PARENT SESSION
Poster Session #45: Gas Exchange.
Friday, August 10, 2001. Presentation from 10:30 AM to 12:00 PM. Exhibition Hall

Comparative study of canopy quantum yields among four AmeriFlux sites with different species and environmental conditions.

Hui, Dafeng¹, Luo, Yiqi¹, Katul, Gabriel², Ellsworth, David², Monson, Russ³, Wilson, Kell⁴, Wofsy, Steve⁵, ^{1 2 3 4 5}

ABSTRACT- Quantum yield (QY) is one of the important parameters influencing canopy carbon influxes. However, our knowledge of canopy QY is limited due to the difficulty in measuring canopy carbon flux. A recently developed technique, eddy-covariance, provides continuous measurements of carbon fluxes at canopy level. We selected four AmeriFlux sites: Niwot Ridge Forest (coniferous), Duke Forest (coniferous), Harvard Forest (deciduous) and Walker Branch Watershed (deciduous) for the analysis of canopy QY. We used an empirical rectangular hyperbolic equation to estimate daily canopy QY and maximum daytime canopy carbon flux (Pmax) in 1999. Results showed that both canopy QY and Pmax displayed strong seasonal variations and species differences. At Niwot Ridge site, QY increased from 0 to 0.035 in summer and decreased in winter. Pmax increased from 0 to 20 mol m^-2s^-1 at week 20 and kept at this level till week 44, then dropped to 0 again. Canopy QY at Duke Forest gradually increased to 0.02 and kept at that level for about 20 weeks, then declined as leaf area declined. Pmax increased to about 40 mol m^-2s^-1 at week 20 and decreased later. Estimated canopy QYs at Walker Branch and Harvard sites were similar, but Pmax at Walker Branch was higher than that at Harvard site. Besides PAR, temperature showed a strong influence on canopy QY. The seasonal change of canopy QY and its sensitivity to temperature may contribute to the interannual variation of measured ecosystem carbon flux.

KEY WORDS: canopy quantum yield, carbon flux, forest