Observational uncertainties in net ecosystem CO2 exchange: the effect of complex terrain.
Monson, Russell*,1, Yi, Chuixiang1, Anderson, Dean 2, Sun, Jeilun3, Lamb, Brian4, Burns, Sean1, 3, 1 University of Colorado, Boulder2 US Geological Survey, Lakewood3 National Center for Atmospheric Research, Boulder4 Washington State University, Pullman
ABSTRACT- Measurements of net ecosystem CO2 exchange (NEE) using continental tower flux networks provide a critical constraint in models of regional and global carbon budgets. Uncertainty exists in these measurements due to the effects of complex terrain and vegetation gradients. Using an array of seven towers distributed across a mountain landscape, we estimated that a significant error exists in the five-year record of measured net ecosystem CO2 exchange. The error was due to the previously ignored influence of advective CO2 flux divergence. When this error was rectified by explicit consideration of the advective flux components, the forest was predicted to exhibit a 38% higher potential for carbon sequestration than previously thought. Horizontal and vertical CO2 flux divergence during the summer growing season was inversely correlated with surface friction velocity (u*) below an upper limit, indicating that the advective fluxes were of the greatest magnitude and most frequent when the atmosphere was relatively stable as occurs during nighttime periods. Both horizontal and vertical advective CO2 fluxes were caused by nighttime, downslope drainage flows that occur at the site. We released SF6 tracer upslope from the primary measurement tower. This experiment revealed the nature of the drainage flows to be thin layers of air that move close to the ground as they carry respired CO2 downslope, beneath the tower flux sensors. Our results represent the first comprehensive assessment of observational uncertainties in net ecosystem CO2 exchange due to the effects of complex terrain. Regional and global CO2 budgets are inherently dependent on measurement networks of NEE to provide observational constraint on models and inventory procedures. The uncertainties in these observational constraints must be carried into the uncertainties of the carbon budgets themselves. Complex terrain can induce atmospheric flows and mean CO2 concentration gradients that are not normally accounted for in regional budgets. Native forest ecosystems often occur in hilly or mountainous terrain and it these ecosystems that often reflect the principal carbon sinks in a region and those most in need of accurate accounting during the measurement of NEE.
Key words: carbon, ecosystem
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