台灣東北部三種相鄰林分林下光照環境之比較

A Comparison of Understory Light Environment Among Three Adjacent Forest Stands in Northeastern Taiwan

本研究在東北部福山地區的天然闊葉林以及鄰近的人工柳杉林與被闊葉樹入侵之人工柳杉林等三種林分，利用半球面影像技術分析林下光照環境，以評估人工柳杉林在放棄經營後其林下光照變化的情形及其對林下植物生長的影響。研究結果顯示，林下直射光與散射光立地係數皆以天然闊葉林最高，人工針葉林次之，被闊葉樹入侵之人工針葉林最低。林下直射光與散射光兩者之立地係數變動範圍亦以天然闊葉林最大，被闊葉樹入侵之人工針葉林最小。此可能係因人工柳杉林造林密度高，而空間配置具明顯規律，故光照穿透林冠的比率低且變動範圍小。而人工柳杉林放棄經營被闊葉樹入侵後，林冠更為鬱閉，光照穿透進一步受阻，故變動的範圍亦向下壓縮。然而即使在被入侵之柳杉林所估測得之光照穿透林冠比率仍皆在4 % 以上，且半數以上的微棲地更高達10 % 以上，高於多數苗木的光合作用補償點，故應不致因其光照較低而降低林下植物的多樣性。此外研究亦發現林下直射光立地係數在同一林分不同坡向間的分布有顯著差異，以南向坡最高，且其差異高於不同林分間的差異，故在比較不同林分間的光照環境時，應特別注意取樣的空間分布。 We examined understory light environment among adjacent natural hardwood forest, and Cryptomeria japonica plantations with and without invasion of hardwood species in the Fushan area to evaluate the consequence of the abandonment of plantation on understory light environment and the growth of understory plants. The result indicates that both direct and indirect site factors (DSF and ISF) and their variation were highest in natural hardwood forest and lowest in the Cryptomeria plantation invaded by hardwood species. The low light environment and its low variation in Cryptomeria plantation is likely related to its high stand density and regular arrangement in space. The invasion of hardwood species further compacted the canopy and decreased the level and range of understory light. Even in the forest stand with the lowest understory light environment, the rate of light penetration through canopy is greater than 4%, and more than 50% of understory microsites having light levels greater than 10% of that in the open. The light level is greater than the compensation point of most trees species as such light should not be a limiting factor for understory biodiversity. We also found that the understory DSF differed significantly among transects of different aspects within a stand and the within stand difference was greater than the among stand difference. Therefore, comparisons of light environment among forests should take into spatial variation account.

We examined understory light environment among adjacent natural hardwood forest, and Cryptomeria japonica plantations with and without invasion of hardwood species in the Fushan area to evaluate the consequence of the abandonment of plantation on understory light environment and the growth of understory plants. The result indicates that both direct and indirect site factors (DSF and ISF) and their variation were highest in natural hardwood forest and lowest in the Cryptomeria plantation invaded by hardwood species. The low light environment and its low variation in Cryptomeria plantation is likely related to its high stand density and regular arrangement in space. The invasion of hardwood species further compacted the canopy and decreased the level and range of understory light. Even in the forest stand with the lowest understory light environment, the rate of light penetration through canopy is greater than 4%, and more than 50% of understory microsites having light levels greater than 10% of that in the open. The light level is greater than the compensation point of most trees species as such light should not be a limiting factor for understory biodiversity. We also found that the understory DSF differed significantly among transects of different aspects within a stand and the within stand difference was greater than the among stand difference. Therefore, comparisons of light environment among forests should take into spatial variation account.