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2013 Annual Report

1a. Objectives (from AD-416):
The semi-arid grasslands of the western Great Plains, mixed-grass prairie and shortgrass steppe, provide a tremendous array of ecosystem services, including livestock forage, a diversity of native plants and animals, resistance to biological invasion, and carbon storage. Global change is expected to dramatically change grasslands and associated ecosystem services, but the nature of its impacts, and the mechanisms underlying those impacts, remain difficult to predict. In water-limited ecosystems, elevated CO2 and warming can have particularly strong and complex effects because, in addition to their direct effects, they alter water availability. Two main objectives will drive our research program over the next five years to understand how these changes might impact the ecosystem services of western rangelands. The first objective is to assess effects of predicted global changes on ecosystem services in a northern mixed-grass prairie. This will be accomplished by determining the effects of temperature, CO2 and precipitation on plant productivity, plant diversity, forage quality, community composition, weed invasion and the ability of native plant communities to recover from disturbance. The biogeochemistry underlying these responses will be studied to improve our understanding of ecosystem responses and to improve algorithms in biogeochemical models like Daycent. We will also evaluate whether and how responses of invasive species differ from those of native species. Our second objective is to develop knowledge and tools that allow rangeland managers to minimize greenhouse gas emissions. We will determine how temperature, CO2 and precipitation influence land-atmosphere exchanges of trace gases and soil carbon (C) storage, and evaluate the relative importance of water, nitrogen (N) and C limitation in regulating C storage. We will use this information plus additional soil C and CO2 flux data from long-term grazing experiments to determine the potential to mitigate greenhouse gas emissions through grazing management, and assess tradeoffs between mitigation and rangeland productivity.

1b. Approach (from AD-416):
To address our first objective concerning the responses of rangelands to global changes, we will use a well-replicated Free Air CO2 Enrichment (FACE) and warming experiment to determine how global change influences the northern mixed-grass prairie. We will examine responses of plant production and quality, composition of native plant communities, carbon and nitrogen cycling, and plant invasion. To understand the mechanisms underlying these responses, we will make extensive use of gas exchange, stable isotope, soil water and nitrogen monitoring, and computer simulation methods. We will use additional treatments to learn how seasonality of precipitation influences the northern mixed-grass prairie, and how the magnitude of those effects compares to effects of CO2 and warming. To address our second objective concerning greenhouse gas mitigation tools, we will measure soil respiration and fluxes of nitrous oxide (N2O) and methane (CH4) using static chambers, and net ecosystem CO2 exchange (NEE) using dynamic chambers within plots of the FACE, warming and irrigation manipulative experiment. Results from the static and dynamic chambers will allow us to quantify CO2-enrichment and warming effects on soil trace gas fluxes and ecosystem level CO2 fluxes, and how these fluxes are related to soil moisture and other environmental factors. We will also take advantage of three ongoing NP215 long-term grazing studies to assess the effects of grazing management strategies (stocking rate and season of use) on the size and dynamics of soil C and N pools, and the potential of these strategies to mitigate greenhouse gas emissions in NMP and SGS. We will use natural variation in precipitation to determine the relative influence of above- and below-average years of precipitation on C and N pool changes. The insights provided by these experiments will help scientists and land managers adapt management practices to sustain ecosystem services in the face of global change, and provide critical information for policy makers.

