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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

2006 Annual Report


1.What major problem or issue is being resolved and how are you resolving it (summarize project aims and objectives)? How serious is the problem? Why does it matter?
The concentration of carbon dioxide (CO2) gas in the atmosphere has increased by about 37% since the Industrial Revolution and is expected to rise to twice the pre-Industrial level within the century. Earth's climate also has changed during the last century, possibly as a result of the increase in CO2 and other greenhouse gases, fostering predictions that climatic change will intensify in the future. Economic costs of climate change may be astronomical. Such an enormous expense argues for limiting the increase in atmospheric CO2 concentration, either by reducing CO2 emissions or by removing more CO2 from air. The alternative of removing more CO2 from air may be the most cost-efficient, but the pool of additional carbon, C, that terrestrial ecosystems could accommodate and even the current size and spatial distribution of the terrestrial C sink remains uncertain. Rangelands cover about 40% of the land area in the U.S., so even small shifts in C balance per unit of rangeland area could significantly impact atmospheric CO2 levels. The capacity of these ecosystems to sequester additional C, however, is being altered by changes in plant productivity caused by the continuing rise in CO2 and by associated shifts in temperature and in the timing and intensity of rainfall events and by changes in land use and the composition of vegetation. Our overall mission is to resolve uncertainty regarding effects of atmospheric, climatic, and recent vegetation changes on the C cycle of rangelands as a basis for mitigating climatic change and improving management. Specific objectives are to:.
1)determine effects of CO2 enrichment, anticipated changes in rainfall, and soil type on the C balance of tallgrass prairie,.
2)determine how history of cultivation and density of invasive woody plants affects the vertical distribution and sizes of pools of organic C on rangelands with relatively-high annual rainfall and,.
3)determine whether effects of changes in precipitation and temperature on the C balance of western rangelands may be predicted from the response of C exchange rates to short-term variation in temperature and rainfall. Effects of CO2 enrichment and anticipated changes in rainfall on the C cycle are being studied using novel, elongated growth chambers. Soil and plant samples from grasslands with different densities of woody plants are being analyzed to determine the impact of woody encroachment on rangeland C. Measurements of CO2 exchange between rangelands and the air at 8 sites in the western U.S. are being used to assess the sensitivity of rangeland C balance to climatic variability.

This research addresses several goals of the action plan for the Global Change National Program - NP 204. These include Goals I.1 and I.4 (Quantify rates of soil C storage with different land use practices and of CO2 fluxes of different ecosystems), I.5 (Determine effects of CO2 enrichment on mechanisms of soil C and N interactions), III.1 (Determine global change effects on the ecology, hydrology, and productivity of North American rangelands), and IV.2 (Assess the changing water requirements of agricultural crops under CO2 enrichment).

Less than half of the C that humans release into the air each year stays in the atmosphere. Part of the C appears to vanish into land, likely taken up by plants and soils. Understanding where this C is going, how much additional C can be stored, and how sensitive C cycling is to changes in climate, atmospheric CO2 concentration, and vegetation is directly or indirectly relevant to every citizen.


2.List by year the currently approved milestones (indicators of research progress)
Objective 1: 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.

FY 2006 Milestone: Measure effects of CO2 concentration, precipitation, and soil type on plant water dynamics.

FY 2006 Milestone: Measure effects of CO2 concentration, precipitation, and soil type on leaf gas exchange.

FY 2006 Milestone: Measure effects of CO2 concentration, precipitation, and soil type on nitrogen (N) mineralization in soil and plant tissue N concentration.

FY 2007 Milestone: Prepare publications describing effects of CO2, precipitation, and soil type on plant water dynamics, leaf gas exchange, and soil and plant nitrogen (N) status.

FY 2007 Milestone: Measure root biomass of forbs and grasses of tallgrass prairie using soil cores collected to 1.5 m in each of 80 monoliths along a controlled environment chamber. Compile data on aboveground biomass of grasses and forbs.

FY 2007 Milestone: Determine effects of CO2 enrichment, anticipated changes in rainfall, and soil type on evapotranspiration rates and other components of ecosystem water balance using soil water measurements on each of 80 monoliths along a controlled environment chamber.

