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PI-Submitted Research Highlights for
Terrestrial Ecosystem Science Program

Inter-annual variability of net and gross ecosystem carbon fluxes

Dennis Baldocchi
University of California, Berkeley

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Jan 8, 2018

A Review

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
We need long term carbon flux measurements for many reasons. Most importantly is to study 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 CO2, experiencing variation in rainfall and different degrees of nitrogen deposition, air pollution and disturbance from humans, diseases and pests. This behavior, with co-occurring with other Earth system changes such as increasing global temperatures, 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 twenty years, there is a growing number of papers that have published long-term records (5 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 to 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.

Contacts (BER PM)
Daniel Stover
SC-23.1
Daniel.Stover@science.doe.gov (301-903-0289)

(PI Contact)

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

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

Publications
Dennis Baldocchi, Housen Chu, Markus Reichstein, Inter-annual variability of net and gross ecosystem carbon fluxes: A review, In Agricultural and Forest Meteorology, Volume 249, 2018, Pages 520-533, ISSN 0168-1923, 10.1016/j.agrformet.2017.05.015.
(http://www.sciencedirect.com/science/article/pii/S0168192317301806)

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