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Title: Turbulence considerations for comparing ecosystem exchange over old-growth and clear-cut stands for limited fetch and complex canopy flow conditions

Author: Wharton, Sonia; Schroeder, Matt; Paw U, Kyaw Tha; Falk, Matthias; Bible, Ken;

Date: 2009

Source: Agricultural and Forest Meterology. 149: 1477-1490

Publication Series: Scientific Journal (JRNL)

Description: Carbon dioxide (CO2), water vapor, and energy fluxes were measured using eddy covariance (EC) methodology over three adjacent evergreen forests in southern Washington State to identify stand-level age-effects on ecosystem exchange. The sites represent Douglas-fir forest ecosystems at two contrasting successional stages: old-growth (OG) and early seral (ES). We show an alternative approach to the usual friction velocity method for determining periods of adequate atmospheric boundary layer mixing based on the ratio of mean horizontal or vertical windflow to a modified turbulent kinetic energy scale. This new parameter in addition to footprint modeling showed that daytime CO2 fluxes in small clearcuts (<10 ha ) can be measured accurately with EC if micrometeorological conditions are carefully evaluated. Peak midday CO2 fluxes at OG were measured in April in both 2006 and 2007 before budbreak when air and soil temperatures and vapor pressure deficit were relatively low, and soil moisture and light levels were favorable for photosynthesis. At the early seral stands, peak midday CO2 fluxes were measured in June and July, whereas spring-time CO2 fluxes were much smaller. Overall, we measured lower evapotranspiration, higher midday CO2 fluxes, and higher Bowen ratios at the old-growth forest than at the ES sites during the summer months. Eddy covariance studies such as ours add critical land-atmosphere exchange data for an abundant, but rarely studied Douglas-fir age class.

Keywords: old growth, clearcut, Douglas-fir, ecosystem exchange, fetch, footprint modeling, turbulence statistics

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Wharton, S.; Schroeder, M.; Paw U, K.T.; Falk, M.; Bible, K. 2009. Turbulence considerations for comparing ecosystem exchange over old-growth and clear-cut stands for limited fetch and complex canopy flow conditions. Agricultural and Forest Meterology. 149: 1477-1490.

 


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