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

Abstract

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 553 mm yr−1 over the study period with an annual rainfall and fog deposition of 4893 and 288 mm yr–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 1–3 h, and the sap flow rate tended to peak later at midday. The midday canopy conductance was higher in the periodically wet days (10.6 mm s–1) as compared with 7.6 mm s−1 in the dry days. Consequently, the dry‐reduced water use strategy led to much lower annual ET with respect to the available energy (~46%) and high precipitation input (~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 © 2012 John Wiley & Sons, Ltd.

Upscaling transpiration in diverse forests: Insights from a tropical premontane site

“…Although R s and VPD were strong drivers of transpiration responses, we also found evidence that leaf wetness suppressed transpiration until a significant threshold of dryness was surpassed. This is important given how frequently leaves were wet in this system, and also suggests that transpiration may be suppressed by leaf wetness in other humid regions (Alvarado‐Barrientos et al, ; Berry, Gotsch, Holwerda, Munoz‐Villers, & Asbjornsen, ; Chu et al, ; Gotsch et al, ; Reinhardt & Smith, ; Ritter, Regalado, & Aschan, ). We observed that transpiration during the two driest months with full records (February and March) was about 26% lower than the two wettest months (July and September, see Figure ).…”

The impact of climate change on water fluxes in a Macaronesian cloud forest

“…Although plants constrain water losses through stomata closure, laurel forest tree species have been previously acknowledged as having low stomatal control and thus follow a profligate water use strategy (González–Rodríguez et al, ; González–Rodríguez et al, ), and this was modelled here via the optimized parameter values of R g 0 and k υ . This feature is also shared by the Sequoia forests in the cloud‐immersed coastline of California and Oregon (Burgess & Dawson, ) and the yellow cypress montane cloud forests in Taiwan (Chu et al, ). For instance, large maximum conductance values up to 300 mmol m ‐2 s ‐1 were maintained for υ of 2–3 kPa in L. novocanariensis (formerly Laurus azorica (Seub.)…”

“…and k υ . This feature is also shared by the Sequoia forests in the cloudimmersed coastline of California and Oregon (Burgess & Dawson, 2004) and the yellow cypress montane cloud forests in Taiwan (Chu et al, 2014). the actual T were obtained during summer (Figure 13), a period during which registered fog water collection was higher (Figure 6a,b) and the site was immersed in the cloud bank ( Figure 5), hence the impact of fog disappearance is expected to be greater.…”

“…By contrast, the colder and wet environment established during the frequent fogs has been shown to provide the forest with the necessary conditions to down‐weigh water losses by tree transpiration (Ritter, Regalado, & Aschan, ), whereas benefiting with the associated carbon gain (Ritter & Regalado, ). Additionally, Chu et al () indicated that, after wet periods, the rain and fog water intercepted by the vegetation in a yellow cypress montane cloud forest acted mainly as a prior evaporation source during the next morning, and thus transpiration was halted or lagged. That rain and fog water deposited on the canopy remain in the vegetation for rather long periods (12–24 hr) have been also reported for some Lauraceae forests (Dietz, Leuschner, Hölscher, & Kreilein, ) and other fog‐affected stands (Klemm, Milford, Sutton, Spindler, & van Putten, ).…”

Detection of ground fog in mountainous areas from MODIS (Collection 051) daytime data using a statistical approach

“…This also holds true for the cloud forest of Taiwan (Hsieh, 2002). While cloud forest areas of Taiwan are already the subject of intensive research (cf., e.g., Mildenberger et al, 2009;Chu et al, 2012), Taiwan's cloud forest has never been completely mapped on a country-wide scale. The most comprehensive information about the extent of cloud forest in Taiwan available today is given by Li et al (2013), based on the National Vegetation Database of Taiwan.…”