Exploration of the eco-hydro-climatic system of mountain cloud-fog forests (MCFs)

MCFs are defined as forests situated on mountainous terrain, perpetually immersed in low clouds and fog. These conditions foster a unique environment characterized by low light, high humidity, and a diverse eco-hydro-climatological system. MCFs alter the water and energy supply by intercepting cloud/fog and providing distinct ecosystem services. Crucial economic crops, such as tea and coffee, often grow in fog-cloud-forest regions, underlining the ecological and economic importance of MCFs. However, climate change and land-use changes can directly affect the water, energy, and carbon cycles, as well as cloud and fog conditions. Thus, a significant challenge in MCF research lies in understanding the complex interactions among ecology, hydrology, and climate, as well as the ecosystem’s vulnerability, especially under a changing climate. This project represents the first attempt to evaluate fog formation and its impacts on the earth system under climate change through direct measurements and numerical model simulations. We aim to explore the characteristics of the asymmetric evapotranspiration that composes the montane’s unique hydro-climatological cycle.

We utilized the datasets from Chi-Lan’s flux tower (located in northeast Taiwan, Figure 1), focusing on the diurnal variations of heat fluxes. The early peak of latent heat flux at Chi-Lan might result in a slower increase in near-surface temperature. The small diurnal temperature ranges, combined with water vapor accumulation from valley wind and local evapotranspiration, often saturate the air by around 3 p.m., thereby favoring fog formation. Subsequently, the canopy can intercept fog water in the afternoon. High relative humidity allows the wetness of the canopy surface to persist throughout the night, followed by evaporation of the canopy surface water the next morning. We further utilized a land surface model to demonstrate the critical role of canopy water in regulating latent heat flux. The model results reveal that canopy evaporation dominates the latent heat flux in Chi-Lan due to the abundance of canopy water. In summary, the unique hydro-climatological cycle of the montane cloud-fog forest comprises characteristics such as asymmetric latent heat flux, small diurnal temperature range, frequent fog occurrence, and ample canopy water, as summarized in the schematic plot below.