Influence of vegetation and seasonal forcing on carbon dioxide fluxes across the Upper Midwest, USA: Implications for regional scaling

Carbon dioxide fluxes were examined over the growing seasons of 2002 and 2003 from 14 different sites in Upper Midwest (USA) to assess spatial variability of ecosystem–atmosphere CO2 exchange. These sites were exposed to similar temperature/precipitation regimes and spanned a range of vegetation types typical of the region (northern hardwood, mixed forest, red pine, jack pine, pine barrens and shrub wetland). The hardwood and red pine sites also spanned a range of stand ages (young, intermediate, mature). While seasonal changes in net ecosystem exchange (NEE) and photosynthetic parameters were coherent across the 2 years at most sites, changes in ecosystem respiration (ER) and gross ecosystem production (GEP) were not. Canopy height and vegetation type were important variables for explaining spatial variability of CO2 fluxes across the region. Light-use efficiency (LUE) was not as strongly correlated to GEP as maximum assimilation capacity (Amax). A bottom-up multi-tower land cover aggregated scaling of CO2 flux to a 2000 km 2 regional flux estimate found June to August 2003 NEE, ER and GEP to be 290 89, 408 48, and 698 73 gC m , respectively. Aggregated NEE, ER and GEP were 280% larger, 32% smaller and 3% larger, respectively, than that observed from a regionally integrating 447 m tall flux tower. However, when the tall tower fluxes were decomposed using a footprint-weighted influence function and then reaggregated to a regional estimate, the resulting NEE, ER and GEP were within 11% of the multi-tower aggregation. Excluding wetland and young stand age sites from the aggregation * Corresponding author. Present address: University of Wisconsin-Madison, Atmospheric & Oceanic Sciences Dept., AOSS 1549, 1225 W Dayton St, Madison, WI 53706, USA. Tel.: +1 608 265 9292; fax: +1 608 262 0166. E-mail address: [email protected] (A.R. Desai). avai lable at www.sc iencedi rec t .com journal homepage: www.e lsev ier .com/ locate /agr formet 0168-1923/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.agrformet.2007.08.001 Author's personal copy