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USGS Mendenhall Postdoctoral  Research Fellowship Program

S2. Assessing the Impact of Global Change on Landscape- to Regional-Scale Carbon Stocks in Dryland Ecosystems

Ecosystem carbon stocks and cycling play a critical role in the global climate system (Schimel, 2001; Houghton and others, 2009). Changes in ecosystem carbon stocks can directly influence the rate and magnitude of global temperature rise by altering atmospheric CO2 concentrations. Increases in ecosystem carbon stocks indicate CO2 removal from the atmosphere and provide a negative feedback to rising global temperatures, whereas decreases in ecosystem carbon stocks indicate CO2 addition to the atmosphere and a positive feedback to climate change (Heimann and others, 2008; Chapin and others, 2008).

Dryland ecosystems represent nearly half (47.2 percent) of the global land mass, and store roughly 27 percent of global ecosystem carbon stocks (Lal, 2004; Trumper and others, 2008). However, because dryland ecosystems have relatively low carbon density (carbon stock per area), the potential contribution of dryland systems to regional and global carbon budgets has received little attention. For example, although carbon stocks in dryland ecosystems account for 31 percent of total stocks in North America (Trumper and others, 2008) and numerous site-level studies have characterized the controls over carbon stocks and cycling in dryland ecosystems (Bradley and others, 2006; Jobbagy and Jackson, 2000; Jackson and others, 2005; Barger and others, 2011; Grote and others, 2010), relatively few studies have attempted to assess the regional-scale impact of global change on these stocks. Thus, we know relatively little about how these large pools of carbon will respond to combined changes in climate, land use practices and invasive species.

In particular, because ecosystem carbon dynamics are strongly related to vegetation activity, changes to the distribution of vegetation types could greatly affect carbon stocks and cycling in these ecosystems. We know that vegetation has the potential to change in response to many factors, notably climate change, land use practices and invasive species (Vitousek, 1994), and vegetation distribution expected to change even more in the future (Intergovernmental Panel on Climate Change, 2007). The overall goal of this project is to quantify how changes in the distribution of vegetation types resulting from global change will impact carbon stocks in arid and semi-arid ecosystems of the southwest United States.

Within the broad goal of assessing the carbon stock consequences of altered vegetation distributions, the postdoctoral scientist will have wide latitude to identify appropriate research. For example, there are exciting opportunities to regionally assess existing site-level carbon cycling results, apply ecological simulation models and (or) conduct landscape- to regional-scale spatial analyses. Although a variety of specific approaches are feasible and numerous existing relevant datasets available, we anticipate that quantifying current and future carbon cycling over large areas will entail compiling and building upon three general types of previous results:

  1. Knowledge about how carbon stocks vary across dominant vegetation types in the region: Ecosystem carbon stocks vary substantially and consistently among different vegetation types. Specific knowledge about these differences can be gleaned by reviewing published literature describing observational and manipulative field experiments that measured carbon stocks and fluxes in various vegetation types of dryland ecosystems, potentially via a formal meta-analysis. In addition, this review can be augmented by the extensive field campaigns and long-term datasets maintained by U.S. Geological Survey (USGS) scientists on the mentoring team.

  2. Estimates of how rapidly carbon stocks change following conversion from one vegetation type to another: Although the amount of carbon stored in soils, which represents a substantial portion of total carbon stocks, is linked to above-ground vegetation, the relationship between vegetation and soil carbon remains poorly elucidated. Estimates of soil carbon change with vegetation change can be obtained through published results including experiments that manipulated vegetation type and assessed soil responses, and from observational studies of sites representing chronosequences of time since vegetation conversion. In addition, ecological simulation modeling can be used to estimate how carbon stocks will change over time following vegetation conversion. The postdoctoral scientist will have the opportunity to conduct model simulations and/or synthesize previous results.

  3. Insight into how distribution of vegetation types may be impacted by global change: This project will require insight about the current and potential future distribution of vegetation types. Current distributions are available from a number of sources, most notably the national gap analysis program (GAP). The impact of altered climatic conditions, land-use practices and biological invasions on vegetation distribution can be estimated through bioclimatic envelope mapping, predicted trajectories of land-use patterns, and predictions of important biological invasions.

  4. Landscape to regional analysis of potential changes in carbon stocks: With these three components in hand, the postdoctoral scientist will be in a position to conduct spatial analysis, likely in a GIS framework, to estimate landscape- to regional-scale carbon stocks under current conditions and predict how those carbon stocks are likely to change in response to future global change.

