The observed responses of ecosystem CO2 exchange to climate variation from diurnal to annual time scale in the northern America. C. Yi, K.J. Davis, The Penn State University P.S. Bakwin, NOAA/CMDL T. Zhou, Beijing Normal University D.D. Baldocchi and colleagues, UC Berkeley S. Wofsy and colleagues, Harvard University

Interannual variability in the rate of accumulation of atmospheric CO 2

Issues What climatic factors control the interannual variability in NEE?What climatic factors control the interannual variability in NEE?

Part I: Climate variables and NEE at various time scales (NB, WB, WL, HF) Respiration and temperature Correlation between nighttime tower flux and air temperature is very high on daily, monthly and seasonal time scales. Correlation breaks down on interannual scales.

Respiration and temperature Northern OBS tower (NB) Manitoba, Canada Wofsy, Munger et al. Boreal black spruce forest

Respiration and temperature WLEF TV tower (WL) Northern Wisconsin, USA Davis, Bakwin et al. Mixed forest/wetland mosaic

Respiration and temperature Harvard Forest (HF) Massachusetts, USA Wofsy, Munger et al. Deciduous forest

Respiration and temperature Walker Branch tower (WB) Baldocchi, Wilson et al. Tennessee, USA Deciduous forest

Why does the temperature- respiration relationship break down on annual time scales? Hypotheses: Annual respiration is proportional to annual litter production which is a weak function of temperature? Temperature sensitivity is limited at the seasonal extremes (summer, winter).

Soil respiration  Litter production (Ta, P, Rn, fertilizer, …) Soil carbon pool

Respiration in autumn is much higher than in spring even though temperature is similar between these two seasons.

Seasonal distribution of temperature sensitivity B, (R e =Ae BT ). Spring B is the largest except at the NB site. There is no correlation between T and respiration in winter except at the WB site. Season NB B R 2 T WL B R 2 T HV B R 2 T WB B R 2 T Spring Summer Autumn Winter

Climate variables and NEE at various time scales (NB, WB, WL, HF) NEE of CO 2 and precipitation Correlation between NEE and precipitation is very poor on daily, monthly and seasonal time scales. Correlation becomes strong for interannual time scales.

NEE and precipitation Northern OBS tower (NB) Manitoba, Canada Wofsy, Munger et al. Boreal black spruce forest

NEE and precipitation WLEF TV tower (WL) Northern Wisconsin, USA Davis, Bakwin et al. Mixed forest/wetland mosaic

NEE and precipitation Harvard Forest (HF) Massachusetts, USA Wofsy, Munger et al. Deciduous forest

NEE and precipitation Walker Branch tower (WB) Baldocchi, Wilson et al. Tennessee, USA Deciduous forest

Climate variables and NEE at various time scales (NB, WB, WL, HF) NEE and net radiation Correlation between NEE and net radiation is strong on all time scales.

NEE and net radiation Northern OBS tower (NB) Manitoba, Canada Wofsy, Munger et al. Boreal black spruce forest

NEE and net radiation WLEF TV tower (WL) Northern Wisconsin, USA Davis, Bakwin et al. Mixed forest/wetland mosaic

NEE and net radiation Harvard Forest (HF) Massachusetts, USA Wofsy, Munger et al. Deciduous forest

NEE and net radiation Walker Branch tower (WB) Baldocchi, Wilson et al. Tennessee, USA Deciduous forest

Climate variables and NEE at various time scales (NB, WB, WL, HF) NEE and PAR Correlation between NEE and PAR is good on short time scales, and becomes worse on annual time scale.

NEE and PAR Northern OBS tower (NB) Manitoba, Canada Wofsy, Munger et al. Boreal black spruce forest

NEE and PAR WLEF TV tower (WL) Northern Wisconsin, USA Davis, Bakwin et al. Mixed forest/wetland mosaic

NEE and PAR Harvard Forest (HF) Massachusetts, USA Wofsy, Munger et al. Deciduous forest

NEE and PAR Walker Branch tower (WB) Baldocchi, Wilson et al. Tennessee, USA Deciduous forest

Summary Dependence of NEE on climatic factors is not consistent across time scales. Net radiation and precipitation become more correlated with NEE on annual time scale. Dryness=Rn/(L*P) may be used as an annual controlling parameter on interannual variability of NEE of CO2.

Discontinuous permafrost exists Water stress is not critical Soil thaw is critical; this depends on Rn Drought leads to more release of CO2 With abundant soil moisture, available energy is critical for CO2 uptake. As dryness>0.95, water stress becomes critical is the second year of drought is the third year of drought. (?) Drought has strong effect on interannual variability in NEE at WB. NB WL HV WB

HV-Harvard Forest (US,92-99) TH-Tharandt (Germany, 97-99) WL-WLEF (US, 97-99) WB-Walker Branch (US,95-98) NO-Norunda (Sweden,96-97) LW-Little Washita (US,97-98) LO-Loobos (Netherlands,97-98) HL-Howland (US, 96-97) HE-Hesse (France, 98-99) Across many sites Average per site over several years

Acknowledgements Funding and personnel support: –DoE – NIGEC – Midwest and Great Plains –NOAA CMDL –NASA – EOS Validation –DoE – TCP/TECO –NSF/NCAR –USDA-FS