FY 2008 Milestone: Prepare publications describing effects of CO2, rainfall, and soil type on root biomass of forbs and grasses of tallgrass prairie. Transfer findings to producers.

FY 2008 Milestone: Prepare publications describing effects of CO2, rainfall, and soil type on evapotranspiration rates and other components of the water balance of tallgrass prairie. Transfer result to policy makers.

FY 2008 Milestone: Sample soil from tallgrass prairie exposed to different CO2 and rainfall treatments to determine treatment effects on C balance. Analyze measurements of soil respiration and C content and plant production to assess treatment effects on C cycling.

Objective 2: 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.

FY 2005 Milestone: Collect soil samples to 2 m depth from grasslands with different densities of woody plants.

FY 2005 Milestone: Harvest aboveground biomass of herbaceous species in grasslands with different densities of woody plants.

FY 2005 Milestone: Record woody density and the sizes of individuals of each woody species in each study plot.

FY 2006 Milestone: Remove roots from soil and begin to measure concentrations of organic carbon and nitrogen in soils collected from grasslands with different densities of woody plants.

FY 2006 Milestone: Measure the carbon concentration in tissues of herbaceous species. Estimate the biomass and carbon content of woody plants growing in grassland plots by felling and measuring similarly-sized trees located offsite.

FY 2007 Milestone: Complete chemical analyses on soils and roots collected from grasslands with different densities of woody plants. Complete C analyses on wood.

FY 2008 Milestone: Collate C analyses of soils and plants from grasslands with different densities of woody plants. Prepare manuscripts describing woody effects on rangeland C balance. Transfer results to policy-makers.

Objective 3: 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.

FY 2006 Milestone: Assemble data of CO2 fluxes (NEE), temperature, and precipitation from rangeland sites in the western U.S.

FY 2007 Milestone: Test relationships between annual means of CO2 fluxes and means of climate variables. Apply statistical tests (homogeneity of slopes model) to multi-year flux data from each of 8 rangeland sites in the western U.S. to determine effects of variability in precipitation and temperature on annual variability in CO2 fluxes.

FY 2008 Milestone: Complete statistical analyses of the sensitivity of rangeland CO2 fluxes to variability in temperature and precipitation.

FY 2009 Milestone: Prepare manuscripts describing the sensitivity of CO2 fluxes to variability in temperature and precipitation at rangeland sites in the western U.S.


4a.List the single most significant research accomplishment during FY 2006.
Increased carbon loss to respiration limits carbon sequestration on rangelands at elevated CO2: Economic and social consequences of climate changes that may result from the continuing rise in atmospheric carbon dioxide (CO2) concentration could be enormous. Plants may slow these changes by removing CO2 from air via photosynthesis, a process that is stimulated by higher CO2 concentrations, but plants and the microbes in soil that decompose dead plant material also return CO2 to air via the process of respiration. ARS scientists at the Grassland, Soil & Water Research Laboratory in Temple, TX measured CO2 uptake and release on grassland in central Texas exposed to CO2 levels that spanned pre-Industrial (subambient) to elevated concentrations to determine how atmospheric CO2 affects respiration and its sensitivity to seasonal changes in photosynthesis. Respiration rates of grassland were greater at elevated than subambient CO2 levels because both carbon input (net photosynthesis) and respiration per unit of carbon input increased with CO2 concentration. By increasing respiration rates, CO2 enrichment may reduce the net amount of carbon that plants remove from air and retain in terrestrial ecosystems. Research addresses Component 1 (Carbon Cycle and Carbon Storage) of the Global Change National Program (NP 204) and Objective 1 of the current Action Plan (Determine how atmospheric CO2 enrichment affects plant production and other components of the carbon cycle on tallgrass prairie).


