Abstract
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 km2 regional flux estimate found June to
August 2003 NEE, ER and GEP to be 290 89, 408, 48, and 698, 73 gC m-2, respectively.
Aggregated NEE, ER and GEP were 280% larger, 32% smaller and 3% larger, respectively, than
that observed from a regionally integrating 447m 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 worsened the comparison to observed fluxes. These results provide insight on the range
of spatial sampling, replication, measurement error and land cover accuracy needed for
multi-tiered bottom-up scaling of CO2 fluxes in heterogeneous regions such as the Upper
Midwest, USA.
