U.S. Department of Energy, Office of Science

Program for Ecosystem Research

Research Project   Threshold responses to interacting global changes in a California grassland ecosystem

Principal investigator:   Christopher B. Field

Project goal

The project uses a "model ecosystem" (a California annual grassland near Stanford University that has been the subject of intensive global change research for many years) to assess the possible role of future warming, elevated carbon dioxide concentration, increased precipitation, and increased nitrogen deposition (alone and in combination) on the possibility of ecosystem state (or structure) changes from the "original" low-fertility annual grassland to a woody ecosystem, a thistle patch, or a high-fertility grassland containing legumes (symbiotic dinitrogen assimilators).

Ecosystem being studied

California annual grassland.


The climatic change variables are being manipulated in the field at the research site.

Why this is important

State changes in ecosystems would be likely triggers for the most significant changes in ecosystem structure and functioning that could occur as a result of ongoing and future climatic changes (and associated changes in atmospheric composition). This research project will provide some of the only field experimental data aimed directly at studying potential links between climatic changes and ecosystem state changes.

For decision makers, including the public, this research project can play a critical role in deepening the appreciation of the potential effects of climatic change on terrestrial ecosystems.


The research is building on the existing infrastructure of the Jasper Ridge Global Change Experiment (JRGCE) by extending the existing studies with two new experiments that focus on understanding the conditions under which ecosystems may shift from one type to another, as well as the mechanisms that may be responsible for such shifts.

The first new experiment involves 2 x 2 m plots in the grassland with herbivores removed, litter removed, tilling of the plots (complete disturbance), maintenance of ample soil moisture, and shading. These experiments are conducted with only native vegetation and with additions of seeds of shrubs and trees, seeds of invasive thistles, and seeds of dinitrogen-fixers. Measurements of state changes in the community composition will be conducted. Measurements of plant physiology and nitrogen and phosphorus cycling in the soil will also be carried out, using established methods. The treatments are being used to explore the ecological conditions/states/changes sufficient to allow a state change in this grassland ecosystem.

The second new experiment introduce six "state-changing" species (two shrubs/trees, two invasive thistles, two dinitrogen-fixers) into half of the replicates of the 16 treatments of the ongoing JRGCE. (The JRGCE was initiated in 1997 and involves two levels [one being ambient] each of atmospheric carbon dioxide concentration, temperature, precipitation, and nitrogen deposition.) Results of the first experiment were used to guide the method of introducing the species into the JRGCE plots (seed density, disturbance, and any protection from herbivory). Following introduction of the state-changing species, plot-scale measurements of plant community composition, standing biomass, leaf area, tissue composition, soil moisture, and soil nitrogen and phosphorus levels were made with existing techniques and technologies.


Christopher B. Field, Carnegie Institution of Washington, Department of Global Ecology

Nona Chiariello, Stanford University (subaward)

Harold A. Mooney, Stanford University (subaward)

Peter M. Vitousek, Stanford University (subaward)

Funding period:   July 2006 to present