The Science

Interannual variability in net carbon exchange in terrestrial ecosystems is large relative to its long-term mean. Furthermore, ecosystem photosynthesis contributed more to net carbon exchange than respiration.

The Impact

Long-term carbon flux measurements are needed for many reasons. Most importantly, these measurements enable the study of ecosystems on ecosystem time scales, which exceed decades. Long-term flux studies are needed to provide information on whether or not, and, if so, how fast, ecosystem metabolism may be responding to a changing world that is warmer, bathed in more carbon dioxide (CO₂), experiencing variation in rainfall and different degrees of nitrogen deposition, air pollution, and disturbance from humans, diseases, and pests. This behavior, co-occurring with other Earth system changes such as increasing global temperatures, a changing hydrological cycle and rising atmospheric CO₂ levels, will contribute to critically important longer time series measurements.

Summary

As the lifetime of regional flux networks approach 20 years, there are a growing number of papers that have published long-term records (five years or more) of net carbon fluxes between ecosystems and the atmosphere. Unanswered questions from this body of work are: (1) how variable are carbon fluxes on a year to year basis? (2) what are the biophysical factors that may cause interannual variability and/or temporal trends in carbon fluxes? and (3) how does the biophysical control on this carbon flux variability differ by climate and ecological spaces? To address these questions, researchers surveyed published data from 59 field study sites that reported on five or more years of continuous measurements, yielding 544 site-years of data.

A disproportionate fraction of the yearly variability in net ecosystem exchange was associated with biophysical factors that modulated ecosystem photosynthesis rather than ecosystem respiration. Yet, there was appreciable and statistically significant covariance between ecosystem photosynthesis and respiration. Consequently, biophysical conditions that conspired to increase ecosystem photosynthesis from one year to the next were associated with an increase in ecosystem respiration, and vice versa; on average, the year-to-year change in respiration was 40% as large as the year-to-year change in photosynthesis. The analysis also identified sets of ecosystems that are on the verge of switching from being carbon sinks to carbon sources. These include sites in the Arctic tundra, the evergreen forests in the Pacific Northwest, and some grasslands, where year-to-year changes in respiration are outpacing those in photosynthesis.

Principal Investigator

Dennis Baldocchi
University of California, Berkeley
baldocchi@berkeley.edu

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

This research was supported by funding from the Office of Biological and Environmental Research, within the U.S. Department of Energy (DOE) Office of Science, through the Terrestrial Ecosystem Science program and its support of the FLUXNET and AmeriFlux projects. Funding for the AmeriFlux Management Project was provided by the DOE Office of Science under Contract No. DE-AC02-05CH11231. Funding for FLUXNET was under contract DESC0012456.

References