All four of the above components offer exciting opportunities for advancing knowledge and generating scientific products. The specific details of how the research project would be focused among the components and how the integrative analysis is conducted can be determined by the postdoctoral scientist and mentoring team, taking into consideration the postdoctoral scientist’s expertise, interests and long-term goals. Conducting an integrative project of this nature will provide the postdoctoral scientist with a broad perspective on the state of knowledge and levels of uncertainty inherent in generating ecosystem carbon estimates over large areas. Although all of the data collection components (1–3 above) could be satisfied by compiling results from previous work, the postdoctoral scientist will have the opportunity to focus on one or more components and address particular knowledge gaps and/or pursue specific research interests. Collected data could then be synthesized within the modeling framework to offer predictive insight into future dryland carbon stocks (4 above).

Barger, N.N., and others, 2011, Woody plant proliferation in North American drylands: A synthesis of impacts on ecosystem carbon balanc: Journal of Geophysical Research, v. 116, G00K07, doi:10.1029/2010jg001506.

Bradley, B.A., Houghton, R.A., Mustard, J.F., and Hamburg, S.P., 2006, Invasive grass reduces aboveground carbon stocks in shrublands of the Western US: Global Change Biology, v. 12, p. 1815–1822.

Chapin, F.S., Randerson, J.T., McGuire, A.D., Foley, J.A., and Field, C.B., 2008, Changing feedbacks in the climate-biosphere system: Frontiers in Ecology and the Environment, v. 6, p. 313–320.

Grote, E.E., Belnap, J., Housman, D.C., and Sparks, J.P., 2010, Carbon exchange in biological soil crust communities under differential temperatures and soil water contents: implications for global change: Global Change Biology, v. 16, p. 2763–2774.

Heimann, M., and Reichstein, M., 2008, Terrestrial ecosystem carbon dynamics and climate feedbacks: Nature, v. 451, p. 289–292.

Houghton, R.A., Hall, F., and Goetz, S.J., 2009, Importance of biomass in the global carbon cycle: Journal of Geophysical Research, v. 114, G00E03, doi:10.1029/2009jg000935.

Intergovernmental Panel on Climate Change, 2007, in Solomon, S., and others, eds., Climate change 2007: The physical science basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change: Cambridge, Cambridge University Press.

Jackson, R.B. , and others, 2005, Trading water for carbon with biological sequestration: Science, v. 310, p. 1944–1947.

Jobbágy, E.G., and Jackson, R.B., 2000, The vertical distribution of soil organic carbon and its relation to climate and vegetation: Ecological Applications, v. 10, p. 423–436, doi:10.1890/1051- 0761(2000)010[0423:tvdoso];2.

Lal, R., 2004, Carbon sequestration in dryland ecosystems: Environmental Management, v. 33, p. 528–544, doi:10.1007/s00267-003-9110-9.

Schimel, D.S., and others, 2001, Recent patterns and mechanisms of carbon exchange by terrestrial ecosystems: Nature, v. 414, p. 169–172.

Trumper, K., Ravilious, C., and Dickson, B., 2008, Carbon in drylands: Desertification, climate change and carbon finance: A UNEP-UNDP-UNCCD Technical Note for Discussions at CRIC 7, Istanbul, Turkey, 03–14 November, 2008.

Vitousek, P.M., 1994, Beyond global warming: ecology and global change: Ecology, v. 75, p. 1861–1876.

Proposed Duty Station: Flagstaff, AZ

Areas of Ph.D.: Ecology, botany, soil science, biogeochemistry, geology, wildlife biology (candidates holding a Ph.D. in other disciplines but with knowledge and skills relevant to the Research Opportunity may be considered).

Qualifications: Applicants must meet one of the following qualifications: Research Ecologist, Research Botanist, Research Biologist, Soil Scientist

(This type of research is performed by those who have backgrounds for the occupations stated above. However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Human Resources specialist.)

Research Advisors:John Bradford, (925) 523-7766,; Sasha Reed, (406) 243-4325,; Michael Duniway, (435) 719-2330,; Jayne Belnap, (435) 719-2333,

Human Resources Office Contact: Candace Azevedo, (916) 278-9393,

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U.S. Department of the Interior, U.S. Geological Survey
Direct inquiries to Rama K. Kotra at
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