CO2 and water vapor fluxes above a subtropical mountain cloud forest—The effect of light conditions and fog

From the 4 August through 22 September 2006, the turbulent vertical fluxes of CO2 and water vapor were measured above a cypress tree (Chamaecyparis species) plantation within the subtropical mountain cloud forest range of NE Taiwan. The study site is characterized by a high frequency of orographic fog. During the day, humid air is transported uphill and causes condensation and fog as it cools. CO2 and water vapor fluxes were measured on two instrumented micrometeorological towers aligned along the valley axis. Despite the large reduction of solar radiation during foggy conditions by 64% as compared to clear situations, the reduction of the CO2 uptake is rather small at tower 1, for which Chamaecyparis is the dominating species within the footprint area. The Chamaecyparis species perform well under these extreme conditions of reduced light. A large section of the footprint area of tower 2 is vegetated with Cryptomeria japonica. CO2 flux comparison at both towers during foggy and non-foggy conditions, and direct tower-totower comparisons showed a significant difference statistically in the CO2 fluxes. While the photosynthesis rate of Chamaecyparis was hardly reduced during fog that of Cryptomeria japonica decreased strongly during foggy conditions. Cryptomeria japonica is less well adapted to the low light conditions. The net water vapor flux was dominated by evaporation of intercepted fog. Transpiration contributed only little to the water vapor flux during the 7-week experimental field study.