探討各種生長收穫模式對柳杉人工林蓄積量模擬結果之分析

Investigating simulation results of different growth/yield models on the stocking estimation of Japanese Cedar (Cryptomeria japonica) Plantations

陳柏蓉(Po-Jung Chen)

指導教授 ： 鄭舒婷

國立臺灣大學/生物資源暨農學院/森林環境暨資源學研究所/碩士(2020年)

https://doi.org/10.6342/NTU202003825

摘要

森林經營涉及層面相當廣泛，與社會、經濟、環境、生態相互依存，有著密不可分的多元關係。隨著國際上對於永續經營的重視，聯合國提出許多與森林經營相關的永續發展目標（Sustainable Development Goals, SDGs），更彰顯出森林資源永續管理之重要地位。然而臺灣林業自1991年頒布「臺灣森林經營管理方案」以來，走入以森林資源保育為主之趨勢，同時期人工林經營亦幾近停滯。近年來政府因應國內外永續發展趨勢，藉由國產材推廣、林業永續多元輔導等來振興林業的生產，在這重啟的過程，林業資源盤點為訂定經營方針的基石。
　　現行林務局轄屬經濟林的林木資源中，屬柳杉人工林（Cryptomeria japonica）為早期大量推廣造林，且至今還存有廣大面積與蓄積之樹種，累積相當多之生長收穫研究，為了解不同生長收穫模式對林分蓄積推估可能產生的不確定性範圍，本研究以柳杉作為研究樹種，透過次級資料的蒐集，包括林業統計、發表過之文獻期刊，利用前人之研究，將林務局轄屬柳杉經濟林之面積與蓄積量資料進行轉換，以林齡作為自變數，推估胸高直徑、樹高，並比較不同公式組合下產生之差距變化，建立各齡級林木株數與立木材積估算之基礎數據。接著，本研究建立林分系統動態模型，考量實際統計數據所得之林木生長率、枯損率、人為砍伐及少數新植之柳杉人工林，估算單木、林分、全林三個層級隨著時間在蓄積量上產生的連續性變化。
　　研究結果發現，使用不同公式組合推估樹高與立木材積，會因組合之基本樣態不同而使產生之曲線趨勢有所差異。在樹高表示上，樹高生長趨緩之估算區間在20-30公尺左右；在立木材積式表現上，初期之表現差距並不明顯，但在過了30齡級後，差距逐漸拉大，至老年立木材積估算範圍由1.24至1.86 m3/tree，最多可差至1.5倍以上；利用不同生長收穫式以林分系統動態模擬2009-2100年柳杉林分動態，結果顯示隨著時間的拉長，林分蓄積差距有越來越大的趨勢，若從新造林林分每年蓄積變化量來看，隨著林齡的增長，每單位蓄積增加量會先上升至林齡35-55以後慢慢轉為下降，表示林木在蓄積量、碳吸存的表現上漸為趨緩。
　　這樣的結果也突顯在進行生物量或碳吸存的估算時，若以過去所建立之經驗式來推估樹高或立木材積，對於不同之地點、時間及未來預估能力都有其限制；蓄積量估算之不確定性之範圍為何，對林業決策或施業精度而言，為一重要之資訊，應加以評估。此外，隨著柳杉林齡的老齡化，推估公式的適用性也有待考驗，若估算出來之數值都與實際情形存有相當大的落差時，恐會造成錯誤之決策，進而影響森林資源的永續利用。因此，本研究建議：不論是國產材之生產或是碳匯量之評估，林業相關單位在提供資源盤點資訊時，應將生產蓄積等不確定性範圍納入考量，作為振興林業可以著力的重點方向之一，以提昇經營者對於林分資訊的掌握程度，有效評估誤差風險，達成永續森林經營之目標。

關鍵字

柳杉人工林 ； 生長收穫式 ； 蓄積量 ； 系統動態 ； 永續森林經營

並列摘要

Forest management involves a variety of multiple aspects closely associated with and interdependent to society, economy, environment and ecology. With increasing international awareness on sustainable management, the United Nations （UN） proposed several targets related to the forest management in the “Sustainable Development Goals （SDGs）”, which further demonstrates the importance of sustainable management of forest resources. However, since the “Taiwan forest operation and management program” promulgated in 1991, the focus of forest management has been shifted from economic harvesting to resources conservation, and plantation management has ever since been nearly stagnated. Recently, Taiwanese government tries to cope with global and domestic sustainable development trends, and has been devoted to promote domestic timber supply and sustainable forestry counseling to revive the forest production. As such, forestry resources inventory acts as the fundamental base for policy making and forestry planning.
Among the Forestry Bureau of Taiwan jurisdictional plantation resources, the Japanese cedar （Cryptomeria japonica） plantation is one of the economic species of which large plantation areas and stockings remains since early initial afforestation periods. There have been a great amount of growth- and yield-related research studies of Japanese cedar. This research took Forestry Bureau of Taiwan jurisdictional Japanese cedar plantation as the study species to understand the uncertainties arising from different modeling equations in growth and yield on stocking estimations. The gathered secondary data of areas and surveyed stockings from available forestry statistics reports and published papers were transformed to estimate numbers of Japanese cedar trees in each age class and their associated diameter at breast height （DBH）, tree height, and wood stem volume by different equations specific to Japanese cedar in Taiwan. A total of twelve combinations of allometric equations were applied to evaluate uncertainties of stocking estimations. After that, a system dynamics stand modeling was constructed considering tree growth, mortality rate, cutting, and actual afforestation based on actual data from forestry statistics reports to simulate the accumulated stocking dynamics at individual, stand, and total levels.
The results showed that using different allometric equations would result in different estimations of trends in tree heights and wood stem volumes. For example, tree height growth slowed down at height between 20 to 30 meters, while trends of wood stem volume did not show obvious variation in the early stage until ages of over 30, estimation differences started to increase. Estimations of the wood stem volume ranged from 1.24-1.86 m3/tree for older age classes, for which the greatest differences can be as large as 1.5 times. Based on the total stand stocking simulations by system dynamics stand modeling from 2009 to 2110, the differences of stand stocking of Japanese cedar forest accumulated with time by various combination of allometric equations. According to the current annual increment （CAI） simulations on the new afforestation in 2016 （stand 0）, the unit stocking increased at beginning, and then started to decline during ages of 35 to 55, demonstrating decreases in the total volume of stocking as well as carbon storages as the plantation ages.
Such results also clearly showed the potential issues when estimating biomass or carbon sequestration using empirical equations of tree height or tree volume, as these equations were usually limited to their locations and specific time frame, and would impose a constraint on the predicting power of future forecasting. In addition, uncertainties of stocking estimations are critical information for forestry decision making and implementation accuracy. Furthermore, the suitability of the equations on aging Japanese cedar should be tested. If there were obvious gaps between simulated conditions and the reality, it could result in wrong decision-making, and affect the sustainable use of the forest resources. As a result, this study suggests that, whether for production of domestic timber or assessment of carbon sequestration, the forestry-related authority units should take the uncertainty of stocking estimations into consideration and provide such information on the inventory of forest resources. This should be taken as one of the key points for revitalizing forestry, so that stand information can be reasonable obtained by managers to effectively estimate risks associated with this uncertainty to achieve the goal of sustainable forest management.

並列關鍵字

Japanese cedar plantation ； growth and yield equations ； stocking ； system dynamics ； sustainable forest management

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