溪頭柳杉及紅檜老齡人工林之林木生長量、枯落物量與生態系統碳儲存量

柳杉是台灣造林歷史中，中海拔地區重要的造林樹種之一，根據林務局第四次森林資源調查統計報告，全台灣柳杉林的材積蓄積量已佔全人工林總材積蓄積量的18.4 %，但隨著林業政策變遷，如今的柳杉林已少有伐採和林分更新。因此，本研究選擇溪頭地區6個林齡介於46～97年生之老齡柳杉造林林分及1個林齡88年生的老齡紅檜造林林分，藉由老齡人工林樣區之試驗調查，用以了解現有老齡林分之林分現況和於2017至2019年間之林分生長變化。研究目的主要為 (1)調查溪頭這些不同林齡的老齡柳杉造林地林分之林分特性及生長動態 (2)了解老齡柳杉與紅檜人工林之連年生長量、枯落物量與生態系統碳儲存量。研究結果顯示，各柳杉林2017年度的生長量為0.91～2.65 Mg C ha-1 yr-1，2018年為0.81～2.14 Mg C ha-1 yr-1，2019年為0.88～1.92 Mg C ha-1 yr-1；紅檜林則分別為0.62、1.42、1.53 Mg C ha-1 yr-1。由於枯落物收集進行期間，溪頭地區皆無受到颱風影響，因此所得之柳杉林枯落物量較過去研究來得低，各柳杉林枯落物量2017年為2,191～2,810 kg ha-1 yr-1，2018年為2,134～3,234 kg ha-1 yr-1，2019年為3,507～4,812 kg ha-1 yr-1；紅檜林則分別為6,814、5,248、5,307 kg ha-1 yr-1。各林分之生態系統碳儲存量 (229～343 Mg C ha-1) 皆以生立木為最大宗，佔生態系統碳儲存量70%以上，柳杉林分林木碳量為203～255 Mg C ha-1、紅檜林分為144 Mg C ha-1；表層0～30 cm土壤有機碳量含量次之，土壤有機碳儲存量為53～78 Mg C ha-1，約佔生態系統碳儲存量25%。林木碳量與土壤有機碳量兩者合計佔生態系統碳儲存量95%以上。連年生長量之結果顯示溪頭老齡柳杉仍然在持續生長吸存二氧化碳，即使是林齡97年生的老齡柳杉林分，其生長量也並非停滯，甚至高過一些林齡較為年輕的老齡林分。雖然各林分樹高已接近樹高生長上限，但胸高直徑仍持續增加，使得林分蓄積量隨著胸高直徑的成長增加，這些老齡林分仍可繼續累積碳儲存量。老齡森林也代表了一個巨大的碳庫，將來這些林分如不進行伐採或遭受擾動損毀，仍然可持續吸存大氣中的二氧化碳固定於林木之中，對於減少碳排放有其貢獻存在。

關鍵字

柳杉；紅檜；老齡人工林；林分生長量；枯落物量；生態系統碳儲存量

並列摘要

Cryptomeria japonica (Japanese cedar) is an important plantation species in Taiwan. According to the Fourth Forest Resources Survey Report of the Forestry Bureau, the volume stocks of Japanese cedar plantations accounted for 18.4% of the total plantations’ stocks. However, with the change in policies, there is less harvesting and forest regeneration in the Japanese cedar plantations nowadays. In this study, six old Japanese cedar stands with stand age from 46 to 97 years old and one stand of Taiwan red cypress for 88 years old were selected, to investigate their stand characteristics and annual stand growth in 2017 to 2019. The objects of this study are to (1) investigate current stand characteristics and growth dynamics of these old-growth Japanese cedar stands of different ages in Xitou, and (2) understand the annual growth, litterfall mass, ecosystem carbon stocks, and distribution of carbon stocks in these old-growth Japanese cedar and Taiwan red cypress stands. The study results show that the growths of the Japanese cedar stands are 0.91-2.65 Mg C ha-1 yr-1 in 2017, 0.81-2.14 Mg C ha-1 yr-1 in 2018, 0.88-1.92 Mg C ha-1 yr-1 in 2019, and the growth of the Taiwan red cypress is 0.62, 1.42, 1.53 Mg C ha-1 yr-1 in 2017 to 2019, respectively. Because none of the areas in Xitou were affected by typhoons during the experiment, the mass of litterfall in the Japanese cedar stands are lower than in the previous studies. The mass of litterfall in the Japanese cedar stands are 2,191-2,810 kg ha-1 yr-1 in 2017, 2,134-3,234 kg ha-1 yr-1 in 2018, 3,507-4,812 kg ha-1 yr-1 in 2019, and the Taiwan red cypress is 6,814, 5,248, 5,307 kg ha-1 yr-1 in 2017 to 2019, respectively. In each stand, live standing trees has the largest amount of carbon stocks in the ecosystem (229-343 Mg C ha-1), accounting for more than 70 % of the total carbon stocks. 203-255 Mg C ha-1 in Japanese cedar stands and 144 Mg C ha-1 in Taiwan red cypress stand. The amounts of soil organic carbon range between 53 and 78 Mg C ha-1 and rank the second pools for the ecosystem carbon stocks, accounting for 25% of the total carbon stocks. These two C stocks account for more than 95% of the total ecosystem carbon stocks. The annual growth results show that the old-growth Japanese cedar stands in Xitou are still growing. The growth of 97 years old Japanese cedar stand is not stagnant, even higher than some younger stands. Although the tree height of each stand is close to its upper limit, these stands will continue to accumulate carbon stocks as their DBH grows in the future. The old-growth forest also represents a huge carbon stock. If they are not harvested or disturbed, they will continue to absorb and fix atmospheric carbon dioxide to reduce carbon emissions.