關刀溪森林生態系干擾地植群更新之研究

Abstract

在森林生態系演替過程中，崩塌與火燒是重大之干擾；擾動後土壤種子庫及小苗庫是動態變化的起點，在更新及演替過程中扮演著重要的角色。本研究就關刀溪森林生態系內之1999年九二一地震崩塌地、松風山（1995、2001年）與杜鵑嶺（1988年）火燒地，進行土壤種子庫、小苗及上木調查，以瞭解干擾之影響、植群之天然更新和演替趨勢。 崩塌地與非崩塌地對照樣區之種子庫組成皆以野牡丹科與玄參科較多，四類習性 (喬木、灌木、草本與藤本)中以灌木型種子較多。各崩塌地小苗主要以草本型（五節芒等）及喬木型（山胡椒等）所佔比例較高；在崩塌初期，陽性先驅物種（如山胡椒、五節芒等）較為優勢，日後逐漸由灌木（銳葉柃木、野牡丹等）或喬木（木荷）小苗佔優勢；各崩塌地之小苗密度及變化受表土層穩定度、表土層存留量及周圍植群所影響。在非崩塌地對照區之小苗密度則以喬木型（山胡椒等）與灌木型（野牡丹科）較高，優勢種為喬木型的香桂、小葉赤楠等及灌木型的柏拉木、台灣厚距花等。各非崩塌地對照樣區之小苗密度呈季節性變化，在乾季（12月至翌年4月）時較低。各崩塌地及非崩塌地對照樣區之小苗萌發月份集中於4∼10月，高峰期在雨季初期（6月）；在每年4∼8月（雨季）時，崩塌地小苗死亡率增加，其中以草本與喬木型小苗死亡比例較高。 自2002年2月至2003年3月調查期間內，崩塌地及非崩塌地對照樣區之月平均氣溫隨海拔上昇而下降；非崩塌地對照區之平均相對濕度顯著高於崩塌地，且兩者皆有明顯的季節變動；崩塌地之月平均光度（830.2 lux）顯著高於非崩塌地對照樣區（313.8 lux）。崩塌地小苗物種之分布主要受坡向與海拔高度的影響，西南向之L3與海拔最高之L4，其小苗物種組成異於其他崩塌地；陽性植物如山胡椒、五節芒等之小苗，則隨機出現在大部分崩塌地樣區。由崩塌地與非崩塌地對照樣區之土壤種子庫、小苗庫及上木組成三者間相似度之比較，推測崩塌地之植群更新主要藉由干擾區外之種實，而未受干擾區之植群更新來源則為林下小苗庫。 在松風山與杜鵑嶺火燒地，地被植群以陽性植物（五節芒、野桐等）及具萌蘗能力物種（蘇鐵蕨、天台烏藥等）為主；上木以生長快速之陽性樹種（山胡椒、野桐等）、具萌蘗能力的闊葉樹種（天台烏藥與埔里杜鵑）及台灣二葉松較為優勢。在未火燒地區，地被組成以天台烏藥、雨傘仔等較為優勢，上木則以天台烏藥、石櫟等為優勢種。隨著樹冠層鬱閉，陽性植物、台灣二葉松與蘇鐵蕨皆呈衰退趨勢，日後若無火燒再次發生，終將被淘汰而形成闊葉樹種的極相社會。

During the process of succession in forest ecosystem, landslide and fire are important disturbances. The soil seed bank and seedling bank are the beginning of the dynamic change after disturbance, and play important roles in the process of regeneration and succession. This study aims to realize the change of vegetation and regeneration after disturbance by investigating the soil seed bank, seedling bank and upper canopy composition of 921 landslide plots, burned areas of Song-Fong Shan and Du-Jan Lin in the Guandaushi forest ecosystem. In the landslide plots and unlandslided check plots, the seed bank consists of abundant seeds of Melastomataceae and Scrophulariaceae; shrub seeds are more than those of tree, herb, and vine. At landslide plots, most seedlings are herb (Miscanthus floridulus, etc.) and tree (Litsea cubeba, etc.); shade-intolerant species (Litsea cubeba, Miscanthus floridulus, etc.) are dominant in the early stage, and gradually displaced by shrub (Eurya acuminata, Melastoma candidum, etc.) and tree (Schima superba); the seedling density and fluctuation are affected by soil stability, top-soil persistence and surrounding vegetation. In the unlandslided check plots, seedlings of tree (Litsea cubeba) and shrub (Melastomataceae) are most abundant; the dominant species of seedling are tree (Cinnamomum subavenium, Syzygium buxifolium, etc.) and shrub (Blastus cochinchinensis, Pachycentria formosana, etc.). The seedling density at unlandslided check plots shows seasonal change and is lower in dry season (December to next April). For all plots, the seed germinating concentrates during April to October, especially in the early rain season (June). The death rate of seedling of landslide plots increases during April to August (rain season). Seedlings of herb and tree show higher proportion of death. During February 2002 to March 2003 study period, monthly average temperature of all plots decrease with the increase of elevation. Monthly average relative humidity of unlandslided check plots is higher than that of landslide plots, and all are changed with seasons. Monthly average light of landslide plots (830.2 lux) is significantly higher than that of unlandslided check plots (313.8 lux). In landslide plots, the dispersion of seedling species is affected by aspect and altitude. The species of seedlings at the southwestern aspect (L3) and at the highest altitude plots (L4) are different from that of other landslide plots. The seedlings of shade-intolerant species, for example Litsea cubeba, Miscanthus floridulus etc., are randomly appeared in most landslide plots. Comparing the similarity of soil seed bank, seedling bank and upper canopy composition in landslide plots and unlandslided check plots, we presume that plant regeneration at landslide plots is mainly by seeds outside disturbance area, but that at undisturbed plots is mostly from seedling bank under canopy. In burned areas of Song-Fong Shan and Du-Jan Lin, ground vegetation is mostly shade-intolerant (Miscanthus floridulus, Mallotus japonicus, etc.) and sprouting species (Brainea insignis, Lindera aggregata, etc.); overstory is dominated by fast-growing shade-intolerant species (Litsea cubeba, Mallotus paniculatu, etc.), sprouting broad-leaved tree (Lindera aggregata and Rhododendron breviperulatum), and Pinus taiwanensis. In unburned areas, ground vegetation is mostly Lindera aggregata and Ardisia cornudentata, upper canopy is mostly Lindera aggregata, Pasania glabra etc. As canopy is getting closer, shade-intolerant species, Pinus taiwanensis, Brainea insignis are decreasing. If no more fires, these species would be eliminated and succeed to climax of broad-leaved trees.