坡地近地表含水率空間分布控制因子於淺層崩塌發生前後之變動

Abstract

崩塌是臺灣常見之山區擾動類型，崩塌干擾容易對森林水土環境造成改變，尤以陡峭、森林覆蓋之源頭集水區更為容易。近地表含水率之空間分布為森林水循環中重要的一環，本研究調查林業試驗所福山研究中心內之一處源頭集水區，在2016年發生崩塌後6年，以高空間解析度之調查近地表含水率與地形、植生、土壤物理性質之空間分布，依據干擾程度分為高、低干擾區後，並比較其與崩塌前同樣區所蒐集資料，瞭解崩塌擾動對含水率空間分布變異的影響，以及主要控制含水率分布的環境因子轉變。 崩塌後主要擾動區域位於高干擾區，其導致坡度變緩、上坡貢獻面積減少、地形濕潤指數分布的連接性中斷，大量林木與土壤的流失也顯示在植生指數與飽和水力傳導度的下降。土壤深度、介質孔隙率及有效孔隙率則在高、低干擾區無明確分區增減的趨勢。崩塌也導致原先位於高干擾區之地表逕流消失，崩塌後僅剩地表湧水分布。高干擾區原先具有之地表逕流消失，以及地形、土壤擾動皆使得高干擾區難以隨平均含水率增加而增加濕潤區域，因此使得崩塌後坡地可達之平均含水率下降，低含水率分布區域增加，空間分布異質性也下降。低干擾區則是維持崩塌前後相似的平均含水率與含水率空間變異程度。在崩塌後水土環境分布皆改變的結果下，崩塌後近地表含水率與多數環境因子空間分布之相關性減弱，且主要控制因子減少。在乾燥狀態時，崩塌後以植生密度之負向控制含水率分布能力最高，坡度次之；到中等狀態時，主要因子之控制性皆較乾燥狀態時下降。由於干擾程度的差異，因此儘管全區主要控制因子之控制性皆在崩塌後減弱，但受影響而減少主要控制因子的情形仍以高干擾區較低干擾區明顯。 本研究顯示崩塌對森林源頭集水區近地表含水率分布的擾動，導致坡地變的乾燥且含水率空間分布異質性降低，並在崩塌後6年環境間關聯性仍低，說明崩塌後除了以傳統植生恢復的角度觀察，近地表含水率的空間分布特性及相關水土環境變動，也應作為重要的評估項目。

Landslides in Taiwan's forested hillslopes can significantly impact the soil and water environment, particularly in steep headwater catchments. This study aimed to investigate the effects of a landslide that occurred in 2016 on near-surface soil moisture distribution using high-resolution data before and after the event. The study site is located at Taiwan Forestry Research Institute, Fushan Research Center. We also examined the influence of the landslide on topography, vegetation, and soil properties that affect soil moisture. The study site was divided into highly disturbed and low disturbed areas for analysis. After the landslide, noticeable changes occurred in the highly disturbed area, including a reduction in slope steepness, upslope contributing area, and connectivity of the topographic wetness index. Due to the loss of vegetation and soil matrix, decline in the vegetation indexes and saturated hydraulic conductivity were also observed. Other variable changes in porosity, soil depth or a decrease in effective porosity were observed in both highly and low-disturbed areas. The landslide had a significant impact on near-surface soil moisture distribution. The landslide resulted in the absence of surface runoff that distributed in the highly disturbed area, and was replaced by a few small springs. Both mean soil moisture and spatial variation decreased after the landslide and were due to the absence of surface runoff, disturbance of topography and soil in the highly disturbed area. On the other hand, low disturbed area maintained similar spatial variation after the landslide. The correlation between near-surface soil moisture distribution and its controlling factors weakened after the landslide in both highly and low-disturbed areas. Under dry conditions, vegetation density has the highest negatively controlling ability of soil moisture, second was the slope. Under moderate conditions, the controlling ability of both vegetation density and slope decrease controlling ability than under dry conditions. The highly disturbed area is still affected more than the low disturbed area. To summarize, landslides have a significant impact on near-surface soil moisture in forested headwater areas, leading to a reduction in mean soil moisture and spatial variation. The correlation between soil moisture and its controlling factors remains weak even six years after the landslide. Therefore, when addressing landslide-affected hillslopes, it is crucial to consider the spatial distribution of near-surface soil moisture and environmental changes alongside traditional restoration approaches.