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United States Department of Agriculture

Agricultural Research Service


Location: Grassland, Soil and Water Research Laboratory

2008 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 completed the second season of an experiment to determine how atmospheric carbon dioxide (CO2) enrichment interacts with soil type to affect plant production and other aspects of the carbon (C) cycle on tallgrass prairie. Effects of CO2 likely are mediated through changes in water balance and soil nitrogen (N) availability to plants. Forbs and grasses of tallgrass prairie in central Texas were planted into intact monoliths of three soil types. A novel field chamber is being used to expose vegetated monoliths to a continuous gradient in CO2 spanning pre-Industrial to anticipated elevated concentrations. CO2 enrichment increased leaf photosynthesis rates and reduced stomatal conductance of dominant grasses and forb. Consistent with data from leaves, greater CO2 reduced rates of water loss from plant communities early in the growing season and increased the annual production of aboveground biomass. The shape of the response curve of biomass to CO2 differed among soil types, possibly indicating that relative availabilities of water and nitrogen for plants differed among soil types. The rate of CO2 efflux from soil (JCO2), the combined CO2 output from heterotrophic and autotrophic respiration, was greater for monoliths of Houston than Austin and Bastrop series soils during the most active portion of the second growing season. JCO2 increased with increasing CO2 on all soils, but the correlation between CO2 and JCO2 was strongest on Bastrop soils. Because plant production is greater on Bastrop and Houston soils than the Austin soil, differences among soils in the response of JCO2 to CO2 likely reflect interactions among plant biomass and soil organic matter quality and quantity along the CO2 gradient. The total of inorganic N extractable from soil did not differ with CO2 on the Austin soil but increased with greater CO2 on Houston and Bastrop soils. Results imply that the response of plant production to increased CO2 is more limited by N availability on the Austin soil than on Bastrop and Houston soils. Root length density did not vary with CO2 on any soil, suggesting no differences in root exploration of the soil volume that might affect N limitation of aboveground productivity. (NP 204; Component III, Objective 3).

Leaf beetles of the genus Diorhabda are being released to biologically control the exotic and invasive tree Tamarix (saltcedar). Beetles collected from Crete increased and spread rapidly near Big Spring, TX, following release. By November 2007, they had defoliated saltcedar on the entire 20 acre release site. About 25% of trees in the center of this site have been killed. During FY08, beetles spread 5 miles along Beals Creek and defoliated ca. 100 acres of saltcedar in the center 3-mile area. In areas defoliated for 3 years, native grasses and forbs have naturally and abundantly revegetated under the former saltcedar canopy. Vegetation surveys indicate that beetles had not fed on other plants at Big Spring. At other release sites in Texas, beetle establishment has been low but a new release protocol is producing improved results. Baseline bird surveys continue. (NP 304; Components IX.A.1, 2).

1. Carbon dioxide enrichment slows water loss from grassland: The concentration of carbon dioxide (CO2) gas in the atmosphere has increased by about 37% since the beginning of the Industrial Revolution and is expected to reach twice the pre-Industrial level within the century. Studies on single leaves have shown that CO2 enrichment usually reduces the rate of water loss per unit of leaf area, but our understanding of how plant and environmental variables, like total leaf area and air temperature, interact with CO2 to regulate water loss from plant stands is more limited. Scientists at the Grassland, Soil & Water Research Laboratory in Temple, Texas, grew tallgrass prairie plants in intact monoliths of soil that were exposed to pre-Industrial to elevated CO2 concentrations. Monoliths were weighed daily to determine how CO2 effects on water loss varied with total leaf area and environmental variables, including soil water content, light, and air temperature. Increasing CO2 generally reduced water loss per unit of leaf area, but the CO2 effect was greatest at relatively-low temperatures and low leaf area. Rising CO2 likely will increase forage production for livestock by slowing water loss from grasslands, although water savings will be greatest early in the growing season when temperatures are mild and the plant canopy is re-establishing. (NP 204; Component III, Objective 3)

2. Increased precipitation variability shifts the balance between carbon uptake and carbon loss in grassland: 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, with still uncertain implications for key processes controlling terrestrial ecosystem structure and function. Changes in the size of precipitation events may cause differential responses in the processes controlling the uptake and loss of carbon (C), and therefore could alter C sequestration in terrestrial ecosystems. A scientist at the Grassland, Soil & Water Research Laboratory in Temple, Texas, together with university collaborators, determined that more extreme precipitation patterns (longer intervals between events combined with larger events) shift the C balance of 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 C-sequestration expected to result from increasing atmospheric CO2 concentration, but may also lower plant water status and productivity. These results highlight the need for improved forecasts of precipitation patterns and for field experiments in which CO2, temperature, and precipitation are manipulated in combination in order to improve forecasts of ecosystem C dynamics. (NP204; Components I and III)

