Litterfall and Element Fluxes in a Natural Hardwood Forest and a Chinese-fir Plantation Experiencing Frequent Typhoon Disturbance in Central Taiwan

Chinese fir (Cunninghamia lanceolata) is the most important forest plantation species in subtropical Asia and is rapidly replacing natural forests. Such land-use change may affect ecosystem nutrient cycling through changes in litterfall nutrient flux. Tropical cyclones often cause pulses of litterfall. Previous studies, however, have mostly focused on the effects of a single cyclone with little effort examining the effects of repeated cyclones. We examined litterfall in a natural hardwood forest and a Chinese-fir plantation in central Taiwan experiencing an average of one typhoon per year. The natural hardwood forest had 54 percent higher annual litterfall (11,400 kg/ha/yr) than the Chinese-fir plantation (7400 kg/ha/yr). Four typhoon-affected months (typhoon period) contributed to approximately 60 percent of the litterfall and litterfall element flux in the natural hardwood forest and 80 percent in the Chinese-fir plantation, with contributions from individual typhoons varied by more than twofold. Litterfall N and P concentrations were significantly higher in typhoon period than in non-typhoon period, likely the result of limited retranslocation. Precipitation was a better predictor of quantity of typhoon-associated litterfall than wind velocity. Both types of forests in southeastern China beyond the reach of typhoons have litterfall peaks in the dry season. In contrast, we measured higher litterfall during the typhoon period than during the dry season, suggesting that in regions with frequent cyclones, cyclones drive temporal variation of litterfall. Global climate change is affecting the frequency and intensity of cyclones; therefore, knowledge of typhoon-litterfall dynamics is indispensable for understanding the effects of climate change on ecosystem nutrient cycling. 台灣中部颱風擾動頻繁之天然闊葉林及杉木人工林枯落物及其營養元素通量 杉木是亞熱帶亞洲地區最重要的造林樹種,且正快速取代天然林而擴張中。此土地利用的改變,可能改變枯落物營養的通量進而影響生態系的營養循環。熱帶氣旋常造成大量的枯落物,但之前的研究多聚焦單一熱帶氣旋的影響,甚少探討重覆的熱帶氣旋擾動所造成的影響。台灣中部每年平均經歷一個颱風,本研究比較本區一天然闊葉林與一相鄰的杉木人工林的枯落物型態。結果顯示天然林的年枯落物量(11,400 kg/ha/yr)比杉木人工林(7,400 kg/ha/yr)高54%。四個受颱風影響的月份(颱風時期)的枯落物量及枯落物營養元通量,在天然林佔年總量的60%,在人工杉木林則佔80%,而不同颱風月間的差異達2倍。天然林落葉中氮與磷的濃度在颱風時期顯著高於非颱風時期,此應與颱風時期營養未充分再吸收有關。研究結果亦顯示降雨量比風速更能預測颱風造成的枯落物量。在中國東南部這兩類森林枯落物的高峰均在乾季,但本研究中颱風時期的枯落物量高於乾季,顯示在颱風頻繁的地區颱風擾動主導了枯落物在時間上的變動情形。全球氣候變遷正改變熱帶氣旋的頻率與強度,因此充分了解颱風與枯落物變動間的關係,為評估氣候變遷將如何影響生態系營養循環所不可或缺的一環。