On the relationship between cloudiness and net ecosystem carbon dioxide exchange in a peatland ecosystem
2005
Letts, Matthew G. | Lafleur, Peter M. | Roulet, Nigel T.
The relationship between incident light intensity and net ecosystem CO₂ exchange (NEE) was examined at a low-shrub bog located near Ottawa, Canada. Shrub height was 0.25 m and maximum leaf area index was 1.3 at the peatland. Light intensity was expressed as a clearness index (CI), where values approaching zero indicate heavy overcast conditions and values approaching unity represent cloudless conditions. Light saturation with respect to net CO₂ uptake at the canopy scale occurred at low CI, near 0.3. Contrary to reports from some forest ecosystems, in which peak NEE occurred with CI in the range 0.4-0.7, at the peatland there were no differences in NEE across all ranges of CI above 0.3. At the same time, CI in the range of 0.4-0.7 was infrequent and of short duration relative to clear skies and thick overcast conditions. Finally, we show that the use of half-hourly average radiation measurements to determine CI can lead to significant overestimation of the CI index in the range between 0.4-0.7.
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