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United States Department of Agriculture

Agricultural Research Service


Location: Grassland, Soil and Water Research Laboratory

2009 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. Identify and evaluate candidate biological control agents in Europe and Asia through exploration in collaboration with ARS foreign labs in Montpellier and Beijing and cooperators in Israel, Turkmenistan, Kazakhstan, Turkey, and China; conduct host-range and biological testing at foreign locations and in quarantine at Temple, TX. Develop methodologies for efficient releases and establishment in the field, and for determining and avoiding or mediating the factors that limit control; monitor effectiveness of approved foreign insects in cooperation with colleagues in western states; assess long-term impacts of declining weed densities on population dynamics of native riparian and rangeland communities.

3.Progress Report
We analyzed measurements of CO2 exchange from eight native rangeland ecosystems in the western U.S. in order to determine effects of environmental variability on rangeland carbon (C) balance. The C balance of rangelands 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. Among-year variation in the response of C balance to the environment contributed relatively more to variability on dry than wetter rangelands. Carbon balance likely varies among years because of year-to-year changes in the biological processes that regulate CO2 uptake and release. Simulation models are important tools for understanding and predicting effects of rising CO2 and climate change on the productivity and C storage of grasslands. With collaborators from Duke University, we developed a simulation model that accurately captures effects of rising CO2 on plant C uptake and water loss, two fundamental factors determining plant productivity and C storage capacity. The model simulates grassland responses to varying temperature and precipitation, providing a comprehensive understanding of grassland plant responses to climate scenarios expected for the Southern Plains. Plants have evolved sensitive mechanisms to minimize water loss during CO2 uptake. With collaborators from Duke University, we have shown that environmental effects on the ratio of CO2 uptake to water loss can be characterized using an optimization approach. This technique will permit better estimates of C gain on periodically water-limited ecosystems, like grasslands, and thereby extend our capacity to predict effects of climate change on these ecosystems. Saltcedar biological control is increasing rapidly at Big Spring, TX, following the first successful release of Diorhabda beetles south of the 37th parallel along Beals Creek. The area defoliated by beetles increased from 2 acres in 2005 to 6.4 mi and 143 acres along the creek in 2008, plus 10 satellite colonies within a 7 x 13 mile area. During FY-2009, the beetle dispersion and defoliation increased rapidly, estimated to be 15 miles and 1,000 acres by September. The saltcedar trees continue to die, about 25% dead after 3 years of defoliation. Local grasses and forbs have recovered naturally and luxuriantly at several locations within 1 or 2 years after the saltcedar defoliation with no herbicides or other saltcedar controls. The 5 years of data provide a basis for analyzing interactions between saltcedar control and ecosystem recovery. During spring 2009, Diorhabda beetles were released at 31 new locations in Texas: Uzbek beetles on the Pease, Upper Colorado and Pecos Rivers, Crete beetles on the Colorado, Pecos and Rio Grande Rivers, and Tunisian beetles on the Pecos and Rio Grande. Beetles are increasing rapidly along the Rio Grande, Colorado and Pease Rivers, but have not overwintered and establishment is not yet confirmed. Weaker establishments are located near Big Spring and on the Pecos River.

1. Greater plant diversity stabilizes forage production on grasslands: Plants are the foundation of all life on Earth, providing food that is consumed either directly or indirectly by humans and animals. We must better understand how to maximize plant growth or production while reducing year-to-year variability in plant production if we are to support an ever-expanding human population. A scientist at the Grassland, Soil & Water Research Laboratory, together with university collaborators, transplanted grassland plants in various combinations into field plots in central Texas to determine whether the amount of plant production and its variability among years depended on the number of plant species in each plot. The amount of plant material produced each year was greater on average and varied less among the 8 years of measurements in plots planted with more species partly because species responded differently to year-to-year changes in weather. Years during which some species grew poorly were years during which other species grew well. Our results indicate that we may both increase the amount of plant material produced and reduce variability in plant production among years by increasing the number of plant species present in grasslands that currently contain few species.

2. Plant diversity is smaller in grasslands with exotic than native plant species: Plant species typically are fewer and exhibit greater differences in relative abundances on grasslands dominated by plants that were introduced to the U.S. (exotics) than by plants that originated in this country (natives). It is not known, however, whether these components of plant diversity are inherently lower in exotic than native plant communities or result from greater soil fertility, more frequent disturbances, or other factors often associated with exotic invasions. In order to determine where exotic species themselves cause lower diversity, a scientist at the Grassland, Soil & Water Research Laboratory, together with university collaborators, measured changes in diversity over time in experimental plant communities planted to either all exotic or all native perennial plants common to the Blackland Prairie region of central Texas. Species diversity declined more rapidly in exotic than native plant communities because species that were most productive when grown alone dominated other species in communities of exotics, whereas less-productive species were favored in communities of native species. Perhaps because of their long history of association, native species appear to better partition available plant resources and thereby to co-exist than do their exotic counterparts. Our results indicate that introduced plants may be reducing the number and variety of plant species on grasslands.

