柳杉人工林小區域皆伐式疏伐九年後林下光照環境和植物組成之變化

Abstract

森林疏伐是對林地內樹木進行部分的砍伐或移除，可增加木材產量，是重要的森林經營手段。疏伐對於森林結構、環境可以造成立即的變化。曾有研究在臺灣中部人倫林道之柳杉 (Cryptomeria japonica) 人工林利用半球面影像檢驗25%及50%疏伐對林下光環境的立即影響。發現疏伐使林下光照量和林下光環境變異均顯增加，且增加量在50%疏伐區遠大於25%疏伐區；在未疏伐區，林下微棲地光照度在疏伐前後相關高，在疏伐區域，林下微棲地光照在疏伐前後相關性偏低，而且疏伐50%的森林較疏伐25%的森林，林下微棲地光照度在疏伐前後相關性下降更多。由於林下光照遠低於一般植物生長之最適光照，故此林下光照之改變對林下植物之生長及多樣性應有重大影響。雖然林下光照應會因疏伐後林冠漸漸閉合而減少(回復)，但需要多久時間，又林下植物之多樣性一旦發生變化，是否能回復未有研究加以探討。 本研究經疏伐過後九年，再次調查林下光照環境以及木本植物和林下植物的組成。結果顯示疏伐九年後影響仍在，但此差異已遠小於剛疏伐完後的差異，且部分差異可能與疏伐無關，另外疏伐對林下光環境變異程度的影響大於對光立地係數的影響。林下植物的部分，散射光立地係數和林下植物多樣性之間具顯著的正相關，可能因為研究區多雲霧，故散射光立地係數較直射光立地係數更能反應真實的日輻射大小。木本植物的部分，不同疏伐強度樣區的木本植物群落相似性均高，但隨疏伐程度越高柳杉優勢度越低且多樣性有越高的趨勢。

關鍵詞：疏伐、光環境、半球面影像技術、林下植物組成Artificial thinning is an important forest management practice that removes some canopy trees to increase timber production. A research examined the effects of 25% and 50% thinning on understory light environments using hemispherical photography at a Cryptomeria japonica plantation in Central Taiwan. The study reported that thinning at both intensities (25 and 50%) significantly increased understory light variability and availability, and the increments were larger at 50% thinning than 25% thinning plots. Understory light availability in the control plots exhibited high correlations between pre-thinning and post-thinning measurements; in contrast in the thinning treatments the correlations were weak. The diminishment in correlation was greater in 50% thinning plots than 25% thinning plots. For most plants, understory light is lower than optimal, so that changes of understory light have major impact on the growth and diversity of understory plants. The enhanced understory light will diminish with time as canopy closure gradually increases. However, it is not clear if the changed understory plant diversity, would recover to pre-thinning conditions. This study re-examined the effect of 25% and 50% thinning on understory light environments and surveyed understory plant diversity and composition nine years after the thinning. The results indicated that that thinning effects on understory light nine years after thinning were still evident, but much smaller than the effect immediately after the thinning. Moreover, the effect on the variability was greater on the availability of understory light. Plant diversity was found to be correlated with indirect site factor but not direct site factor. Possibly, in this frequently cloudy site, indirect site factor could better reflect real total solar radiation. Plant community similarity between three thinning intensities were high. However, higher thinning intensity was associated with lower dominance of Cryptomeria japonica, and higher plant diversity.

Key words: thinning, light environment, hemispherical photography, understory plant composition.