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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

2005 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? What 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 ©) 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 the milestones (indicators of progress) from your Project Plan.
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 policymakers.

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.What was the single most significant accomplishment this past year?
Nitrogen limitation constraints carbon sequestration on rangelands: Rangelands may reduce the increase in atmospheric carbon dioxide (CO2) concentration during the coming century by sequestering carbon (C) in soil, but the amount of additional C that can stored probably will depend on whether nitrogen (N) availability remains adequate to sustain plant growth. To determine how CO2 affects soil C and N, ARS scientists at the Grassland, Soil & Water Research Laboratory in Temple, TX, in cooperation with scientists from Duke and Washington State Universities, measured C and N dynamics in an intact grassland that was exposed for four years to a gradient in CO2 spanning pre-Industrial to predicted concentrations. 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 limited any increase in net sequestration at elevated CO2 by reducing the amount of C in older soil organic matter. 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.


4b.List other significant accomplishments, if any.
Annual plants regulate grassland productivity and species composition: Vegetation in many grasslands has been altered by heavy grazing and other disturbances and by plant invasions, yet we understand little of how changes in the composition of vegetation affect plant productivity and other of the services that grasslands provide. ARS scientists at the Grassland, Soil & Water Research Laboratory in Temple, TX, removed two groups of annual plants (annual broomweed, annual grasses) from a heavily grazed grassland in central Texas to determine whether these temporary residents of disturbed sites influence the delivery of ecological services (plant productivity for grazers, regulation of the nitrogen cycle) on this grassland either directly by affecting process rates or indirectly by regulating the composition of perennial plant species. Biomass production and nitrogen (N) accumulation by vegetation declined in the year following the removal of annual grasses in approximate proportion to the contribution of these plants to biomass production and N accumulation in unmanipulated grassland. The decline in production was short-lived, however, because biomass production and N accumulation by perennial forbs (broad-leaf herbaceaous plants) increased during the second year to compensate for the loss of annual plants. Because forbs differ from the perennial grasses that usually dominant in this grassland in functioning and in value as a forage for cattle, annual plants may influence the services derived from this and similar grasslands more by regulating the composition of vegetation than by directly affecting plant productivity.

Species diversity influences grassland productivity: Humans are reducing the number of plant species in many ecosystems and are creating greater disparity in the abundances of remaining plants, with some species becoming very abundant and others becoming rare. Recent studies have shown that the loss of plant species may reduce plant productivity in grasslands, but effects of the increasing disparity in species abundances on grasslands is not known. To determine effects of both species number and species abundances on grasslands, ARS scientists at the Grassland, Soil & Water Research Laboratory in Temple, TX, in cooperation with a scientist at Iowa State University, experimentally varied the number of plant species and their abundances in small plots. Plant production was greater in plots with many species than in plots with few species, but effects of species number on production were the same whether species were equally abundant or whether some were abundant and others were rare. Some species were lost or became extinct from plots, however, and species extinctions were greater in plots in which species abundances were inequitable than in plots in which species were equally abundant. Our results indicate that the increasing disparity in abundances among plant species may accelerate species loss and an accompanying decline in grassland productivity.


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


5.Describe the major accomplishments over the life of the project, including 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 increases in atmospheric CO2 and temperature 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 mitigating climatic change and improving management. Given the enormous social and economic consequences of C accumulation in the atmosphere, this research is directly or indirectly to every citizen. To date in the current project, we have shown that CO2 enrichment did not affect soil C in grassland because nitrogen (N) availability to plants declined and losses of formerly-stored (old) C offset increases in new C at elevated CO2. 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. 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. The potential impact of these results on public policy is enormous.

This research accomplishment addresses two goals of the action plan for the Global Change National Program - NP 204. These include Goals I.5 (Determine effects of CO2 enrichment on mechanisms of soil C and N interactions) and III.1 (Determine global change effects on the ecology, hydrology, and productivity of North American rangelands).

