福山試驗林孔隙特性與孔隙-非孔隙林下苗木組成之研究

Abstract

林冠孔隙的出現提供額外的空間和光等資源，讓林下耐陰樹種有機會加速生長向林冠邁進，因此孔隙可促進冠層樹種的更新，並維持森林物種多樣性。熱帶與溫帶地區，已有大量關於孔隙的研究，但卻少有研究在亞熱帶森林進行。台灣位處亞熱帶，又位於北太平洋西岸，颱風侵襲最頻繁的地區，使森林因應颱風擾動而產生結構上的適應。本研究的目的即在探討福山，一個經常受颱風擾動的亞熱帶森林，其孔隙的分布和特性與其它地方對孔隙的研究結果是否有所不同；如頻繁的颱風使樹木定期落葉，提供林下相對較高的光量，可能使耐陰樹種的小苗無需依賴孔隙即能向上生長至林冠，因此森林是否不需依賴孔隙進行更新？本研究在福山試驗林一號試驗集水區設置七條由稜線延伸至溪谷的樣線，調查所經過的孔隙，內容包含孔隙面積、年齡，及計算森林的更新時間（turnover time），並與1995年前人在1994年的六個颱風過後，於福山進行的孔隙研究做一比對。研究中亦調查苗木組成及測量林下光，以比較孔隙和非孔隙林下的苗木組成與光環境的關係。研究共記錄17個孔隙，孔隙面積在1-36 m2之間，數量和面積皆較過去的研究為少和小。林份更新時間以形成1年與9年內的孔隙做計算分別為197年和398年，遠較過去的研究所估算的51年和175年為長。林下光照在孔隙和非孔隙間沒有顯著差異，此結果反應在林下苗木於孔隙和非孔隙的組成相似性高，且非孔隙林下擁有孔隙中所有出現的苗木物種，顯示颱風擾動頻繁的福山林木似不需依賴孔隙進行更新。Forest gaps can promote canopy tree regeneration thereby maintain forest species diversity. Gaps provide space and light for rapid growth of understory plants, especially shade-tolerant species, thus help them reach the canopy. This regeneration process has been well described in tropical and temperate forests, but is rarely studied in subtropical forests. Forests in Taiwan which is located at the west side of North Pacific, is influenced by frequent typhoons as such the forests have structural adaptation to typhoon disturbance. This study examined gap characteristics in a subtropical forest of northeastern Taiwan, the Fushan Experimental Forest. Frequent typhoon disturbance causes regular defoliation, and thereby increases understory light availability allowing seedlings of shade-tolerant species to reach the canopy. It is questionable that in such a forest, if gap dynamics also plays an important role on canopy tree regeneration. To address this question, 7 transects were set up at watershed #1 in Fushan Experimental Forest. The gaps intersected the transect lines were surveyed and gap characteristics including area size, age, and turnover time were compared to the study conducted in 1995 following the disturbance of 6 typhoons of 1994. Seedling composition and understory light environment were also investigated to examine their relation to gap and non-gap understory. A total 17 gaps were recorded and the area of individual gaps ranged from 1 to 36 m2. Both the numbers and size of the gaps were less/smaller than those reported before. The calculated turnover time is 197 and 398 years when gaps< 1 and ≦ 9 years, respectively, were include and is much longer than the 51 and 175 years reported in the 1995 study. Understory light availability showed no significant difference between gap and non-gap understory and this contributed to the high similarity of seedling composition between the two environments. Moreover, non-gap understory encompasses all seedling species recorded in gaps. Thus, seedling regeneration in Fushan may not rely on gaps.