台灣中部柳杉林之長期林分蒸散量推估:熱消散樹液流法的校正及其於野外資料的應用

Abstract

為了解氣候變遷及森林經營對水循環的影響，長期林分蒸散量的推估是不可或缺的。樹液流量測方法是推估林分蒸散量的方式之一，且其由 Granier 在1985年校正的熱消散樹液流方法於現今廣為所用。然而，一些前人研究進行校正實驗發現並非所有樹種都適用於Granier的經驗公式，且柳杉為台灣山區森林重要樹種之一，但還無研究進行校正實驗測定Granier樹液流方式對量測柳杉樹液流的精確度。並且甚少研究檢測推估林分蒸散時，樹木生長的影響。因此本研究目的為 1) 使用樹幹片段進行校正實驗以確認Granier樹液流方法對量測中部台灣柳杉之樹液流的精確度；2) 利用近7年野外量測之樹液流資料檢測真實邊材長度與樹木生長對林分蒸散量推估的影響；3) 將校正實驗之結果應用於野外資料以改增進的林分蒸散量推估的精確度；4) 研究長期林分蒸散及氣象資料之年間變異。校正實驗結果顯示使用 Clearwater 公式修正可以大幅提升推估的精確度，因此真實邊材長度的量測是很重要的。且校正實驗結果顯示 Granier 經驗公式推估的結果會低估真實流量約30%。由於真實邊材長度的量測十分重要，我們在野外樹液流量測樣區進行染劑注射實驗，且量測樹木生長量，用以檢測邊材長度的改變對蒸散量推估的影響，亦檢測由樹木生長所造成的邊材面積的改變對長期蒸散量推估的影響。在本研究中，由樹木生長造成的邊材面積的改變與邊材長度的改變對每年林分蒸散量推估的影響不大，因為 1) 在本研究期間樹木生長量小，及2) 因染劑注射實驗修正之邊材長度造成2-4公分之樹液流速變大，雖2-4公分樹液流速變大使推估量增加，但其會與邊材面積減少所造成的推估降低互相抵銷。而為了得到可良好推估蒸散的方法，我們嘗試將校正實驗之結果使用五種不同方法應用於野外資料。結果顯示五種方式的差距出乎預期之大。藉由比較其各自的樹液流日變化曲線，以及比較五種方式計算之年林分蒸散量與潛在蒸發量，得到年林分蒸散量推估的可能範圍大致為原本推估方式的1.5至3倍。最後，本實驗推估近7年的林分蒸散量。林分蒸散量的年間變異很小，因為林分蒸散量主要受水蒸氣壓虧缺及太陽輻射影響，而其年間變異亦不大。溪頭的氣象狀況在過去近7年間似乎維持滿穩定的狀態。

Long-term stand scale transpiration estimation is indispensable for understanding climate change impacts and the effects of forest management on the water cycle. Sap flow measurement is a robust approach for estimating stand transpiration; a thermal dissipation method calibrated by Granier (1985) has already been widely implemented. However, some experimental sap flow calibration studies have claimed that Granier’s empirical formula is not universally applicable. Japanese cedar is a dominant tree in the mountain forests of Taiwan; however, no experimental calibration studies have been conducted to determine the accuracy of the Granier sap flow method for this species. Additionally, the potential for incorporating tree growth effects on stand transpiration into the formula has not been examined. Therefore, this study aimed to 1) determine the accuracy of the Granier sap flow method on Japanese cedar in central Taiwan by conducting a calibration experiment using stem segments; 2) examine the effect of actual sapwood depth and tree growth on transpiration estimates based on near 7-year field measurements; 3) apply the results from the calibration experiment to field data to improve the accuracy of transpiration estimates; and 4) investigate inter-annual variation due to long-term transpiration and meteorological factors in a Japanese cedar plantation in Xitou, central Taiwan. The results of the calibration experiment showed that the application of Clearwater formula substantially improved the accuracy of transpiration estimates, indicating the importance of accurate sapwood depth measurements. The calibration experiments showed that the Granier formula underestimated actual water uptake by approximately 30%. Thus, a dye injection experiment was conducted using samples from the field study site; tree growth was also measured to examine the effects of changes in sapwood depth and area due to tree growth on long-term transpiration estimates. These effects were small, because 1) there was little tree growth during the study period and 2) the 2–4 cm sap flow rate became high due to the correction of sapwood depth for dye injection. However, the increase in the inner sap flow rate was balanced by the decrease in sapwood area due to the sapwood depth correction. To determine the better method to estimate stand transpiration, the calibration results were applied to field data using five different methods. Contrary to expectations, there were distinct differences among the results produced by these methods; stand-scale transpiration, was found to be 1.5–3 times larger than that obtained using the original method. We then conducted stand transpiration estimates for the near 7-year field data. There was little inter-annual variation in stand transpiration, because stand-scale transpiration was mainly affected by vapor pressure deficit and solar radiation, which exhibited little inter-annual variation. Meteorological conditions in Xitou appear to have been stable over the past 7 years.