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, and university administrators. 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)the mentoring of a Temple High School student during development of his award winning Science Fair project during the Fall 2004/Spring 2005 and.
2)participation as a speaker at a tallgrass prairie field day. The high school student was awarded first place at his school science fair and at regional competition. Attendees at the field day (producers and lay persons) were briefed on global change issues, techniques for restoring tallgrass prairie, and the role of plant species diversity in sustaining functional prairies.


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 various aspects of the ecology of tallgrass prairies at the Mezynski tallgrass prairie conference and tour; Riesel, TX (Oct. 2004).

-Oral presentation of the Unit’s global change research program to a local RV club following the group’s tour of the Grassland, Soil and Water Research Laboratory (Dec. 2004).

-"Range Scientist Receives Award for Blackland Research Work," Temple Daily Telegram, 16 March 2005.

-Oral presentation of the Unit’s global change research program to a global change class from Southwestern University (April 2005).

-Oral presentation of the Unit’s global change research to elementary students and teachers at the Future Scientist Student Outreach Initiative Program, followed by a field tour of our Lysimeter/Monolith CO2 Gradient plant growth chamber; Temple, TX (May 2005).

-Field tour of our Lysimeter/Monolith CO2 Gradient plant growth chamber for the Governor’s Drought Preparedness Council; Temple, TX (May 2005).


Review Publications
Hickman, K.R., Derner, J.D., Polley, H.W. 2004. Response of species diversity within plant functional groups to altered precipitation regimes in tallgrass prairie. Ecological Society of America Abstracts. V. 89. p. 219.

Wilsey, B.J., Polley, H.W. 2004. Realistically low species evenness does not alter grassland species richness-productivity relationships. Ecological Society of America Abstracts. p. 547.

Polley, H.W., Wilsey, B.J., Tischler, C.R. 2004. Is the outcome of species interactions sensitive to species relative abundances?. Ecological Society of America Abstracts. p. 403.

Derner, J.D., Tischler, C.R., Polley, H.W., Johnson, H.B. 2005. Seedling growth of two honey mesquite varieties under CO2 enrichment. Rangeland Ecology and Management. 58:292-298.

Wilsey, B.J., Polley, H.W. 2004. Realistically low species evenness does not alter grassland species-richness-productivity relationships. Ecology. 85(10):2693-2700.

Ramirez-De Leon, H., Ocumpaugh, W.R., Burson, B.L., Tischler, C.R., Rooney, W.L. 2004. Heritability for seedling vigor in switchgrass [abstract]. American Society of Agronomy. 2004 CDROM.

Tischler, C.R., Polley, H.W., Johnson, H.B. 2003. Effects of disturbance and soil fertility on competitiveness of weedy invaders of Texas pastures [abstract]. American Society of Agronomy Abstracts. 2003 CDROM.

Tischler, C.R., Derner, J.D., Polley, H.W., Johnson, H.B. 2004. An 'Alamo' switchgrass population with reduced seed dormancy. In: Randall, J., Burns, J.C. Proceedings of The Third Eastern Native Grass Symposium, October 1-3, 2002, Chapel Hill, North Carolina. p. 292.

Derner, J.D., Tischler, C.R., Polley, H.W., Johnson, H.B. 2004. Effects of elevated co2 on growth responses of honey mesquite seedlings from sites along a precipitation gradient. pp. 156-160. Wildland Shrub Symposium Proceedings. Laramie, WY.

Polley, H.W., Tischler, C.R. 2005. Carbon dioxide enrichment influences the expression of genetic variation for seedling growth in honey mesquite. In: Society for Range Management 58th Annual Meeting and Trade Show, February 5-11, 2005, Fort Worth, Texas. 2005 CDROM.

Polley, H.W. 2005. Influence of CO2 enrichment on plant community and soils systems in mesic rangelands. In: Society for Range Management 58th Annual Meeting and Trade Show, February 5-11, 2005, Fort Worth, Texas. 2005 CDROM.

Tischler, C.R., Ocumpaugh, W.R. 2004. Kleingrass, Blue Panic, and Vine Mesquite. In: Moser, L.E., Burson, B.L., Sollenberger, L.E., editors. Warm-season (C4) Grasses. Agronomy Monograph 45. Madison, WI: American Society of Agronomy. p. 623-649.

Last Modified: 9/10/2014
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