, Volume 119, Issue 1–3, pp 85–108 | Cite as

Combined global change effects on ecosystem processes in nine U.S. topographically complex areas

  • Melannie D. HartmanEmail author
  • Jill S. Baron
  • Holly A. Ewing
  • Kathleen C. Weathers


Concurrent changes in climate, atmospheric nitrogen (N) deposition, and increasing levels of atmospheric carbon dioxide (CO2) affect ecosystems in complex ways. The DayCent-Chem model was used to investigate the combined effects of these human-caused drivers of change over the period 1980–2075 at seven forested montane and two alpine watersheds in the United States. Net ecosystem production (NEP) increased linearly with increasing N deposition for six out of seven forested watersheds; warming directly increased NEP at only two of these sites. Warming reduced soil organic carbon storage at all sites by increasing heterotrophic respiration. At most sites, warming together with high N deposition increased nitrous oxide (N2O) emissions enough to negate the greenhouse benefit of soil carbon sequestration alone, though there was a net greenhouse gas sink across nearly all sites mainly due to the effect of CO2 fertilization and associated sequestration by plants. Over the simulation period, an increase in atmospheric CO2 from 350 to 600 ppm was the main driver of change in net ecosystem greenhouse gas sequestration at all forested sites and one of two alpine sites, but an additional increase in CO2 from 600 to 760 ppm produced smaller effects. Warming either increased or decreased net greenhouse gas sequestration, depending on the site. The N contribution to net ecosystem greenhouse gas sequestration averaged across forest sites was only 5–7 % and was negligible for the alpine. Stream nitrate (NO3 ) fluxes increased sharply with N-loading, primarily at three watersheds where initial N deposition values were high relative to terrestrial N uptake capacity. The simulated results displayed fewer synergistic responses to warming, N-loading, and CO2 fertilization than expected. Overall, simulations with DayCent-Chem suggest individual site characteristics and historical patterns of N deposition are important determinants of forest or alpine ecosystem responses to global change.


Ecosystem models DayCent-Chem Carbon sequestration Greenhouse gases Nitrogen deposition Climate warming Nitrate 



Funding was provided by the EPA Clean Air Markets Division, the National Park Service Air Resources Division, and the US Geological Survey. We thank Amanda Elliot Lindsey for the graphics, and Lois St. Brice for her help with the Acadia simulations. We are grateful to the editor and the anonymous reviewers for very helpful comments. This is a product of the USGS Western Mountain Initiative.


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Copyright information

© US Government 2014

Authors and Affiliations

  • Melannie D. Hartman
    • 1
    Email author
  • Jill S. Baron
    • 2
  • Holly A. Ewing
    • 3
  • Kathleen C. Weathers
    • 4
  1. 1.Natural Resource Ecology LaboratoryColorado State UniversityFort CollinsUSA
  2. 2.U.S. Geological Survey, Natural Resource Ecology LaboratoryColorado State UniversityFort CollinsUSA
  3. 3.Program in Environmental StudiesBates CollegeLewistonUSA
  4. 4.Cary Institute of Ecosystem StudiesMillbrookUSA

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