Satellite Observations of Decadal Scale CO2 Fluxes Over Black Spruce Forests in Alaska Associated with Climate Variability

We improved the satellite-based empirical model (Kitamoto et al, 2007) by adding additional effective parameters on vapor pressure deficit (VPD) and photosynthesis, and estimated the CO₂ budget within black spruce forests in Alaska. In the stand scale validation, our modified model successfully reproduced the observed gross primary productivity (GPP), ecosystem respiration (RE), and net ecosystem exchange (NEE) by the eddy covariance measurement. The 10-day average of the model output was highly correlated with the observed GPP (r²=0.9), RE (r²=0.9), and NEE (r²=0.7). We used the modified model to estimate the regional GPP, RE, and NEE of black spruce forests over Alaska from 1982 to 2003 by using the normalized difference vegetation index (NDVI) from the Advanced Very High Resolution Radiometer (AVHRR) and climate data. The estimated regional averages of GPP, RE, and NEE were 2,172, 2,008, and -164 g CO m^-2 y^-1 during the past 22 years. Our model analysis showed that GPP was mainly affected by spring air temperature, whereas RE was affected by summer air temperature, indicating that the sink strength of the black spruce forests was controlled by the seasonality in air temperature between spring and summer. Path analysis enforced the notation that spring warming increased the CO₂ sink, but summer warming decreased the sink.