4b.List other significant research accomplishment(s), if any.
Precipitation event sizes and intervals control grassland productivity: Climate models predict increased precipitation variability for Central Plains grasslands, but our ability to predict likely responses of grasslands to greater variability in rainfall is limited. An ARS scientist at the Grassland, Soil & Water Research Laboratory in Temple, TX, with collaborators from Kansas State University applied sixteen different irrigation treatments that varied in watering interval and total quantity to mesocosms containing tallgrass prairie species assemblages to assess effects of rainfall variability on aboveground production and soil respiration of tallgrass species and on photosynthesis of the dominant C4 grass Andropogon gerardii. Results demonstrated.
1)strong relationships between precipitation event size and interval and the ratio of ecosystem processes regulating carbon cycling to rainfall quantity (rainfall use efficiency; RUE), and.
2)that RUE was related directly to the responsiveness of these processes to precipitation variability. These results are the first to demonstrate that interactions between rainfall event size and interval may be more important than the total annual quantity of rainfall in determining ecosystem function under future precipitation scenarios. Research addresses Component IV (Changes in Weather and the Water Cycle at Farm, Ranch and Regional Scale) of the Global Change National Program (NP 204) and Objective 3 of the current Action Plan (Determine whether climate change effects on ecosystem C balance of western rangelands may creditably be predicted from the response of C cycling processes to seasonal and inter-annual variation in temperature and precipitation).

Photosynthesis and growth responses to rainfall variability in two C4 grasses in tallgrass prairie: Future climate scenarios project increased precipitation variability, but it is not clear how such a change will impact critical pools and fluxes of carbon in grasslands. In particular, will leaf-level photosynthesis respond similarly to shifts in soil moisture when rainfall is more variable, or will increased precipitation variability change the photosynthesis-soil moisture relationship? An ARS scientist at the Grassland, Soil & Water Research Laboratory in Temple, TX, with collaborators from Kansas State University measured leaf photosynthesis and numerous other leaf attributes of Andropogon gerardii and Sorghastrum nutans, both C4 tallgrasses, in field plots under rain exclusion shelters receiving either ambient levels of precipitation quantity and variability (ambient treatment) or increased variability in rainfall (longer dry intervals and larger rain events, but the same total precipitation as the ambient treatment). Increased precipitation variability altered the photosynthesis-soil moisture relationship in S. nutans, but not in A. gerardii, demonstrating for the first time that morphologically and systematically similar species may respond differently to shifts in soil moisture variability. Species composition of grassland communities must be considered when considering climate change impacts on carbon uptake. Research addresses Component IV (Changes in Weather and the Water Cycle at Farm, Ranch and Regional Scale) of the Global Change National Program (NP 204) and Objective 3 of the current Action Plan (Determine whether climate change effects on ecosystem C balance of western rangelands may creditably be predicted from the response of C cycling processes to seasonal and inter-annual variation in temperature and precipitation).

CO2 enrichment magnifies intra-specific variation in seedling growth of an invasive shrub: The invasive shrub honey mesquite reduces grass production and the utility of grasslands for grazing. The continuing rise in atmospheric carbon dioxide (CO2) concentration could stimulate mesquite growth by increasing the biochemical rate at which mesquite plants convert atmospheric CO2 to plant carbon, but CO2 effects may not be expressed uniformly among mesquite genotypes. ARS scientists at the Grassland, Soil & Water Research Laboratory in Temple, TX, measured effects of doubling CO2 concentration on seedling growth of 14 mesquite genotypes collected from across the shrub's geographic distribution in the southwestern U.S. Seedling biomass one month following emergence was 3% to 75% greater at elevated than ambient CO2 depending on genotype, but CO2 enrichment did not favor the largest or fastest-growing genotypes at ambient CO2. CO2 enrichment could accelerate mesquite encroachment on grasslands by increasing seedling growth, but CO2 effects on mesquite are not predictable from seedling size or growth rate at the current concentration. Research addresses Component III (Agricultural Ecosystem Impacts) of the Global Change National Program (NP 204) and Objective 2 of the current Action Plan (Determine how invasive woody plants affect pools of organic carbon in mesic grassland).