3. Inter-annual variability in climate regulates the carbon balance of northern mixed-grass prairie: Air temperature may increase as carbon dioxide (CO2) and other greenhouse gases accumulate in the atmosphere. The rate at which CO2 accumulates in air is sensitive to climatic effects on the balance between CO2 uptake by growing plants and CO2 release by respiring plants and animals (carbon balance). Scientists at the Grassland, Soil & Water Research Laboratory in Temple, TX, and Northern Great Plains Research Laboratory in Mandan, ND, measured climatic factors (like temperature and precipitation) and the exchange of CO2 between air and prairie in North Dakota for 5 years in order to determine effects of climatic variability on carbon balance of northern mixed-grass prairie. Inter-annual variability in climate accounted for about 20% of total variation in net CO2 exchange on both grazed and ungrazed prairie. The effect of a given change in climate on CO2 balance differed among years, however. The carbon balance of these grasslands cannot be predicted without knowledge of year-to-year variation in both climate and CO2 exchange-climate relationships. (NP 204; Component III Objective 3)

4. The response of grassland forage production to rainfall variability depends on the identity of dominant plants rather than plant diversity: The reliability of forage production from one year to the next is thought to depend partly on species diversity, implying that forage production will vary more among years in severely human-impacted than more-diverse grasslands. Scientists at the Grassland, Soil & Water Research Laboratory in Temple, Texas, together with a university collaborator, measured impacts of natural variability in rainfall on forage production for a low-diversity (restored) and a high-diversity (remnant) tallgrass prairie at each of two locations. Species diversity was greater by a factor of two or more in remnant than restored prairies, but variability in grassland biomass did not differ between prairie types because biomass of the dominant grass of restored prairies varied less than did biomass of other abundant species. Effects of rainfall variability on forage production depend more on characteristics of dominant forage species than on species diversity alone. (NP 204; Component III Objective 3)

5. Biological control of saltcedar a success at Big Spring, TX: Exotic saltcedars (Tamarix spp.), small trees introduced from Asia and the Mediterranean area during the 1800s, have invaded western riparian areas from the Great Plains to the Pacific and from Canada into northern Mexico, where they cause major environmental and economic losses. They displace native plant communities, degrade wildlife habitat, promote wildfires, reduce livestock grazing and water for irrigated agriculture, increase soil surface salinity and interfere with recreational uses of parks and waterways; conventional controls are expensive, require repeated applications and damage many non-target plants in natural areas. A scientist at the Grassland, Soil and Water Research Unit in Temple, TX, released the leaf beetle Diorhabda elongata from Crete, Greece, into the open field along Beals Creek near Big Spring, TX, after 3 years testing and obtaining release permits. The beetles dispersed 5 miles and defoliated nearly all the saltcedar in the center 3 miles along Beals Creek, estimated at 100 acres. In the center 1.6 acres, the beetles have killed 25% of the saltcedar trees, and have permitted natural revegetation by local grasses and forbs; no feeding or damage whatsoever has occurred to any other species of plants. This is the first successful control in the Southern Plains area and demonstrates the potential of biological control for providing permanent, low cost, and safe control of saltcedar, recognized as the worst riparian weed in the western U.S. (NP304; Components IX.A.1,2)

6.Technology Transfer

Number of Non-Peer Reviewed Presentations and Proceedings10
Number of Newspaper Articles and Other Presentations for Non-Science Audiences3

Review Publications
Polley, H.W., Wilsey, B.J., Derner, J.D. 2007. Dominant species constrain effects of species diversity on temporal variability in biomass production of tallgrass prairie. Oikos. 116:2044-2052.

Polley, H.W., Frank, A.B., Sanabria, J., Phillips, R.L. 2008. Interannual variability in carbon dioxide fluxes and flux-climate relationships on grazed and ungrazed northern mixed-grass prairie. Global Change Biology. 14:1620-1632.

Polley, H.W., Johnson, H.B., Fay, P.A., Sanabria, J. 2008. Initial response of evapotranspiration from tallgrass prairie vegetation to CO2 at subambient to elevated concentrations. Functional Ecology. 22:163-171.

Fay, P.A., Kaufman, D.M., Nippert, J.B., Carlisle, J.D., Harper, C.W. 2008. Changes in grassland ecosystem function due to extreme rainfall events: implications for responses to climate change. Global Change Biology. 14:1600-1608.

Everitt, J.H., Yang, C., Fletcher, R.S., Deloach Jr, C.J. 2007. Comparison of QuickBird and SPOT 5 satellite imagery for mapping giant reed. Journal of Aquatic Plant Management. 46:77-82.

Last Modified: 9/10/2014
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