臺灣東北部不同齡級與地點之森林其結構差異及風害探討

老齡林 (Old-growth forest) 是屬於消長後期的森林狀態，普遍至少有150-200年以上未受大型擾動的生長期，對於穩定生態系統扮演著相當重要的角色。台灣北部低海拔的福山試驗林由於受到當地原住民保護，過去三四百年未受到人為大型破壞性擾動，以森林年分來認定可以說是一塊天然老齡林。然而據觀察與前人研究卻顯示福山老齡林少見枯立或枯倒木，木質殘體佔地上喬木生物量的比例偏低；樹冠高度低於鄰近植被相似地區；林冠孔隙 (gap) 小且多，使得林下光照量在孔隙與非孔隙間沒有顯著差異，研究假說認為此森林結構可能皆為福山地區高頻率但樹木低死亡率的颱風擾動所造成，並且異於一般老齡林之結構特性，反而相似於消長前期的森林結構特徵。因此本研究比較高頻率颱風擾動的福山地區，其老齡林與當地80年次生林的森林結構是否相近。同時亦在較內陸的烏來哈盆地區之森林進行調查，比較離東岸不同距離之森林，受颱風不同程度的影響對於森林結構以及風害程度之差異。蒐集前人文獻，並且搭配野外調查於福山與烏來哈盆進行森林結構探究。結果顯示福山老齡林木質殘體累積量雖低於全球標準，但與當地次生林相比仍顯著高出許多，且組成類型也有相當的不同。然而在樹冠高度、幹密度與喬木地上部生物量上則與次生林有相似的現象，甚至在胸高斷面積之平均數值低於當地次生林，有偏向消長早期森林的趨勢，可能與經常受到颱風擾動有很大的關聯。而在距東岸不同地點之森林的結構比較中，顯示位內陸的烏來哈盆其樹冠高度顯著高於福山地區的森林。活樹木質密度，則是福山顯著高於烏來哈盆地區，可能皆為颱風長期不同程度的擾動兩地選汰後所呈現的面貌。而在木質殘體存量的部分，臺灣各地區皆有較低的現象，似乎是東亞亞熱帶地區的共同現象。最後在2015年夏季颱風風害比較上，福山地區皆新增4-5個林冠孔隙，烏來哈盆則在颱風前後調查皆無紀錄；且福山與哈盆次生林其樹木死亡率皆高於福山老齡林，顯示地區差異造成颱風不同程度的擾動，以及不同消長階段的森林在面對颱風擾動時，皆會有不同程度的風害影響。然福山和烏來哈盆與全球相比，仍皆有著相當低的樹木死亡率，似乎是高頻率颱風擾動地區森林長期適應下的結果。綜合以上，希冀能有助於預測未來全球氣候變遷，熱帶氣旋擾動頻率及影響程度增加對森林生態系結構與功能的影響。

關鍵字

颱風；老齡林；森林結構；木質殘體；福山試驗林

並列摘要

Old-growth forests are forests at late-succession stages that do not experience stand replacing disturbance for at least 150-200 years. Old-growth forests plays an important role on stabilizing ecosystems. The Fushan Experimental Forest (FEF) of northeastern of Taiwan, has not experienced large-scale anthropogenic disturbance for at least 300-400 years so that can be considered as an old-growth forest. Yet, literature and field observation indicate that FEF has a smaller amount of rare snags and logs compared to other old-growth forests. The canopy height FEF is shorter than nearby forest located at leeward side of the Snow Mountain. It also has more but smaller gaps compared to many mature forests and light availability is not different between gap and non-gap understory. Typhoon disturbance likely plays a key role in characterizing the structure of FEF which is distinctively different from commonly conceived for old-growth forests. Many of the structure features of FEF seem to be more similar to those of forests at earlier succession stages. In this study, I compare and contrast the structure of the FEF and a nearby 80-yr forest via both meta-analysis and field survey. I also locate a secondary forest in Hapen that is located 10 km further from the eastern coast than Fushan, to compare and contrast their structural differences in relation to successional stages and typhoon disturbance. The results indicate that the amount of woody debris at FEF is larger compared to the nearby 80-yr forest. The proportions of different components of woody debris also differ between the two forests. But FEF is similar in canopy height, stem density, and aboveground biomass to the nearby 80-yr forest. The basal area at FEF is lower than the nearby 80-yr forest. Compared to the two stands at Fushan, the hapen forest has higher canopy and lower wood density, probably caused by the different severity of typhoon disturbance. The three forests all have smaller amounts of woody debris compared most mature forests around the globe may be characteristic to subtropical forests of eastern Asia. The two 2015 typhoons added 4-5 gaps in Fushan forests, compared to none recorded in the Hapen forest. The secondary forest in Fushan and Happen both have greater mortality than FEF. The results suggest that distance relative to the typhoon routes and successional stage both play an important role in charactering forest structure and that all the forests experiencing frequent typhoon disturbance have low typhoon-induced mortality. The result from this study might be used to predict structural changes of forests that are experiencing increasing rates and intensity of tropical cyclone disturbance as a result of climate change.