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.

The observed responses of ecosystem CO2 The observed responses of ecosystem CO2 exchange to climate variation from diurnal exchange to climate variation from diurnal

to annual time scale in the northern to annual time scale in the northern America.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

Interannualvariabilityin the rate ofaccumulationof atmosphericCO2

IssuesIssues

• What climatic factors control the What climatic factors control the interannual variability in NEE?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-Why does the temperature-respiration relationship break respiration relationship break

down on annual time scales?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).

0SdC

dt

Litter production - Soil respirationSdC

dt

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, (Re=AeBT ).

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 R2 T

WL 

B R2 T

HV 

B R2 T

WB 

B R2 T

Spring 0.0351 0.47 -5.1 0.0844 0.50 5.2 0.0445 0.38 5.1 0.0632 0.60 13.3

Summer 0.0676 0.36 13.3 0.0526 0.23 16.2 0.0255 0.08 17.0 0.0283 0.10 22.3

Autumn 0.0603 0.83 -1.3 0.0801 0.45 7.8 0.0231 0.18 8.0 0.0547 0.66 14.1

Winter 0.0126 0.08 -21.8 0.0142 0.04 -7.0 0.0283 0.08 -3.3 0.0340 0.46 4.4

Climate variables and NEE at various time scales

(NB, WB, WL, HF)

NEE of CO2 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

SummarySummary

• 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 existsWater stress is not criticalSoil 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.1998 is the second year of drought. 1999 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 sitesAverage 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