PARENT SESSION
Oral Session # 19: Biogeochemistry II: Grasslands.
Presiding: JT Lennon
Tuesday, August 5. 8:00 AM to 11:30 AM, SITCC Meeting Room 105.

NSF-I.R.C.E.B. Interannual climate variability and ecosystem processes in tallgrass prairie: Effects on ecosystem CO₂ fluxes.

Arnone, John^*,1, Schorran, David¹, Johnson, Dale², Wallace, Linda³, Luo, Yiqi³, Coleman, James¹, Schimel, David⁴, Verburg, Paul¹, ¹ Desert Research Institute, Reno, NV² University of Nevada, Reno, NV³ University of Oklahoma, Norman, OK⁴ National Center for Atmospheric Research, Boulder, CO

ABSTRACT- Mechanistic numerical models currently in use can reliably estimate the effects of intra-annual, or seasonal, effects on terrestrial ecosystem CO₂ exchange. However models become less robust when estimating interannual, or year-to-year, effects because the response times and lagged behavior of key ecosystem processes are poorly understood. In a 4-year experiment using the EcoCELL whole-ecosystem controlled-environment gas exchange facility at DRI, we are quantifying the responses to an anomalously warm year of interlinked ecosystem processes that are involved in modulating the ecosystem CO₂ fluxes. We excavated, containerized, and installed twelve 13,000 kg intact tallgrass prairie soil monoliths with their respective plant communities into four separate EcoCELLs–three monoliths per EcoCELL–in the fall of 2001. Starting in February 2002, have been exposing all monoliths to an "average" Oklahoma climate. In February 2003, we increased the air temperature in two of the EcoCELLs by 4°C to simulate an anomalously warm year while maintaining an "average" climate (air temperature) in the other two EcoCELLs. Mean daily net ecosystem CO₂ exchange (NEE) in 2002 increased from wintertime lows (Feb. 2002 NEE: 0.39 mol CO₂ m^-2 s^-1 uptake) to relatively large rates of uptake (June 2002: 4.34 mol CO₂ m^-2 s^-1) as plant communities developed, followed by immediate and dramatic declines in NEE after aboveground harvest (September 2002: 0.22 mol CO₂ m^-2 s^-1). Results to date indicate "normal" responses of NEE to plant phenology, and significant initial responses to experimental warming (Feb. 2003: -0.12 mol CO₂ m^-2 s^-1, CO₂ efflux).

Key words: net ecosystem productivity, heterotrophic respiration, ecosystem carbon balance, net primary productivity