3. Progress Report:
Research under project 5409-110-005-00D is guided by two objectives: (1) Assess effects of predicted global changes on ecosystem services in northern mixed-grass prairie, and (2) Develop knowledge and tools that allow rangeland managers to minimize greenhouse gas emissions. Both objectives are centered on the Prairie Heating and CO2 Enrichment (PHACE) Experiment in which atmospheric carbon dioxide (CO2) concentration, temperature and soil water are all being manipulated to further our understanding of how semi-arid rangelands respond to multiple global change factors. The second objective includes measurements of soil carbon within a separate, long-term grazing experiment. A core group of scientists from ARS, the University of Wyoming, Colorado State University, and the Biometeorology Institute in Florence, Italy, plus several graduate students and post docs continue to collaborate on this unique project. This year, one of the two lead scientists retired and a post-doc was hired to help complete the project. Our results to date suggest that the effects of elevated CO2 and warmer temperatures depend to a large extent on the combined effects of these two factors on soil plant water relations, and that perennial warm-season (plants possessing the C4 photosynthetic pathway) grasses appear to prosper under these future conditions. Due to higher-than-expected water savings from elevated CO2, our results suggest that productivity in this semi-arid rangeland may be greater under climate warming than previously suspected. However, we are also learning how a number of other plant and soil attributes, particularly the cycling of soil/plant nitrogen (N) may determine the ultimate responses of this rangeland to climate change through competition for soil resources. Experimental work on Dalmatian toadflax (Linaria dalmatica, a perennial invasive perennial forb) demonstrated that elevated CO2 and warming led to a 13-fold increase in its invasion of mixed-grass prairie. Similar results have now been observed for other invasive species, including diffuse knapweed (Centaurea diffusa) and cheatgrass (Bromus tectorum). A Department of Energy grant is extending the PHACE experiment through 2013 and adding additional modeling efforts using results from this experiment to evaluate longer term effects of climate change on carbon (C) cycling. New research added in this final field season of the project focuses on how elevated CO2 and warming influence plant water use, including the depth from which plants obtain water, and traits associated with physiological and morphological drought tolerance. We also continued efforts to use leaf traits to predict global change responses, both within the PHACE experiment and through synthesis of literature from prior global change experiments. In our greenhouse gas mitigation research, we completed soil sampling (postponed from the previous year due to drought) from long-term grazing experiments, to determine the long-term effects of livestock grazing on soil C sequestration and storage. 212 2 A 2008 212 3 A 2008 212 3 B 2008 212 4 C 2008

4. Accomplishments
1. Elevated CO2 increases invasion by increasing both carbon and water in semi-arid rangeland. In dry regions, elevated atmospheric carbon dioxide (CO2) can influence plants both directly, by increasing carbon needed for photosynthesis, and indirectly, by increasing water use efficiency. ARS researchers in Ft. Collins, CO and collaborators from the University of Wyoming, and the University of Western Sydney, Australia, found that direct and indirect CO2 effects combined to dramatically increase success of the invasive plant, Dalmatian toadflax (Linaria dalmatica). Toadflax biomass and seed production were 13-fold and 32-fold higher, respectively, with elevated CO2 than without. These findings show that dry regions are likely to be particularly vulnerable to invasion as climate change proceeds. They will help rangeland scientists and managers predict which species are most likely to invade, and which areas are most likely to be invaded under future climates.

2. Elevated CO2 does not offset decreased streamflow for riparian species. Climate change is expected to decrease streamflow and inhibit riparian (streamside) trees throughout much of the western US. It is possible, however, that elevated atmospheric carbon dioxide (CO2), which increases plant water use efficiency, could compensate for decreased streamflow. ARS scientists from Ft. Collins, CO and collaborators from Colorado State University and the US Geological Survey discovered that while elevated CO2 does increase water use efficiency and growth of riparian trees, these increases do not counterbalance predicted decreases in streamflow. These results suggest that active streamflow management may be needed to maintain riparian plant communities under future climates.

3. Synthetic analysis of climate change effects on invasion. Climate change has been predicted to favor invasive over native species, but it is unclear how commonly this occurs. ARS scientists from Ft. Collins, CO, and collaborators from nine different universities conducted the first meta-analysis of climate change effects on invasion. They found that both warming and elevated atmospheric carbon dioxide (CO2) favor invasive species in aquatic ecosystems. In contrast, general differences in climate change responses between invasive and native species were rare in terrestrial systems, and widely varying responses suggest that ecosystem- and species-specific studies will be needed to predict global change responses. Trends towards stronger invasive responses in more favorable environments, suggest that future research should focus on climate changes that decrease stress or increase resource availability. This research helps scientists and managers predict which ecosystems will be most vulnerable to invasion in the future.

4. Climate change alters relative importance of nitrogen and phosphorus in limiting rangeland production. Forage production, carbon sequestration, and other key processes in rangelands are often limited by nitrogen, phosphorus, or both. ARS researchers in Ft. Collins, CO and collaborators from the University of Wyoming, and the University of Sydney, Australia, found that climate change can alter the ratios of nitrogen to phosphorus. Elevated atmospheric carbon dioxide (CO2) makes nitrogen more limiting, while warming makes phosphorus more limiting, particularly under dry conditions. These findings suggest that phosphorus may be more important in controlling forage production and carbon sequestration in rangelands that suffer decreased precipitation together with warming.

