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PI-Submitted Research Highlights for
Terrestrial Ecosystem Science Program

Slow Ecosystem Responses Conditionally Regulate Annual Carbon Balance Over 15 Years in Californian Oak-Grass Savanna

Dennis Baldocchi
University of California, Berkeley

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The Science                  
Long term carbon flux measurements over Mediterranean-type ecosystems enable us to observe how the metabolism of an ecosystem responses to a wide range of physical, biological and ecological conditions. Biotic and abiotic extremes and legacies can introduce variations to annual ecosystem carbon balance, other than those that might be explained by the fast responses to factors like light and temperature.

The Impact
Spring droughts decreased gross primary productivity and ecosystem respiration.
Extremely wet springs reduced annual gross primary productivity.
Extra water availability enhanced oak photosynthesis during the summer.
Dry autumns and winters decreased ecosystem respiration.
Litter legacy effects occurred in the second year of two consecutive wet springs.

Summary
Many ecophysiological and biogeochemical processes respond rapidly to changes in biotic and abiotic conditions, while ecosystem-level responses develop much more slowly (e.g., over months, seasons, years, or decades). To better understand the role of the slow responses in regulating interannual variability in NEE, we partitioned NEE into two major ecological terms—gross primary productivity (GPP) and ecosystem respiration (Reco). We tested a set of hypotheses on seasonal scales using the flux and environment data collected from 2000 to 2015 in an oak-grass savanna area in California where ecosystems experience a wet winter and spring and a five-month-long summer drought each year. In our results, the spring season (Apr.–Jun.) contributed more than 50% of annual GPP and Reco. An analysis of outliers showed that each season could introduce significant anomalies in annual carbon budgets. The magnitude of the contribution depends on biotic and abiotic seasonal circumstances across the year and the particular sequences. We found that: (1) extremely wet springs reduced GPP in the years of 2006, 2011 and 2012; (2) soil moisture left from those extremely wet springs enhanced summer GPP; (3) groundwater recharged during the spring of 2011 was associated with the snowpack depth accumulated during the winter between 2010 and 2011; (4) dry autumns (Oct.–Dec.) and winters (Jan.–Mar.) decreased Reco significantly; (5) grass litter produced in previous seasons might increase Reco, and the effect of litter legacy on Reco was more observable in the second year of two consecutive wet springs. These findings confirm that biotic and abiotic extremes and legacies can introduce variations to annual ecosystem carbon balance, other than those that might be explained by the fast responses.

Contacts (BER PM)
Dennis Baldocchi
University of California, Berkeley
Baldocchi@berkeley.edu

(PI Contact)
Daniel Stover and Jared DeForest
SC-23.1
Daniel.Stover@science.doe.gov (301-903-0289) and Jared.DeForest@science.doe.gov (301-903-1678)

Funding
This research was conducted at the sites that are members of the AmeriFlux and Fluxnet networks. The research was supported in part by the Office of Science (Terrestrial Carbon Project), U.S. Department of Energy, Grant No. DE-FG02-03Reco63638 and through the Western Regional Center of the National Institute for Global Environ-mental Change under Cooperative Agreement No. DE-FC02-03Reco63613. Other sources of support included the Kearney Soil Science Foundation, the National Science Foundation and the Californian Agricultural Experiment Station. S. Wolf acknowledges support from a Marie Curie International Outgoing Fellowship (European Commission, grant 300083). 

Publications
Ma, S., D. Baldocchi, S. Wolf, and J. Verfaillie. 2016. Slow ecosystem responses conditionally regulate annual carbon balance over 15 years in Californian oak-grass savanna. Agricultural and Forest Meteorology 228-229:252-264. http://dx.doi.org/10.1016/j.agrformet.2016.07.016

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