4c.List significant activities that support special target populations.
None


5.Describe the major accomplishments to date and their predicted or actual impact.
This project, initiated in October 2004, is designed to advance scientific accomplishments resulting from our research group's preceding project. Among the several accomplishments of the preceding project was the finding that carbon, C, may have been stored passively in soil organic matter as CO2 rose to the current concentration, but the capacity of soils to sequester additional C and to slow the continuing rise in atmospheric CO2 apparently is limited. Soil C was lost from grassland that was exposed to lower-than-present CO2 concentrations, but was unchanged at elevated CO2 levels forecast within the century. The current project is designed to resolve some of the remaining uncertainty regarding effects of atmospheric, climatic, and vegetation changes on the C cycle of rangelands as a basis for improving management and mitigating climatic changes that may accompany CO2 enrichment. Given the enormous social and economic consequences of C accumulation in the atmosphere, this research is directly or indirectly relevant to every citizen.

The capacity of any ecosystem to store C depends both on the rate at which C is captured or fixed via plant photosynthesis (carbon input) and on the average period during which fixed C is retained before being respired by plants or soil microorganisms or otherwise lost (residence time of C). To date in the current project, we have shown that soil C in grassland was unchanged at elevated CO2 because CO2 enrichment reduced the residence time of C sufficiently to almost completely offset a large enhancement in C fixation. Respiration rates of grassland (plants plus soil microorganisms) were greater at elevated than pre-Industrial (subambient) CO2 levels partly because CO2 enrichment increased respiration per unit of carbon input. Losses of formerly stored (old) C offset increases in new C at elevated CO2 as soil N was transferred from old soil fractions with high C:N ratios to newly produced plant tissues with low C:N ratios at elevated CO2. This net transfer of N from soil to plants likely increased the response of plant production to higher CO2, but also moderated any increase in net sequestration of C at elevated CO2 by reducing the amount of C in older soil organic matter and stimulating grassland respiration. Our results indicate that little of the additional C fixed by grassland plants at elevated CO2 may be sequestered in soil if N is depleted from older soil organic matter to meet the nutritional demands of more rapidly growing plants.

Climate models predict increased precipitation variability for the Central Plains, but it is not clear how such a change will impact critical pools and fluxes of carbon on grasslands. To date in the current project, we have show that increased variability in rainfall alters the relationship of leaf C uptake (photosynthesis) to soil moisture for some, but not all, dominant grasses of tallgrass prairie, indicating that species composition partially regulates the response of grassland carbon cycling to rainfall variability. We also have shown that rainfall event sizes and intervals between rainfall event may be more important than the total annual quantity of rainfall in regulating key components of the carbon cycle on tallgrass prairie, including aboveground production and soil respiration. The potential impact of our research for public policy is enormous.

This research accomplishment is aligned with two components of the Global Change National Program (NP 204) - Carbon Cycle and Carbon Storage and Changes in Weather and the Water Cycle at Farm, Ranch and Regional Scale - and with Objectives 1 and 3 of the current Action Plan (Determine how atmospheric CO2 enrichment affects plant production and other components of the carbon cycle on tallgrass prairie; Determine whether climate change effects on ecosystem C balance of western rangelands may creditably be predicted from the response of C cycling processes to seasonal and inter-annual variation in temperature and precipitation).

This research also addresses Goal 5 of the ARS Strategic Plan (Protect and enhance the nation's natural resource base and environment) and is particularly relevant to Objective 1 of Goal 5 (Provide science-based knowledge and education to improve the management of forest, rangelands, and pastures) and Performance measure 5.1.1 (Develop ecologically based information, technologies, germplasm, and management strategies that sustain agricultural production while conserving and enhancing the diverse natural resources found on rangelands and pasture lands).


6.What science and/or technologies have been transferred and to whom? When is the science and/or technology likely to become available to the end-user (industry, farmer, other scientists)? What are the constraints, if known, to the adoption and durability of the technology products?
During the past year, formal and informal presentations of scientific information and research results were made to consumer groups, students (graduate, undergraduate, and secondary), lay persons, producers, scientists, university administrators, and Congressional staff persons. Information transferred included: potential impacts of global change on water resources, effects of atmospheric CO2 enrichment on grassland composition and productivity, and the role of plant species diversity in promoting and sustaining grassland function.