3. Rising atmospheric carbon dioxide concentration slows isoprene emissions from plants: Plants release organic compounds into the atmosphere that may react with other chemical constituents in air to regulate the production and lifetime of atmospheric ozone and methane, gases that contribute to what is commonly known as global warming. Isoprene is among the most influential of the volatile compounds released by plants. However, the rate at which isoprene is emitted from plant leaves may depend on the concentration of carbon dioxide gas (CO2) in the atmosphere. Scientists at the Grassland, Soil & Water Research Laboratory in Temple, TX, together with university collaborators, grew trees of two species at lower-than-present and at elevated CO2 concentrations to determine how recent and anticipated increases in atmospheric CO2 concentration affect isoprene emission rates. Isoprene emissions were greater at low CO2 levels representative of the pre-Industrial period than at the current CO2 concentration for eucalyptus, but not for sweetgum. By slowing emissions of isoprene, atmospheric CO2 enrichment may reduce the concentration and lifetime of trace gases like ozone and methane that contribute to global warming.

4. LYCOG – A unique facility for exposing ecosystems to a gradient in atmospheric CO2: In order to predict grassland productivity, diversity, and capacity to support livestock production in the future, it is crucial to understand how these ecosystems respond to rising atmospheric CO2 and how CO2 effects may vary spatially with changes in soil type. Scientists at the Grassland, Soil and Water Research Laboratory validated the utility of the Lysimeter CO2 Gradient (LYCOG) facility as a field tool for applying a continuous gradient in atmospheric CO2 concentration (250-500 ppm) to grassland vegetation while regulating air temperature to track ambient conditions. The facility consisting of elongated chambers also is an advancement over previous CO2 gradient facilities because it includes weighing lysimeters and systems to collect and measure the volume of soil water drainage, thus permitting closure of the soil moisture budget. The LYCOG facility was used successfully to create and regulate the desired CO2 gradient with no major variation in other factors such as air and soil temperature that may confound CO2 effects and complicate their interpretation, nor with major confounding effects from the infrastructure on leaf photosynthesis and soil respiration. Results demonstrate the utility of elongated chambers like LYCOG for determining how atmospheric CO2 enrichment will affect soil water balance and other ecosystem processes.

5. Dominant grasses of tallgrass prairie differ in response to warming and altered precipitation: Two predicted manifestations of climate change are continuing increases in surface temperatures and altered precipitation patterns. The response of dominant plant species to climate changes will play an important role in determining the potential for grasslands and other ecosystems to support livestock grazing and other uses in the future. In order to understand how climate change may affect tallgrass prairie ecosystems, a scientist at the Grassland, Soil & Water Research Laboratory and university collaborators measured the responses of leaf level water balance and carbon uptake to warming and altered precipitation patterns in two C4 grasses. The grasses, Andropogon gerardii (big bluestem) and Sorghastrum nutans (Indiangrass), were grown in plots warmed to 2–4 degrees C above ambient using infrared lamps and watered to create a more extreme precipitation pattern. Both species responded to warming and precipitation treatments, with Sorghastrum experiencing larger effects in carbon uptake than Andropogon. Results imply that the relative abundances of dominant grasses in tallgrass prairie will change rapidly if warmer temperatures and more extreme precipitation regimes develop, with unknown impacts on plant production and other ecosystem processes.

Review Publications
Wilkinson, M., Monson, R.K., Trahan, N., Lee, S., Brown, E., Jackson, R.B., Polley, H.W., Fay, P.A., Fall, R. 2009. Leaf isoprene emission rate as a function of atmospheric CO2 concentration. Global Change Biology. 15:1189-1200.

Nippert, J.B., Fay, P.A., Carlisle, J.D., Knapp, A.K., Smith, M.D. 2009. Ecophysiological responses of two dominant grasses to altered temperature and precipitation regimes. Acta Oecologica. 35:400-408.

Wilsey, B.J., Teaschner, T.B., Daneshgar, P.P., Isbell, F.A., Polley, H.W. 2009. Biodiversity maintenance mechanisms differ between native and novel exotic-dominated communities. Ecology Letters. 12:432-442.

Isbell, F.I., Polley, H.W., Wilsey, B.J. 2009. Biodiversity, productivity and the temporal stability of productivity: Patterns and processes. Ecology Letters. 12:443-451.

Moran, P.J., Deloach Jr, C.J., Dudley, T., Sanabria, J. 2009. Open field host selection and behavior by tamarisk beetles (Diorhabda spp.) (Coleoptera: Chrysomelidae) in biological control of exotic saltcedars (Tamarix spp.) and risks to non-target athel (T. aphylla) and native Frankenia. Biological Control. 50:243-261.

Fay, P.A. 2009. Precipitation variability and primary productivity in water-limited ecosystems: How plants 'leverage' precipitation to 'finance' growth. New Phytologist. 181:5-8.

Isbell, F., Polley, H.W., Wilsey, B.J. 2009. Species interaction mechanisms maintain grassland plant species diversity. Ecology. 90:1821-1830.

Tracy, J.L., Robbins, T.O. 2009. Taxonomic revision and biogeography of the Tamarix-feeding Diorhabda elongata (Brulle, 1832) species group (Coleoptera: Chrysomelidae: Galerucinae: Galerucini) and analysis of their potential in biological control of Tamarisk. Zootaxa. 2101:1-152.

Knapp, A., Beier, C., Briske, D., Classen, A.T., Luo, Y., Reichstein, M., Smith, M., Smith, S.D., Bell, J.E., Fay, P.A., Heisler, J.L., Leavitt, S.W., Sherry, R., Smith, B., Weng, E. 2008. Consequences of more extreme precipitation regimes for terrestrial ecosystems. Bioscience. 58:811-821.

Last Modified: 9/10/2014
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