下層疏伐與行列疏伐之經濟效益分析－ 以花蓮林管處之兩個疏伐地為例

Abstract

行政院農業委員會林務局為鞏固民生木材來源，提供不同規格木材資源之存量，計畫適度的對國有林林木經營區進行疏伐撫育。人工林如果不進行疏伐，將無法產生大徑級之木材，使林木資源之利用受限，因此，林務局督促各林區管理處，依據該林區管理處之林區森林經營計畫實施疏伐撫育。然而施行疏伐作業，必會面臨數個問題，包括如何進行疏伐、進行何種疏伐、何為最適之疏伐時間等。為使其施業後之疏伐空間對留存林分產生最大的效益，建立適當的流程與方法，評估不同疏伐方式所產生之效益是重要的課題。 花蓮林區管理處轄內柳杉造林地之柳杉林齡皆已超過30年，而林分鬱閉擁擠，因此該林區管理處欲施行中後期之疏伐撫育，以利留存木林分之生長。花蓮林區管理處於2008年在玉里事業區27林班，瑞穗林道兩旁之38年生柳杉造林地，辦理中後期疏伐撫育作業。林道上方採取單株下層疏伐，林道下方採取行列疏伐，下層疏伐之株數疏伐率為28.6％，行列疏伐則為40％。本研究以此疏伐案為例，進行下層疏伐與行列疏伐方式之經濟效益分析。 本研究先以徑級分配推估未疏伐與疏伐後之林分狀況，再參考以國立台灣大學實驗林管理處柳杉所建立之生長式，模擬修改其參數以符合林務局花蓮林區管理處轄內柳杉造林地之生長狀況，並由未來材積生長之推估來推測單一輪伐期內下層疏伐與行列疏伐之淨收益。 根據本研究之推估，下層疏伐部分的淨收益為-456,218元，行列疏伐部分的淨收益為-1,206,861元，疏伐木標售之收益低於疏伐成本。而在木材收穫率為87.36％、材積利用率70％、實質折現率0.7％、通貨膨脹率1.08％、木材實質價格隨通貨膨脹率增加與其他相關之假設下，比較林道上方柳杉林分不疏伐與下層疏伐，以及林道下方柳杉林分不疏伐與行列疏伐等四個方案在不同林齡主伐下之淨收益。根據本研究之推估，各方案之淨現值隨著林齡的增加而增加，而柳杉造林地若未疏伐，其淨收益會較疏伐林分之淨收益高，另外，下層疏伐林分之淨收益高於行列疏伐林分之淨收益。以單一輪伐期平均每立方公尺材積之淨收益而言，未疏伐林分平均每立方公尺材積之淨收益高於已疏伐林分平均每立方公尺材積之淨收益，而林道下方行列疏伐平均每立方公尺材積之淨收益遠低於林道上方下層疏伐之平均每立方公尺材積之淨收益。 經本研究之比較分析後，本研究認為花蓮林區管理處現行之柳杉造林地不適合進行疏伐，疏伐會造成輪伐期內林地淨現值之減損，而行列疏伐所造成之損失比下層疏伐還大。另外，本研究建議花蓮林區管理處未來進行疏伐時，應先建立樣區，取得樣區資料，並量測實際的疏伐面積，在缺乏樣區資料與實際疏伐面積資料之經濟評估將造成估測上的誤差。

In order to consolidate the timber supply for the domestic consumption, and to provide the resource stock of different sizes of woods, the Forestry Bureau of the Council of Agriculture plans to conduct certain level of forest thinning in Timber Management Area of National Forest. If the forest thinning is not conducted, the plantation forest will not be able to generate large-diameter woods, and it will restrict the usefulness of timber. So the Forestry Bureau commands the Forest District Offices to conduct forest thinning in accordance with their forest district management plan. Forest District Offices however would face many challenges if they are going to conduct thinning operations. These challenges include how to conduct thinning, which forest thinning method should be used and to decide the best timing for thinning, etc. In order to make the retained trees after thinning to provide the maximum benefits, it is an important topic to establish a proper procedure and method to evaluate the benefits generated by different thinning methods. Cryptomeria (Cryptomeria japonica) plantation at Hualien Forest District Office is more than30 years. The forest stand is dense. Hualien Forest District Office decides to conduct forest thinning to promote the growth of retained trees. In 2008 Hualien Forest District Office conducted forest thinning on both sides of Jeisui logging trail in the 27th Forest Districts of Yuli Business District. For the forest above the logging trail, low thinning method was utilized and the thinning intensity was 28.6％. For the forest under the logging trail, row thinning method was utilized and the thinning intensity was 40％. Based on these two thinning cases, this study analyzes and compares the economic benefits of low thinning and row thinning methods. This study uses the size-class distribution to estimate the state of stand before and after thinning operation. This study then uses the stand growth model developed by the Experimental Forest Bureau of the National Taiwan University, and modifies parameters to fit the stand growth in Plantation Forest of Hualien Forest District Office. And lastly this study uses the estimate of the future growth of the timber volume to estimate the net present value of low thinning and row thinning under a single timber rotation. Based on the estimate of this study, the net present value (NPV) of the net benefits of low thinning is -456,218 NTD dollars, the NPV of the net benefits of row thinning is -1,206,861 NTD dollars. The sale value of thinned timber is below the thinning cost. This study compares the NPVs of four different cases, such as unthinned stand and low thinned stand above logging trail, and unthinned stand and row thinned stand below logging trail, under the assumptions of 87.36％ timber harvest rate, 70% timber utilization rate, 0.7％ real discount rate, 1.08％ inflation rate, and the constant level of the real prices of timber. Based on the estimate of this study, for all four cases the NPVs increase along with the increases of the timber age. And the NPV of unthinned stand is higher than the NPV of the thinned stand. Moreover, the NPV of low thinned stand is higher than the NPV of row thinned stand. Based on a single timber rotation, the NPV per m3 of unthinned stand is higher than the NPV per m3 of thinned stand. And the NPV per m3 of row thinned stand is smaller than the NPV per m3 of low thinned stand. Based on the results of this study, it is therefore concluded that the plantation forest of Hualien Forest District is not appropriate for thinning. Thinning will result in the decrease of the NPV of the forest under one timber rotation, and the decrease of the NPV of row thinning is higher than the decrease of the NPV of low thinning. If Hualien Forest DistricIt decides to conduct forest thinning in the future, it is suggested that it should set up sample plots, to gather stand data before thinning, and to measure the real thinning area. Evaluation without sample plot data and real measure of thinning area will result in evaluation error.