Does canopy wetness matter? Evapotranspiration from a subtropical montane cloud forest in Taiwan

Evapotranspiration (ET) and canopy wetness were measured over a 2-year intensive field campaign at the Chi-Lan Mountain cloud forest site in Taiwan. Eddy covariance and sap flow methods were applied to measure ET and tree sap flow of the endemic yellow cypress (Chamaecyparis obtusa var. formosana). ET was 553mmyr(-1) over the study period with an annual rainfall and fog deposition of 4893 and 288mmyr(-1), respectively. The duration of canopy wetness exceeded actual fog or rain events (mostly in the afternoon), and the intercepted water was evaporated later in the following dry morning. The cumulative wet duration accounted for 52% of time over the study period, which was longer than the duration of rainfall and fog altogether (41%). As it adapted to the extremely moist environment, the yellow cypress behaved in a wet-enhanced/dry-reduced water use strategy and was sensitive to short periods of dry atmosphere with high evaporation potential. During dry days, the sap flow rate rose quickly after dawn and led to conservative water use through midday and the afternoon. During periodically wet days, the canopy was mostly wetted in the morning, and the interception evaporation contributed largely to the morning ET. The initiation of morning sap flow was postponed 1-3h, and the sap flow rate tended to peak later at midday. The midday canopy conductance was higher in the periodically wet days (10.6mms(-1)) as compared with 7.6mms(-1) in the dry days. Consequently, the dry-reduced water use strategy led to much lower annual ET with respect to the available energy (similar to 46%) and high precipitation input (similar to 11%). The moist-adapted ecohydrology we report reveals the vulnerability of montane cloud forests to prolonged fog-free periods. More research is urgently needed to better understand the resilience of these ecosystems and formulate adaptive management plans. Copyright (c) 2012 John Wiley & Sons, Ltd.