5. Grassland plants face tradeoffs between herbivore defense and growth. Understanding tradeoffs between different plant functions is key to understanding plant community diversity and composition. Scientists from ARS in Ft. Collins, CO, and collaborators from the global NutNet experiment, demonstrated that across 39 grasslands from around the world, plants that thrive with high nutrient availability are also strongly inhibited by grazing by large animals. These results will help rangeland scientists predict how grazing and fertilization will influence desirable and undesirable plant species.

6. Elevated CO2 and nitrogen favor decrease forage quality. Increasing atmospheric carbon dioxide (CO2) and nitrogen deposition will require adaptation of rangeland management, yet there is little information about what managers should prepare for. This study by ARS researchers from Ft. Collins, CO showed that forage quality decreases, cool season grasses increase, and warm season grasses decrease under these global changes. The negative effects of global change were not affected by defoliation (simulated grazing), presumably because these plants are well adapted to defoliation, as a result of long evolutionary history of grazing. These results will help ranchers and public land managers anticipate changes in the quality and seasonal availability of forage under future climates.

7. Single-factor climate change experiments should be interpreted cautiously. Climate involves the simultaneous changes in atmospheric chemistry like atmospheric carbon dioxide (CO2) concentrations along with altered temperatures and precipitation patterns, and understanding how these multiple global changes can affect terrestrial ecosystems is complex. Due to fiscal and logistical constraints, most experiments evaluating how intact ecosystems respond to climate change are able to manipulate only one environmental factor at a time, and then interpret those results in the context of a world in which several environmental factors are known to be changing. This present synthesis activity involving scientists from ARS in Ft. Collins, CO, evaluates the results from a limited number of experiments which evaluated the responses of terrestrial ecosystems to manipulations in two global change factors, temperature and CO2. The results indicate that due to complex interactions involving multiple global change factors, the basic responses of ecosystems like plant production or nutrient cycling cannot be simply predicted from the results obtained in single-factor experiments. These findings provide insight into how simulation modeling can be used to help interpret single-factor experiments, as well as fundamental mechanistic information on how changes in nutrient cycling will condition the responses of ecosystems to climate change. Dieleman,W.I.J., S.Vicca , F.A. Dijkstra, F.Hagedorn, M.J. Hovenden, K.S. Larsen, J.A. Morgan, A. Volder, C.S Beier, J. S. Dukes, J. King,S. Leuzinger,S. Linder, Y. Luo, R. Oren, P. DeAngelis, D. Tingey, M. R. Hoosbeek and I.A.Janssens. 2012. Simply additive effects are rare: a quantitative review of plant biomass and soil process responses to combined manipulations of CO2 and temperature. Global Change Biology 18:2681–2693.

8. Abundant spring precipitation events drive semi-arid grassland C uptake. As both the amount and seasonal distribution of rainfall are intrinsic to climate change, scientists are concerned how such changes in the hydroclimate can affect soil moisture and the consequent activity of organisms at the land-atmosphere interface involved in the cycling of atmospheric carbon dioxide (CO2). Such activity can have significant impacts on the amount of CO2 which is released into Earth’s atmosphere and which is partially responsible for climate change. ARS scientists in Fort Collins, CO, installed Bowen ratio micrometeorology stations for monitoring fluxes of CO2 on a native shortgrass prairie and evaluated the effects of rainfall and temperature on soil water dynamics on CO2 fluxes from this semi-arid grassland for three years. More than 95% of CO2 uptake occurred in May and June each year, when plants were actively growing. Precipitation during these two months was most effective at promoting CO2 uptake. Precipitation events greater than approximately 0.4 inch (10 mm) consistently resulted in net uptake in CO2 due to increased photosynthesis, whereas precipitation events less than 0.4 inch resulted in CO2 loss to the atmosphere, from soil microbial respiration. These results indicate that primarily large, spring precipitation events drive carbon uptake in semiarid grasslands of the western Great Plains, and that climate-change driven alterations in seasonal precipitation patterns will impact the capacity of these grasslands to sequester C in their soils.

