Montane cloud-fog forest, generally recognized as a forested ecosystem with frequent fog immersion in montane regions, is of great value to hydrology and ecology.
Considerable cloud and fog droplets are set to become a vital factor in watershed yields and local biome growth, making the forest become a hotspot of species richness and biodiversity (Bruijnzeel et al., 2011; Bruijnzeel, 2001; Bubb et al., 2004; Goldsmith et al., 2013). Recently, such a unique ecosystem is facing a risk of fog disappearance.
Anthropogenic forcing, such as rising temperature and elevated CO2 concentration, may lift the cloud base height and influence water vapor supply from evapotranspiration, thus posing a threat to fog formation (Foster, 2001; Nair et al., 2003; Oliveira et al., 2014; Still et al., 1999; Williams et al., 2015).
Fog can remarkably affect the hydro-climatology in the forest (Anber et al., 2015;
Ataroff & Rada, 2000; Mildenberger et al., 2009). From the energy cycle perspective, fog can strongly block solar radiation, while diffused sunlight may increase due to better scattering ability of small water droplets (Anber et al., 2015; Lai et al., 2006). From the water cycle perspective, the wet environment can reduce the vapor pressure deficit.
Intercepted by the canopy, fog water cannot be negligible from total water input to the ecosystem. The water may account for up to 30% of total precipitation, supporting foliar uptake for some vegetation, especially in some regular dry seasons (Limm et al., 2012).
With the interception water above the stomata, transpiration is commonly reduced although some species can exceptionally maintain photosynthesis because of the xeromorphic traits on leaves (Chu et al., 2014; Goldsmith et al., 2013; Pariyar et al., 2017).
After fog events, evaporation from the canopy water may happen if there is enough solar radiation. The evaporation from the soil is relatively insignificant in the montane
cloud-fog forest and used to be neglected from the water balance equation of LH flux (Klemm et al., 2006). To summarize, combining the energy and water perspective, there is a consensus of a total reduction of LH flux under foggy conditions (Chu et al., 2014;
Goldsmith et al., 2013; Mildenberger et al., 2009).
Among all the cloud-fog forests, the characteristics of hydro-climatology in perhumid montane cloud-fog forest can be much more different. In the perhumid forest characterized by upslope fog, the amount of annual precipitation is twice more than that in typical forests, but the annual LH flux is half less (Bruijnzeel et al., 2011; Chu et al., 2014; Oliveira et al., 2014). Plentiful precipitation can serve as a source of canopy water, making the forest seldom suffer from moisture limitation. Besides, the canopy water usually can last longer on the leaves comparing to the duration of each fog and rain events in this foggy and wet environment. Therefore, canopy evaporation is expected to be a major contributor to LH flux (Chu et al., 2014; Giambelluca et al., 2009). Once the water vapor exchange from the land to the atmosphere, it cools near-surface temperature and moistens the boundary layer. The time scale of canopy evaporation is within one day, which is the shortest response among the other components (transpiration and soil evaporation) in the total LH flux (Wang et al., 2006).
Since the recurring fog also happens in daily timescale, high canopy evaporation in the perhumid montane cloud forest is expected to impact the fog climatology. Previous field studies were mostly done with intensive observation and used to focus on quantifying fog interception and the unidirectional effects of fog on LH flux (Chang et al., 2002; Chang et al., 2006; Klemm et al., 2006; Mildenberger et al., 2009). However, how fog interacts with the LH flux remains unclear from a climatological perspective, in which the long-term observation diurnal analysis is required.
Taiwan, where mountains account for about 60% of the island, is suitable for
studying the hydro-climatology in the montane cloud-fog forest, mainly located at 1500m to 2500m above mean sea level (Schulz et al., 2017; Thies et al., 2015). Long-term flux tower observations were implemented in different types of forests (Chen & Li, 2012; Chu et al., 2014; Klemm et al., 2006; Maneke-Fiegenbaum et al., 2018; Wey et al., 2011). The hydro-climatological characteristics in precipitation and LH flux can reflect the differences between Taiwan’s montane cloud-fog forest and non-cloud-fog forest. Higher annual precipitation but lower LH flux in Chi-Lan (CL) montane cloud-fog forest compared to LienHuaChih (LHC) non-cloud-fog forest can be seen. More importantly, an asymmetric diurnal cycle of LH flux with an early peak at 9 a.m. was found in CL montane cloud-fog forest (Fig. 1.1). This diurnal structure of LH flux is not in the same phase with net radiation, but with a couple of hours earlier than the net radiation. In contrast, such a phenomenon cannot be observed in LHC non-cloud-fog forest.
Our study aims to investigate relations between LH flux and fog in montane cloud-fog forest from diurnal and climatological perspectives. The present study will focus on how the asymmetric LH flux affects near-surface meteorology in montane cloud-fog forests, why the asymmetric LH flux occurs and whether the land or atmospheric forcing controls the emergence of the asymmetric LH flux. We hypothesize that the early peak of LH flux may cool down the temperature in the morning of the montane cloud-fog forest, and canopy water may be a key factor to the early peak of LH flux. The analyses compared the meteorological data from flux tower observations between CL montane cloud-fog forest and LHC non-cloud-fog forest. Besides, we conducted several offline land model simulations to examine the contribution of canopy water to the peak of LH flux in CL montane cloud-fog forest. Sensitivity tests were analyzed to find the controlling factors of the asymmetry of LH flux.