Author/Authors :
Xiao، نويسنده , , Jingfeng and Zhuang، نويسنده , , Qianlai and Law، نويسنده , , Beverly E. and Chen، نويسنده , , Jiquan and Baldocchi، نويسنده , , Dennis D. and Cook، نويسنده , , David R. and Oren، نويسنده , , Ram and Richardson، نويسنده , , Andrew D. and Wharton، نويسنده , , Sonia and Ma، نويسنده , , Siyan and Martin، نويسنده , , Timothy A. and Verma، نويسنده , , Shashi B. and Suyker، نويسنده , , Andrew E. and Scott، نويسنده , , Russell L. and Monson، نويسنده , , Russell K. and Litvak، نويسنده , , Marcy and Hollinger، نويسنده , , David Y. and Sun، نويسنده , , Ge and Davis، نويسنده , , Kenneth J. and Bolstad، نويسنده , , Paul V. and Burns، نويسنده , , Sean P. and Curtis، نويسنده , , Peter S. and Drake، نويسنده , , Bert G. and Falk، نويسنده , , Matthias and Fischer، نويسنده , , Marc L. and Foster، نويسنده , , David R. and Gu، نويسنده , , Lianhong and Hadley، نويسنده , , Julian L. and Katul، نويسنده , , Gabriel G. and Matamala، نويسنده , , Roser and McNulty، نويسنده , , Steve and Meyers، نويسنده , , Tilden P. and Munger، نويسنده , , J. William and Noormets، نويسنده , , Asko and Oechel، نويسنده , , Walter C. and Paw U، نويسنده , , Kyaw Tha and Schmid، نويسنده , , Hans Peter and Starr، نويسنده , , Gregory and Torn، نويسنده , , Margaret S. and Wofsy، نويسنده , , Steven C.، نويسنده ,
Abstract :
The quantification of carbon fluxes between the terrestrial biosphere and the atmosphere is of scientific importance and also relevant to climate-policy making. Eddy covariance flux towers provide continuous measurements of ecosystem-level exchange of carbon dioxide spanning diurnal, synoptic, seasonal, and interannual time scales. However, these measurements only represent the fluxes at the scale of the tower footprint. Here we used remotely sensed data from the Moderate Resolution Imaging Spectroradiometer (MODIS) to upscale gross primary productivity (GPP) data from eddy covariance flux towers to the continental scale. We first combined GPP and MODIS data for 42 AmeriFlux towers encompassing a wide range of ecosystem and climate types to develop a predictive GPP model using a regression tree approach. The predictive model was trained using observed GPP over the period 2000–2004, and was validated using observed GPP over the period 2005–2006 and leave-one-out cross-validation. Our model predicted GPP fairly well at the site level. We then used the model to estimate GPP for each 1 km × 1 km cell across the U.S. for each 8-day interval over the period from February 2000 to December 2006 using MODIS data. Our GPP estimates provide a spatially and temporally continuous measure of gross primary production for the U.S. that is a highly constrained by eddy covariance flux data. Our study demonstrated that our empirical approach is effective for upscaling eddy flux GPP data to the continental scale and producing continuous GPP estimates across multiple biomes. With these estimates, we then examined the patterns, magnitude, and interannual variability of GPP. We estimated a gross carbon uptake between 6.91 and 7.33 Pg C yr− 1 for the conterminous U.S. Drought, fires, and hurricanes reduced annual GPP at regional scales and could have a significant impact on the U.S. net ecosystem carbon exchange. The sources of the interannual variability of U.S. GPP were dominated by these extreme climate events and disturbances.
