臺灣東部山區雲霧森林之微氣候韌性

Abstract

頻繁的雲霧籠罩與高相對濕度在雲霧森林的水文氣候循環中扮演重要的角色。在其獨特的微氣候特徵之下，雲霧森林孕育著多樣的物種，使其被視為全球的生物多樣性熱點。然而，過去有許多研究提及雲霧森林在氣候變遷下的脆弱度，其中有眾多文獻強調了氣候變遷下雲霧減少對雲霧森林的影響。相較於整個熱帶而言，臺灣的雲霧森林在森林面積中佔有相對高的比例，但過去有較少文獻針對臺灣的雲霧森林討論其在氣候變遷下所受到的衝擊。本研究透過分析臺灣東部1980到2021年的降尺度氣候資料，討論雲霧森林與非雲霧森林區域的微氣候在平均態及長期變異上的差異。結果顯示，坐落於中海拔區域的雲霧森林，其白天的相對濕度相較於臺灣東部其他區域有最高的平均量值以及最小的年際變化。當地的高相對濕度可以歸因於其白天大量的水氣來源，以及在克勞修斯-克萊佩龍方程(Clausius-Claypeyron relation)對溫度的非線性影響之下比低海拔地區小得多的飽和水氣量。另一方面，由於雲霧森林及高海拔區域的地表蒸發散量大多取決於當地能量多寡而非可用水量，使當地蒸發散得以調節乾濕年時，地表向大氣的水氣供應。在相近的水氣變化下，雲霧森林相較於高海拔區域更暖的溫度使其白天的相對濕度擁有較小的變化幅度。從相對濕度年際變化的穩定性來看，臺灣東部的雲霧森林展現出相較於其他區域更大的韌性，有別於過去文獻所強調在氣候變遷下雲霧森林的脆弱度。未來研究需更進一步了解外部水氣供應對雲霧森林相對濕度變化的貢獻，並且探討本研究所發現雲霧森林的韌性在未來氣候變遷下是否還將存在。

Frequent immersion of clouds at ground level and high relative humidity (RH) play vital roles in the hydroclimatological cycle of montane cloud forests (MCFs). These unique microclimates support a wide variety of species, making MCFs renowned as global biodiversity hotspots. However, the vulnerability of MCFs to climate change has been frequently mentioned in previous research, with many studies highlighting the impacts of reduced cloud and fog occurrence on these ecosystems that heavily rely on fog. While some of these studies have examined the conditions of MCFs in Taiwan, there remains a dearth of research on this topic. By analyzing downscaled climate data with a spatial resolution of 2km and a temporal resolution of hourly from 1980 to 2021, our study investigated the microclimate characteristics of MCFs in Eastern Taiwan. We compared these characteristics, including their mean state and long-term variations, with those of non-cloud-forested regions that encompassed all land types other than cloud forests. Our findings revealed that the MCFs, located in the mid-altitudes of Eastern Taiwan, exhibited the highest average daytime RH among the analyzed regions, while displaying the smallest interannual variations. The high mean RH can be attributed to the abundant water vapor supply during the daytime and the nonlinear effect of the Clausius-Clapeyron (C-C) relation on temperature that resulted in much smaller saturated water vapor amounts in the MCFs compared to lower elevations. Both the MCFs and high-altitude regions experienced energy-limited conditions, regulating their water vapor supply through evapotranspiration in dry and wet years. However, the warmer temperatures in the MCFs contributed to smaller variations in daytime RH compared to high-altitude regions. The high resilience of RH in Eastern Taiwan’s MCFs, with minimal interannual variations, suggests a contrast to the vulnerability emphasized in previous studies. Further investigations may be necessary to facilitate our understanding of the contribution of non-local water vapor supply to changes in RH and to determine whether the resilience we found in the MCFs will persist in the face of future climate change.