Alternative spatial resolutions and estimation of carbon flux over a managed forest landscape in Western Oregon

Spatially-distributed estimates of biologically-driven CO2 flux are of inte rest in relation to understanding the global carbon cycle. Global coverage by satellite sensors offers an opportunity to assess terrestrial carbon (C) flux using a variety of approaches and corresponding spatial resolutions. An important consideration in evaluating the approaches concerns the scale of the spatial heterogeneity in land cover over the domain being studied. I n the Pacific Northwest region of the United States, forests are highly fra gmented with respect to stand age class and hence C flux. In this study, th e effects of spatial resolution on estimates of total annual net primary pr oduction (NPP) and net ecosystem production (NEP) for a 96 km(2) area in th e central Cascades Mountains of western Oregon were examined. The scaling a pproach was a simple 'measure and multiply' algorithm. At the highest spati al resolution (25 m), a stand age map derived from Landsat Thematic Mapper imagery provided the area for each of six forest age classes. The products of area for each age class and its respective NPP or NEP were summed for th e area wide estimates. In order to evaluate potential errors at coarser res olutions, the stand age map was resampled to grain sizes of 100, 250, 500 a nd 1000 m using a majority filter reclassification. Local variance in near- infrared (NIR) band digital number at successively coarser grain sizes was also examined to characterize the scale of the heterogeneity in the scene. For this managed forest landscape, proportional estimation error in land co ver classification at the coarsest resolution varied from -1.0 to +0.6 depe nding on the initial representation and the spatial distribution of the age class. The overall accuracy of the 1000 m resolution map was 42% with resp ect to the 25 m map. Analysis of local variance in NIR digital number sugge sted a patch size on the order of 100-500 m on a side. Total estimated NPP was 12% lower and total estimated NEP was 4% lower at 1000 m compared to 25 m. Carbon flux estimates based on quantifying differences in total biomass stored on the landscape at two points in time might be affected more stron gly by a coarse resolution analysis because the differences among classes i n biomass are more extreme than the differences in C flux and because the a dditional steps in the flux algorithm would contribute to error propagation . Scaling exercises involving reclassification of fine scale imagery over a range of grain sizes may be a useful screening tool for stratifying region s of the terrestrial surface relative to optimizing the spatial resolution for C flux estimation purposes.