疏伐方式對林分結構複雜度之效應

Abstract

維護生物多樣性已為現今人造林經營中極為重要的目標。一般認為結構複雜度較高的森林因具有豐富的物種棲地，亦應造就較高的生物多樣性。是以如何提高結構多樣性為人造林撫育經營過程中重要之課題，而疏伐作業已被視為增進人造林結構多樣性重要之撫育方法。本研究以南投縣信義鄉同富村和社營林區25 林班，一佔地10 公頃柳杉(Cryptomeria japonica)人造林為例，經由每木定位與利用結構複雜度指標(Structural Complexity Index, SCI) ，分析不同疏伐方法對結構多樣性之效應。首先本研究以蒙地卡羅法模擬三種疏伐處理(株數疏伐率20%、40%、60%)各1000 次株數以瞭解逢機疏伐對林分SCI 之效應。其次， 本研究分析三種疏伐方式（分上層、中層、下層）結合上述三種疏伐率對林分SCI 值之影響，並比較其與逢機疏伐之差異與優劣。而後，以加入該林分前期殘留杉木(Cunninghamia lanceolata)所生之萌櫱，計算SCI 值，以模擬疏伐後林下栽植對林分結構之變化。最後將地形高程納入SCI 計算，以瞭解微地形變化對SCI 之影響。結果顯示隨機疏伐方式難以改善人造林的結構多樣性。選用中層疏伐方式使林木胸高直徑的變異度增加， 進而提高人造林之垂直結構多樣性。在實施疏伐後，於林下栽植苗木亦有助於增加空間結構複雜度。本研究亦顯示並非任何疏伐均會提升林分結構，唯有事前精密的調查與分析方能有助於達成該目標。

Nowadays, an important objective in managing plantation forests is the maintenance of biodiversity. As it is believed that a structurally more complex stand will harbor more diverse habitats, which in turn will promote species diversity, creating structural complexity is central to the management of plantation forests, and thinning is considered as one of the best methods to achieve that objective. Using a 10-ha sugi (Cryptomeria japonica) plantation located in central Taiwan as an example, this study intended to answer which thinning regime would create the highest structural complexity of that stand, as measured by Structural Complexity Index (SCI). Via Monte Carlo simulations, this study first investigated how a random thinning approach under three stem removal rates (20, 40, and 60%) would affect SCI. This study then proceeded to investigate how the binations of three thinning approaches(thinning-from-top, -middle, and -below) and the three stem removal rates would affect SCI. Thirdly, to mimic the effects of after-thinning understory planting on SCI, this study included Chinese-fir (Cunninghamia lanceolata) sproutings in calculating the SCI values. Finally, this study analyzed how the inclusion of topographic variations would affect SCI values. The results showed that a random thinning approach would not enhance structural complexity, and a thinning-from-middle approach would create a structurally more complex stand, due to its effects in increasing the variance of stem diameter. After-thinning understory planting would also increase structural complexity. This study showed that not all thinning regimes would promote structural complexity, some may actually reduce it, and only via detailed inventory and analysis will help us to achieve that objective.