次日尺度雲霧對山地雲霧森林植物反應之影響特徵分析

Abstract

雲霧透過降低入射太陽輻射、抑制地表加熱，並維持高濕度與低水氣壓差（vapor pressure deficit, VPD）環境，強烈影響山地雲霧森林（montane cloud-fog forests, MCFs）的植物反應。這些變化會透過多重且相互競合的作用機制，調節植物與大氣之間的碳水交換：太陽輻射減少會降低可利用光照與地表能量供應，進而抑制光合作用與蒸散作用；相對地，較低的 VPD 會減弱大氣水分需求，有助於維持氣孔開放，進而在一定程度上加強光合作用。然而，這些作用如何在次日尺度上影響植物目前仍不清楚。本研究以臺灣中央氣象署阿里山測站作為代表性山地雲霧森林站點，檢視次日尺度雲霧的時序對植物反應的影響。本研究使用 2000 至 2024 年共 25 年之夏季無雨日的小時觀測資料，利用凝聚式階層分群法將太陽輻射日循環分類為不同雲霧型態。接著，透過群集合成分析比較各雲霧型態對應的微氣候條件，並使用現地資料作為大氣強迫驅動單點 Community Land Model version 5.0 （CLM5.0）模擬，以估算光合作用、蒸散、氣孔導度與地表能量通量。結果顯示，雲霧日具有明顯不對稱的太陽輻射日循環，約 9 LST 後太陽輻射快速下降，並伴隨午後霧頻率增加。相較於晴朗日，雲霧日日出前有較濕冷的背景場，日出後更早生成的雲阻擋太陽輻射，限制地表加熱，再加上當地水氣的累積，使雲霧日較快達到飽和的環境條件。 CLM5.0 單點模擬結果顯示，相較於晴朗日，雲霧日的蒸散、潛熱通量與顯熱通量明顯降低，但光合作用在一天中多數時段仍維持相近水準。蒸散減少主要是因為太陽輻射降低限制了地表能量供應，同時低 VPD 減弱了大氣水分需求；相對地，低 VPD 有助於維持氣孔開放，因而部分抵銷雲霧日時較少可利用光照對光合作用的限制。這些結果顯示，次日尺度雲霧發生時序會透過可利用光照與大氣需求之間的平衡調節植物反應，創造了抑制水分與能量交換的環境，同時在一定程度維持碳吸收。

Cloud-fog strongly influences montane cloud-fog forests (MCFs) by reducing incoming solar radiation, suppressing surface heating, and maintaining humid, low vapor pressure deficit (VPD) conditions. These changes affect vegetation carbon and water exchanges through competing pathways: reduced solar radiation can limit photosynthesis and suppress transpiration by reducing light availability and energy supply, whereas lower VPD can weaken atmospheric water demand and help maintain stomatal opening, thereby partly sustaining photosynthesis. However, how these effects interact at sub-daily timescales remains unclear. This study investigates the impacts of sub-daily cloud-fog timing on vegetation response at the Alishan station, a representative MCF site in Taiwan. We analyze 25 years of hourly non-rainy summer observations from 2000 to 2024 and apply agglomerative hierarchical clustering to classify solar radiation diurnal cycles into distinct sub-daily cloud-fog patterns. Cluster-composite analyses are used to compare associated microclimatic conditions, and a single-point Community Land Model version 5.0 simulation driven by in-situ observations as atmospheric forcing is used to estimate photosynthesis, transpiration, stomatal conductance, and surface energy fluxes. The results identify Cloud-Fog Days as days with a pronounced asymmetric solar radiation cycle, characterized by a rapid decline in solar radiation after 9 LST and enhanced afternoon fog frequency. Compared with Sunny Days, Cloud-Fog Days are associated with a cooler and more humid pre-sunrise background, followed by premature morning cloud development that blocks incoming solar radiation, limits surface heating, and, together with moisture accumulation, promotes an earlier saturated, low-VPD environment favorable for afternoon cloud-fog formation. The CLM5.0 simulation suggests that Cloud-Fog Days substantially reduce transpiration, latent heat flux, and sensible heat flux relative to Sunny Days. Transpiration decreases because reduced solar radiation limits energy supply and low VPD weakens atmospheric demand. In contrast, photosynthesis remains comparable to Sunny-Day levels during much of the daytime, partly because low VPD maintains stomatal opening and offsets the effect of reduced light availability. These findings demonstrate that sub-daily cloud-fog timing regulates vegetation response through the balance between light availability and atmospheric demand, creating conditions that suppress water and energy exchanges while partly sustaining carbon uptake.