On the relationship between cloudiness and net ecosystem carbon dioxide exchange in a peatland ecosystem

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 (Cl), 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 Cl, near 0.3. Contrary to reports from some forest ecosystems, in which peak NEE occurred with Cl in the range 0.4-0.7, at the peatland there were no differences in NEE across all ranges of Cl above 0.3. At the same time, Cl 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 Cl index in the range between 0.4-0.7.