Technology transfer included.
1)a presentation of results from our global change research project to Congressional staff persons affiliated with our U.S. Congressman and the two U.S. Senators from Texas and.
2)advice on species to include in seed mixtures for grassland restoration to the biologist with the Corp of Engineers at Fort Hood military reservation, TX.


7.List your most important publications in the popular press and presentations to organizations and articles written about your work. (NOTE: List your peer reviewed publications below).
Oral presentation describing the contribution of the historical and pre-Industrial increase in atmospheric carbon dioxide concentration to ongoing changes in grassland composition and functioning; Department of Botany and Microbiology, University of Oklahoma, Norman, OK (March 2006; 40 persons).

Oral presentation of the Unit's global change research program to a global change class from Southwestern University (April 2005; 15 persons).


Review Publications
Polley, H.W., Wilsey, B.J., Derner, J.D., Johnson, H.B., Sanabria, J. 2005. Early-successional plants regulate grassland productivity and species composition. In: Ecological Society of America Abstracts, August 7-12, 2005, Montreal, Canada. 2005 CDROM.

Polley, H.W., Mielnick, P.C., Dugas, W.A., Johnson, H.B., Sanabria, J. 2006. Increasing CO2 from subambient to elevated concentrations increases grassland respiration per unit of net carbon fixation. Global Change Biology. 12:1390-1399.

Polley, H.W., Wilsey, B.J., Derner, J.D., Johnson, H.B., Sanabria, J. 2006. Early-successional plants regulate grassland productivity and species composition: a removal experiment. Oikos. 113:287-295.

Polley, H.W., Tischler, C.R., Johnson, H.B. 2006. Elevated atmospheric CO2 magnifies intra-specific variation in seedling growth of honey mesquite: an assessment of relative growth rates. Rangeland Ecology and Management. 59:128-134.

Gill, R.A., Anderson, L.J., Polley, H.W., Johnson, H.B., Jackson, R.B. 2006. Potential nitrogen constraints on soil carbon sequestration under low and elevated atmospheric CO2. Ecology. 87(1):41-52.

Wilsey, B.J., Martin, L.M., Polley, H.W. 2005. Predicting plant extinction based on species-area curves in prairie fragments with high beta richness. Conservation Biology. 1835-1841.

Derner, J.D., Schuman, G.E., Jawson, M.D., Shafer, S.R., Morgan, J.A., Polley, H.W., Runion, G.B., Prior, S.A., Torbert Iii, H.A., Rogers Jr, H.H., Bunce, J.A., Ziska, L.H., White, J.W., Franzluebbers, A.J., Reeder, S.J., Venterea, R.T., Harper, L.A. 2005. USDA-ARS global change research on rangelands and pasturelands. Rangelands. 27(5):36-42.

Polley, H.W., Derner, J.D., Wilsey, B.J. 2005. Patterns of plant species diversity in remnant and restored tallgrass prairies. Restoration Ecology. 13(3):480-487.

Collins, S.L., Calabrese, L., Smith, M.D., Fay, P.A. 2005. Climate drivers and rate of change in mesic and arid grasslands. In: Ecological Society of America Proceedings, August 7-12, Montreal, Canada. 2005 CDROM.

Kaufman, D.M., Harper, C.W., Nippert, J.B., Fay, P.A., Blair, J.M. 2005. Ecosystem responses to rainfall quantity and variability in model grassland assemblages. In: Ecological Society of America Proceedings, August 7-12, 2005, Montreal, Canada. 2005 CDROM.

Fay, P.A., Smith, M.D., Travers, S.E., Nippert, J.B., Blair, J.M., Garrett, K.A. 2005. Ecological responses to precipitation quantity and frequency in grasslands: pattern and process from the gene to the ecosystem. In: Ecological Society of America Proceedings, August 7-12, 2005, Montreal, Canada. 2005 CDROM.

Porporato, A., Vico, G., Fay, P.A. 2006. Superstatistics of hydro-climatic fluctuations and interannual ecosystem productivity. Geophysical Research Letters. 33:15402-15405.

Last Modified: 9/10/2014
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