日本柳杉之樹液流徑向變異性：以臺灣溪頭為例

Abstract

本研究使用著名的Granier熱消散法為基礎，透過考慮樹液通量密度的徑相變異來評估樹液流。本研究樣區位於臺灣南投縣的臺大溪頭實驗林，該樣區為樹齡一致的單一樹種（Cryptomeria japonica）。先前在溪頭進行的樹液流研究已經探討樹液流之方向變異與徑向變異，然而使用Granier探針在不同位置、不同深度檢查徑向變化，無法去除方向變異的變因。因此，本研究採用單點量測之熱消散探針藉此排除方向變異，討論樹液通量密度隨徑向變化對樹液流估算的影響。 結果顯示，不同深度的每日平均樹液通量密度及日變化均顯示有效運送水分的邊材長度達6公分以上，與目視判斷邊材長度3公分有顯著差異。從形成層以下0到6公分有明顯觀測到樹液通量密度，而6到12公分之樹液通量密度則不明顯，其原因可能為量測的極值所限。沿著深度觀察到明顯的晝夜變化峰值延遲，隨著深度的增加而增加。其徑向剖面圖呈現一個伽瑪分布 (Gamma distribution)，在1.5公分左右達到峰值，然後隨著深度增加而減少。與Granier樹液流的估算方法比較，考慮0到12公分的徑向變化的樹液流提高為1.67倍；使用伽瑪方程計算結果較傳統估算法，乾季及濕季的樹液流分別提高為1.59倍及1.83倍。 本研究總結，考慮樹液通量密度的徑向分佈延伸到比目視辨識的邊材深度更深，能解釋被常規方法低估的日本柳杉林分蒸散量。此外，使用移動式探針所求得的徑向剖面與伽瑪數值方程的結果吻合，提供未來估算林分蒸散量更有效率的利用方式。

This study evaluated whole-tree sap flow by considering the radial variation in sap flux density, using the well-known Granier thermal dissipation (TD) method as the basis. The sampling site for this study was the Xitou Experimental Forest of National Taiwan University (NTU) in Nantou County, Taiwan, which had a single species of Cryptomeria japonica of uniform age. Previous sap flow studies at Xitou had examined circumferential and radial variation. However, the radial variation was examined with the Granier sensors at different positions for different depths, which did not remove the circumferential effect. For this reason, this study adopted single-point measurements to detect the change in sap flux density with the radial direction by 4-cm sensors and mobile sensors, excluding the circumferential effect. The daily mean and diurnal pattern measurements of the sap flux density at multiple depths demonstrated that the sample tree effectively transported water even at depths of up to 6 cm or more, significantly differing from the visual determination of sapwood length of 3 cm. The sap flux density varied significantly from 0 to 6 cm beneath the cambium. In contrast, the radial variation in sap flux density from 6 to 12 cm was insignificant, probably due to the detection limit of the sensors. An apparent peak delay of diurnal variations was observed along the depths, which increased as the depth increased. The radial profile exhibited a Gamma distribution, which peaked at around 1.5 cm and then decreased as the radius increased. Considering the measured radial variation in sap flux density from 0 to 12 cm in a 1-cm step, the whole-tree sap flow in the wet season increased 1.67 times more than Granier's TD calculation. In contrast, the correction using the gamma-type function results showed an increase in the whole-tree sap flow by 1.59 times greater in the dry season and 1.83 times greater in the wet season than the conventional Granier's calculation. This study concluded that the radial distribution of sap flux density extending deeper than the visually identified sapwood depth could explain the potential reason for underestimating the transpiration of Japanese cedar by the conventional method. The use of the mobile sensor also corresponded with the results of the equation of gamma function, providing a more efficient use of estimating stand transpiration in the future.