利用樹液流法測量溪頭柳杉人工林之蒸散狀況及變異

Abstract

在森林生態系中，蒸散作用是水文循環中一個重要的過程。在森林中，樹液流測量法是一個測量單株樹木個體尺度蒸散量的可靠方法。雖然已有研究報告指出樹液流在個體內有顯著的空間分布差異，並在估算個體樹木蒸散時造成困難，但卻很少有研究詳細檢視這些空間分布差異在季節上的變化。因此，本研究在位於台灣中部的台大溪頭實驗林的柳杉人工林中實施樹液流測量，試圖釐清樹木個體之內在空間與時間上的變異。本研究使用Granier的熱擴散探針法，在兩顆較小、三棵中等、三棵較大的柳杉上進行測量。測量期間為2010年七月到2011年一月。所得的樹液流資料經過處理，分析樹液流流速與生物計量參數如胸高直徑（DBH）、樹高、邊材厚度及邊材面積之間的關係，並且分析於不同深度及方向所測得的樹液流流速之間的相關性。 研究結果，在樹液流徑向分布變異的分析中，可以得知在整個測量期間當中，於深度2–4 cm處所測得的樹液流流速，約為深度0–2 cm處所測得樹液流流速的50–60 %。而在樹液流的方位分布變異的分析則顯示，在不同方位之間樹液流流速有顯著的差異，一個方位的樹液流流速可能為另一方位的50–200 %。樹液流流速與方向並沒有相關性。但在不同方位所測得的流速，他們之間的關係在整個測量期間中大致上是固定的。另外，藉由一個簡單的計算過程和比較，可以得知樹液流在徑向分布變異與方位分布變異的季節性改變，有可能在估算個體樹木尺度的蒸散量時造成不顯著的影響。總體而言，本研究探討適當的設計，來測量柳杉的個體蒸散量。

Transpiration is an important process in water cycle of forested ecosystem. To measure transpiration in forests, sap flow measurement method can be robust technique for individual tree-scale measurements. Although it have been reported that significant spatial variations in sap flow within-tree, which makes difficult to estimated individual tree-scale transpiration estimates, few studies have examined their seasonal change characteristics. Thus, this study was conducted to clarify within-tree special and temporal variations in sap flow in a Japanese cypress forest, Sitou NTU experimental forest located in Central Taiwan. In this study Granier's thermal dissipation technique is applied to two smaller sized, three middle sized and three larger sized trees, respectively. The measurement was carried out through 2010 July to 2011 January. Sap flow data is analyzed for the relationships between sap flux density and some biometric parameters such as DBH, tree height, sapwood thickness and sapwood area, and the correlation between sap flux density at different depth and azimuthal aspects. Consequently, in radial profile analysis, we found inner sap flow velocities measured at depth of 2–4 cm was approximately 50–60 % of the outermost sap flux density measured at depth of 0–2 cm through the measurement period. In azimuthal variations analysis, we found significant azimuthal variations in sap flux density, and one aspect showed 50–200 % of other aspects. Dependency of sap flux density on direction cannot be found. However, the relationships between one aspect and the other aspects are mostly fairly consistent through the measurement period. We also showed seasonal change of the radial and azimuthal variations in sap flow could have insignificant impacts on accuracy of individual tree-scale transpiration estimates based on a simple numerical exercise. Overall, this study discussed appropriate design for individual-tree scale transpiration estimates for Japanese cypress trees.