carbon sequestration on grasslands and other ecosystems. Scientists at the Grassland, Soil & Water Research Laboratory in Temple, Texas, together with university collaborators found that more extreme precipitation patterns (longer intervals between events combined with larger events) shifted experimental grasslands toward greater net uptake of C and made C fluxes less responsive to variation in event size. More extreme precipitation regimes thus may reinforce increases in grassland C-sequestration expected to result from increasing atmospheric CO2, but may also lower plant water status and productivity. Benefits of greater carbon storage on grasslands likely will be offset by reductions in forage quantity and quality. (NP 204; Component IV, Changes in Weather and the Water Cycle at Farm, Ranch and Regional Scales; Objective 5, Climate and Weather Variability and Extremes)

Photosynthetic traits of C3 and C4 grassland species: Scientists at the Grassland, Soil & Water Research Laboratory in Temple, Texas, together with university collaborators collected data to test a basic hypothesis in grassland ecology, that grasses with the C4 photosynthetic metabolism dominate in the tallgrass prairie because physiological resource use efficiency is greater than in co-occurring plant species with C3 photosynthesis. Leaf level physiological processes related to carbon gain and water loss were measured for seven C3 and C4 species, both in quasi-natural microcosms with high resource availability and in intact grasslands where plants have been shown to experience multiple resource limitations. C3 and C4 species failed to show expected differences in resource use efficiency in field compared to microcosm conditions. Our findings suggest instead that C4 species shift their resource use strategies as conditions change through the growing season. Results provide a partial mechanistic explanation for the seasonal variation in forage quality observed on tallgrass prairie. (NP 204; Component III, Agricultural Ecosystem Impacts; Objective 3, Grazinglands (Range and Pastures))

Species abundances affect grassland productivity: Humans are changing the relative abundances of plant species on much of the landscape so that some species are becoming very abundant and others are becoming rare. Whether the increasing disparity in species abundances is affecting the capacity of grasslands and other ecosystems to meet human needs is not known. To determine effects of changing species abundances on grasslands, scientists at the Grassland, Soil & Water Research Laboratory in Temple, Texas, experimentally varied the proportional contributions of species to the total number of plants in small plots. Aboveground biomass produced by species mixtures varied with changes in species ratios in only one of the six species combinations studied. For three of six species mixtures, however, species abundances determined whether the biomass of mixtures exceeded the biomass expected based on the yield of each species when grown alone. Our results indicate that the productivity of grasslands may be sensitive in the short-term to changes in species ratios caused by grazing, fire, or herbicide application. (NP 204; Component III, Agricultural Ecosystem Impacts; Objective 3, Grazinglands (Range and Pastures))


6.Technology Transfer

Number of non-peer reviewed presentations and proceedings10
Number of newspaper articles and other presentations for non-science audiences1

Review Publications
Nippert, J.B., Fay, P.A., Knapp, A.K. 2007. Photosynthetic traits in C3 and C4 grassland species in mesocosm and field environments. Environmental and Experimental Botany. 60:412-420.

Polley, H.W., Dugas, W.A., Mielnick, P.C., Johnson, H.B. 2007. C3-C4 composition and prior carbon dioxide treatment regulate the response of grassland carbon and water fluxes to carbon dioxide. Functional Ecology. 21:11-18.

Polley, H.W., Wilsey, B.J., Tischler, C.R. 2007. Species abundances influence the net biodiversity effect in mixtures of two plant species. Basic and Applied Ecology 8:209-218.

Last Modified: 9/22/2014
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