以樹液流法評估臺灣溪頭柳杉林水分利用效率

Abstract

目前國內大部分柳杉 (Cryptomeria japonica (L.f.) D. Don) 人工林已超過了研究建議的輪伐期。根據前人研究，當林齡為21年時，年生長量出現最大值，之後隨著林齡的增加而逐漸減少。代表柳杉在超過一定年齡後，生長速度開始減慢，這會降低樹木的碳儲存效率。本研究利用熱消散方法測量樹液流並透過R語言REddyProc套件進行資料填補，以此估算林分蒸散量，並透過實驗了解：（1）蒸散量與森林生產力之間的關係；（2）林齡超過建議的輪伐期是否會導致蒸散作用和生長量減少。我們將12年的蒸散量和森林碳儲存量資料分為兩個區間來討論，並分析水分利用效率 (WUE) 隨時間的變化。 研究期間，溪頭地區的年蒸散量介於110 mm至149 mm，在兩個區間分別為303.7、291.2 Mg per plot，沒有太大差異。碳儲存變化量 (ΔC) 在兩個區間分別為0.52、 0.27 Mg per plot，WUE分別為1.72 、0.92 (g C / L H2O)，兩者都有下降的趨勢。在個體尺度，DBH與WUE則呈低度相關，但有趣的是，蒸散量 (TR)和ΔC呈正相關性(R=0.93)，顯示TR提高確實會提高ΔC。結果顯示，林分蒸散量在兩個區間沒有明顯差異，而 ΔC、WUE 有下降的趨勢，這表示相較TR，ΔC、WUE可能受到林齡的影響。相對其他研究，較低的年均ΔC 顯示林分已進入生長緩慢的階段。DBH較大的個體，往往會有較高的TR、ΔC，但不代表會有更高的WUE。 整體而言，兩區間蒸散量變化不大，但ΔC與WUE均下降，儘管研究樣區的柳杉林仍有碳儲存效益，但已步入生長緩慢的階段，反映出林齡對森林功能的影響。另外，本研究結果顯示用水量愈高的個體ΔC會愈大。本研究補充了臺灣老齡柳杉林WUE觀測資料，提供未來相關研究的基礎參考。

At present, most of the Japanese cedar (Cryptomeria japonica (L.f.) D. Don) plantations in Taiwan have already passed the rotation period recommended by research. According to previous studies, it was found that when the forest age is 21 years, there is a maximum annual growth, and then it gradually decreases as the forest age increases. This means that the growth rate of Japanese cedar begins to slow down after reaching a certain age, which in turn reduces the carbon storage efficiency of the tree. This study used the thermal dissipation method (TDM) to measure sap flow and filled data gaps using the REddyProc package in the R language to estimate stand transpiration. The experiment also revealed: (1) to investigate the relationship between transpiration and forest productivity; (2) to examine whether stand age beyond the recommended rotation period results in reduced transpiration and growth. This study divided 12 years of transpiration and forest carbon storage data into two intervals for discussion and analysis of the changes in WUE over time. During the study period, the annual transpiration in the Xitou area ranged from 110 to 149 mm, which was converted into mass as 303.7 and 291.2 Mg per plot, respectively. The carbon storage change (ΔC) in the two intervals was 0.52 and 0.27 per plot, and the WUE was 1.72 and 0.92 (g C / L H2O), respectively, both of which showed a downward trend. At the individual scale, DBH and WUE were weakly correlated; however, interestingly, transpiration (TR) and ΔC were positively correlated (R=0.93), indicating that an increase in TR would indeed increase ΔC. The results show no significant difference in stand transpiration between the two intervals, while ΔC and WUE showed a downward trend, indicating that ΔC and WUE may be affected by stand age compared to TR. Compared with other studies, this study’s annual average ΔC was lower, showing that the stand has entered a slow-growth stage. Individuals with larger DBH tend to have higher TR and ΔC, but this does not mean that they will have higher WUE. Overall, the transpiration in the two intervals showed little change, but both ΔC and WUE decreased. Although the Japanese cedar forest in the study area still has carbon storage benefits, it has entered a phase of reduced growth, reflecting the impact of age on forest function. In addition, the results of this study indicate that greater water consumption is associated with higher individual ΔC. This study contributes to the limited WUE observational data for Japanese cedar forests in Taiwan and provides a foundational reference for future related research.