考量死亡有機物與木製品碳釋放下之土地期望價值與最適輪伐期

Land Expected Value and Optimal Forest Rotation Considering the Carbon Release of Dead Organic Matter and Harvested Wood Products

過去文獻對於碳交易市場對土地期望價值與最適輪伐期之影響已有相當多的研究，但前人研究缺乏考量製成木製傢俱等用材在未來使用中逐漸分解情況，及土壤碳匯之碳吸存與碳釋放部分，故本研究以杉木為例進行模擬分析，進行四種情境土地期望價值與最適輪伐期之分析，土地期望價值情境包括：（1）僅考慮木材收入、（2）考慮木材收入與地上部碳匯收入、（3）考慮木材收入、地上部碳匯收入、地下部土壤碳釋放支出、（4）考慮木材收入、地上部碳匯收入、地下部土壤碳釋放支出、傢俱用材碳釋放支出。其次，本研究利用敏感度分析確定各項因素變動下對土地期望價值與最適輪伐期之影響。結果顯示在台灣建議碳價格之下四種不同情境之土地期望價值分別為每公頃4,370,167元、6,545,936元、5,924,128元及4,895,334元，碳交易市場的存在可讓土地期望價值上升12%至50%。而從敏感度分析結果可看出土地期望價值受到碳價格、碳釋放比例及折現率三者有甚大影響，碳價格與折現率的上升會縮短最適輪伐期，當碳價格超過每公噸3,573元之後，但考慮土壤與傢俱用材的碳釋放後，可使這樣的情況減緩。以台灣目前的林業現況，僅考慮地上部碳匯收入，最高可讓土地期望價值增加50%，但會增加土地期望價值對於折現率的敏感度，增加林業經營的風險。考量土壤與傢俱用材碳釋放後，可使上述情況得到改善，雖然土地期望價值上升幅度降為12%，但卻大幅降低折現率對於最適輪伐期與土地期望價值的影響力，同時隨著碳價格的上升，越能增加林主將伐材作為傢俱，持續發揮森林碳儲存效益的誘因，甚至能延長最適輪伐期。綜合以上，本研究建議政府未來應建立碳匯市場時，碳價格不宜超過每公噸3,573元，碳匯市場中森林碳匯之計算方式，應同時考慮地上部碳匯收入、土壤之死亡有機物與傢俱用材碳釋放，才能較正確地反映出森林經營完整的經濟效益。

Many studies have discussed the impact of carbon trade on land expectation value and optimal rotation age, but CO2 emissions from harvested forest products are rarely taken into consideration. This study takes the Cunninghamia lanceolata as an example to determine the sensitivity of land expectation value and optimal rotation age to different factors with sensitivity analysis. Land expectation value and the optimal rotation age are estimated for four kinds of situations: (1)Timber value only, (2)Timber value and carbon value of above ground, (3)Timber value and carbon value of above ground and dead organic matter, (4)Timber value and carbon value of above ground, dead organic matter and harvested forest products. The results show that under the Taiwan suggested carbon price the LEV of four kinds of situations are 4,370,167NTD/ha, 6,545,936 NTD/ha, 5,924,128 NTD/ha and 4,895,334 NTD/ha, the carbon trading can make the LEV rise about 12-50%. And the sensitivity analysis show that the LEV is influenced by carbon price, carbon release ratio and discount rate. The optimal rotation age starts to reduce when carbon price is higher than 3,573 NTD. However, the influence is reduced if the timber value and carbon value of above ground are taken into consideration. When the timber value and carbon value of above ground are considered, the land expectation value increases significantly, the optimal rotation age decreases, and the sensitivity of land expectation value to discount rate increases. Considering only carbon value of dead organic matter and harvested forest products suppresses the increase of land expectation value, but significant mitigates the problem that the optimal rotation age reduces and the sensitivity of LEV to discount rate increases. In this situation, the higher the carbon price, the more incentives for forest land owners to keep their harvested forest products in order to keep the carbon in wood, resulting in a longer optimal rotation age. We suggest the government establish a carbon market in Taiwan and the carbon value should include the timber above ground, dead organic matter, and harvested forest products.