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Harvard Forest Symposium Abstract 2009

  • Title: Improved measurements of whole-forest carbon dioxide exchange at eddy flux tower sites and insights into the effect of foliage on storage of CO2 in the sub-canopy air space
  • Primary Author: Julian Hadley (Harvard Forest)
  • Abstract:

    In the spring of 2008 with support from the DOE Terrestrial Carbon Program, we installed a carbon dioxide profile system to measure CO2 concentrations in the air between the soil surface and the eddy flux system at Little Prospect Hill. This data is important because CO2 that is stored below the flux measurement point and is not subsequently released to the atmosphere where it can be measured by the eddy flux system, must be added to the value of CO2 efflux that the flux system measures, when calculating respiration by the ecosystem. The CO2 profile system showed significant accumulation of CO2 between the ground and the forest canopy at night, especially before midnight. Both the rate of CO2 accumulation in the air space below the canopy and CO2 efflux from the canopy top declined during the night, suggesting that air stored below the canopy early in the night was not vented later through the canopy top later, but was removed from the subcanopy air space by another process. This process is very likely gravity-driven down-slope cold air advection (air flow) on this sloping site. During summer, addition of nighttime CO2 accumulation to eddy flux data will lead to increasing the estimate of nighttime CO2 production by the forest by around 1 µmol m-2 s-1, or 20 to 25%. Data from the fall and winter of 2008-2009, which will determine how large a correction must be made to fall and winter CO2 production estimates, are still being analyzed. It is clear however, that because of subcanopy CO2 accumulation, annual estimates of ecosystem carbon storage or net ecosystem production (NEP) will be revised downward.

    The presence or absence of canopy foliage appears to have a marked effect on CO2 accumulation in the air space between the canopy and the ground. We used the ratio of photosynthetically active radiation (PAR) in the forest understory to PAR above the canopy to detect changes in the amount of canopy foliage. Subcanopy CO2 accumulation, especially between 6 pm and midnight, fell markedly between October 10 and October 16. This was the period when autumn leaf abscission began, as evidenced by an increase in understory PAR from 3% to 9% of above-canopy PAR.

  • Research Category: Forest-Atmosphere Exchange