9. Ecosystems exhibit resilience to early 21st century drought. Climate change is underway, and as a result many world regions are predicted to experience increased drought and warming in the present century and beyond. An early 21st century drought permitted an evaluation of how such conditions may affect the functioning of ecosystems worldwide. In a collaborative study involving numerous ARS scientists at Fort Collins, CO and other ARS locations, along with university collaborators, satellite imagery was used to quantify the effects of the early 21st century drought on ecosystem productivity and resilience across two continents. The results indicate that plant productive capacity was maintained through prolonged warm drought by increases in plant water use efficiency during the driest years and resilience during wet years, as indicated by a common water use efficiency. However, extensions of these findings suggest that prolonged drought which results in significant plant mortality will threaten ecosystem resilience. This work provides foundational information for the development of satellite imagery techniques by which scientists can monitor the health of ecosystems world-wide and their susceptibility to climate change.

Review Publications
Dijkstra, F.A., Pendall, E., Morgan, J.A., Blumenthal, D.M., Carrillo, Y., Lecain, D.R., Follett, R.F., Williams, D.G. 2012. Climate change alters stoichiometry of phosphorus and nitrogen in semiarid grassland. New Phytologist. 196:807-815.

Perry, L.G., Shafroth, P.P., Blumenthal, D.M., Morgan, J.A., Lecain, D.R. 2013. Elevated CO2 does not offset greater water stress predicted under climate change for native and exotic riparian plants. New Phytologist. 197:532-543.

Dijkstra, F., Augustine, D.J., Brewer, P., Von Fischer, J. 2012. Nitrogen cycling and water pulses in semiarid grasslands: Are microbial and plant processes temporarily asynchronous?. Oecologia. 170:799-808.

Derner, J.D., Jin, V.L. 2012. Soil Carbon dynamics and rangeland management. In: Liebig, M.A., Franzluebbers, A.J., and Follett, R.F. (eds.). Managing agricultural greenhouse gases: Coordinated agricultural research through GraceNet to address our changing climate. Amsterdam, Netherlands: Academic Press. Book Chapter. p. 79-92.

Dijkstra, F., Morgan, J.A., Follett, R.F., Lecain, D.R. 2013. Climate change reduces the net sink of CH4 and N2O in a semiarid grassland. Global Change Biology. 19:1816-1826.

Dieleman, W., Vicca, S., Dijkstra, F., Hagedorn, F., Hovenden, M., Larsen, K., Morgan, J.A., Volder, A., Beier, C., Dukes, J. 2012. Simple additive effects are rare: A quantitative review of plant biomass and soil process responses to combined manipulations of CO2 and temperature. Global Change Biology. 18:2681-2693.

Lecain, D.R., Morgan, J.A., Hutchinson, G.L., Reeder, J.D., Dijkstra, F.A. 2012. Interactions between elevated atmospheric CO2 and defoliation on North American rangeland plant species at low and high N availability. Grass and Forage Science. 67:350-360.

Ponce Campos, G., Moran, M.S., Huete, A., Zhang, Y., Bresloff, C., Huxman, T., Eamus, D., Bosch, D.D., Buda, A.R., Gunter, S.A., Scalley, T., Kitchen, S., McClaran, M., McNab, W., Montoya, D., Morgan, J.A., Peters, D.C., Sadler, E.J., Seyfried, M.S., Starks, P.J. 2013. Ecosystem resilience despite large-scale altered hydro climatic conditions. Nature. 494:349-352.

Sorte, C.J., Ibanez, I., Blumenthal, D.M., Molinari, N., Miller, L.P., Grosholz, E.D., Diez, J.M., D'Antonio, C.M., Olden, J.D., Jones, S.J. 2012. Poised to prosper? A cross-system comparison of climate change effects on native and non-native species performance. Ecology Letters. 16:261-271.

Lind, E., Borer, E., Seabloom, E., Adler, P., Bakker, J., Blumenthal, D.M., Crawley, M., Davies, K., Firn, J., Gruner, D. 2013. Life history constraints in grassland plant species: A growth-defense trade-off is the norm. Ecology Letters. 16:513-521.