利用複次疏伐規劃改善南投林區柳杉、台灣杉人工林固碳量之研究-多段目標規劃法之應用

Abstract

現今全球氣候變遷對環境產生嚴重影響，森林固碳功能儼然成為重要國際課題，2015年巴黎協定(Paris Agreement)亦指出森林經營應重視碳匯功能，同時仍須兼顧水土保持、森林生態以及滿足社會需求等功能。森林固碳規劃過去多以單次疏伐為主，然前人研究發現複次疏伐有益於提升林分固碳量及森林生態功能，且複次疏伐可提供穩定材積收穫利於林業持續發展。 本研究就南投林區柳杉與台灣杉人工林進行為期三十年之複次疏伐固碳規劃，將土沙沖蝕增量、指標物種數、各期材積和各期面積控制、各期材積收穫總量限制及各齡級面積限制設為經營條件，並以多段目標規劃法(multi-segment goal programming, MSGP)整合線性規劃求出固碳量極大化之最適解。研究結果顯示，複次疏伐和單次疏伐於相同經營條件下，複次疏伐固碳量較單次疏伐方案高。同時，材積收穫亦較單次疏伐規劃方案高，且產生較少土沙沖蝕增量，其疏伐作業方式包含一次輕度和一次中度疏伐以及兩次輕度疏伐，單次疏伐在相同齡級則多以中度疏伐為主。另本研究發現，加強材積平穩、齡級和各期面積控制以及土沙沖蝕增量限制，會使規劃之固碳量降低，然總疏伐材積則未有明顯趨勢。綜上所述，本研究認為複次疏伐可增加林地於時間上的選擇彈性，相較於單次疏伐規劃方案可於較大範圍內搜尋最適解，雖固碳量僅略高於單次疏伐方案，然可提供材積收穫以及森林環境等功能，為森林經營者可考量之疏伐方式。

As climate change has serious impacts on the environment, managing forest for carbon sequestration plays a vital role in the global issue. Recently, Paris Agreement has pinpointed that forest management has important functions on not only carbon sink but also soil and water conservation, forest habitats and satisfaction of social demands. Previous researches have shown that a repeated thinning was capable of improving carbon sequestration and forest ecosystem services. Furthermore, this repetition ensures a stable timber supply in order to maintain forest sustainability. In this study, we aim to maximize carbon sequestration of Japanese Cedar and Taiwania plantations in Nantou national forest district by repeated thinning during a 30-year planning horizon. The thinning scheme is under certain conditions, such as control of soil erosion, preserving target species, regulating the volume of timber removed and the area of forest stand thinned in three planning periods and every age classes. The study applies multi-segment goal programming (MSGP) combined with linear programming to find an optimal solution for maximizing carbon sequestration under various management requirements. The study results indicate that the level of carbon sequestration by repeated thinning is higher than that of non-repeated thinning under the same management requirements. Furthermore, repeated thinning provides more volume of timber removed and less amount of soil erosion in the stand, by means of a single medium thinning mainly and double thinning composed of light and medium ones. The study finds that if stricter restrictions of volume flow, area control, and soil erosion were set, the maximum amount of carbon sequestration would decrease, but the total volume removed by thinning has no obvious change. Finally, the study suggests that, even the effect of carbon sequestration by repeated thinning is slightly higher than non-repeated thinning, repeated thinning can enhance the flexibility of thinning intensity and scheduling over time for searching the optimal solution in a wider range and can provide better outcomes of timber harvest and forest environment for forest managers to make decisions.