Research in the Field lab during the past year continued to address a broad set of questions concerning the global carbon cycle and ecosystem responses to global change. About half the lab worked on experimental projects at the ecosystem scale and half worked on modeling or data synthesis at the global scale. Both groups contribute to the overall goal of developing a framework for assessing the current function and predicting the future function of the integrated global biosphere.

This year marked the conclusion of the first phase of the Jasper Ridge Global Change Experiment. In this project, Field, several members of his lab, and collaborators from Stanford University are examining the responses of grassland ecosystems to 4 major components of global change: warming, elevated atmospheric carbon dioxide, nitrogen deposition, and increased precipitation. This is the first experiment to apply this set of manipulations in all possible combinations to largely natural ecosystems exposed to natural variations in weather, soils, and disturbance. Results from the first phase of the experiment indicate that many combinations of factors elicit new responses that are not simply the mean of the responses to the individual factors. Much of the response that affects ecosystem function occurs through changes in species composition rather than changes in individual physiology or biochemistry. Even some of the single-factor responses have important implications for global change. For example, former postdoctoral fellow Matthias Rillig (now on the faculty at the University of Montana) found that warming increases the proliferation of mycorrhyzal fungi, which assist plants with nutrient uptake but decrease the abundance of a key mycorrhizal protein that is critical for soil stabilization. Stanford Ph.D. student Erika Zavaleta found that elevated carbon dioxide levels increased the survival of shrub seedlings. This is a result with mixed implications for ecosystem services, because shrub invasion decreases the suitability of grasslands for grazing and many other uses while potentially increasing their ability to store carbon. In future phases of this experiment, the lab plans to increase its emphasis on the mechanisms underlying differences between short-term and long-term responses, to develop principles for generalizing results of this experiment to other ecosystems, and to use molecular tools for understanding the behavior of microbial communities in the soil.

At the global scale, the focus in the Field lab has been split between characterizing the function of natural ecosystems and quantifying human impacts, especially on forests. One project, conducted in collaboration with investigators at several universities and NASA labs, is using data from a satellite called SeaWiFS to map carbon dioxide uptake of plants on land and in the oceans. SeaWiFS, launched in September 1997, provides comprehensive global coverage including dramatic changes in ocean primary production (carbon dioxide uptake by ocean plants) between the El Nino of 1997 and 1998 and the La Nina of 1999 and 2000. Surprisingly, changes in ocean primary production, which are driven by changes in the delivery of nutrients through circulation of the water, were more dramatic than changes in land primary production. The latter changes are driven largely by changes in precipitation. In the future, the lab will test the generality of this result and evaluate its impact on the global carbon cycle.

The lab’s research on global forests combined thousands of ground-based observations to develop a picture of carbon storage in forests across the temperate and boreal latitudes (23-67º) of the Northern Hemisphere. Postdoctoral associate Christine Goodale discovered that these forests are storing large amounts of carbon, on the order of 500 million tons per year. However, based on indirect estimates from atmospheric data, this amount is less than expected. The unexpectedly low carbon uptake reflects a large impact of disturbances, especially fires, severe storms, and insect outbreaks. Reconciling the estimates based on methods from forestry and atmospheric sciences requires significant carbon sinks outside the forest sector. The invasion of grasslands by shrubs is one such example. This last phenomenon, something the lab is also addressing at the ecosystem scale, provides a nice example of how research initiated at contrasting scales can converge to produce a new level of understanding.