南仁山溪谷樣區熱帶低地雨林枯木存量與動態

Abstract

近年來，氣候變遷議題逐漸引起大眾關注，而碳循環更受到生態研究者的注目。森林在全球碳循環中扮演重要角色，其中，熱帶森林生物量佔全球陸域初級生產力的30–50 %，並蘊藏約40 %的碳量，故熱帶森林為碳循環研究之焦點。然而，過去熱帶森林碳循環研究僅專注於活體木本植物的生物量變動情形，很少針對枯木量(Coarse woody debris, CWD)的研究。枯木量意指森林內大型枯倒木、枯立木及枯枝條的質量，約佔森林地上部總生物量(即活體樹木生物量和枯木存量之總和)的三分之一，為森林碳循環研究中易被忽略的重要因子。 目前熱帶森林的枯木量研究主要集中在亞馬遜盆地，亞洲地區則相對稀少。臺灣位處熱帶邊緣且易受颱風干擾，至今仍無枯木存量及動態的完整研究。本研究選擇南仁山地區一具有碳循環研究潛力之低地雨林樣區，定量枯木存量以及進行為期一年的動態調查，期望了解南仁山地區之(1)枯木存量、(2)不同季節之輸入量、和(3)枯木之分解速率，以增進對南仁山地區森林碳循環的了解。 於2009年一月以穿越線測量現地枯木體積，並依枯木分級等級估算密度，兩者相乘得枯木存量。隨後每三個月複查穿越線枯木新增量，即得各季節枯木新增量。此外，亦設置分解樣方，複查標定的枯木質量變化，求得分解速率常數值。 以穿越線法測得枯木存量為10.48-12.68 Mg ha-1，其中89.60–99.18%由枯倒木所貢獻，枯立木僅佔0.82–10.40%。複查分解樣區的枯木體積及分解等級，求得分解商數為0.55–2.37 yr-1。季節新增量之年累積輸入量為6.37–8.09 Mg ha-1 yr-1，且高峰值出現於十月，推測可能為夏季颱風干擾造成枝條大量斷落或是樹幹傾倒，致使輸入量大增。與其它熱帶森林相比較，溪谷樣區的枯木存量較低，但年輸入量相似、分解速率較快，故存量較低可能受快速的分解作用所影響，進一步分析影響分解速率的因子後，發現南仁山溪谷樣區的枯木木材基質較不具抗腐性，此特性可能促成枯木快速分解。

Recently the climate change has attracted attentions of many ecologists. Forest plays a significant role in carbon cycle, especially those forests in tropical areas. Tropical forests account for up to 40% carbon stock and 30–50% productivity in the terrestrial ecosystem. Most of previous studies for tropical forests carbon balance only focused on living tree biomass, and few paid attention to coarse woody debris (CWD). Coarse woody debris (CWD), which includes fallen and standing dead wood, and fallen branches, comprised about one third of aboveground biomass. However, in the past, it was usually neglected, which might lead to misunderstanding of forest carbon balance. Thus, conducting studies for the stock and dynamics of CWD can give us a comprehensive view on forest carbon cycle. In tropical areas, the role of CWD has been evaluated in several studies recently; however, most of CWD researches mainly focused on Amazon basin. There are few studies for CWD in Asian rainforests. Thus, I selected a lowland rainforest in the Nanjenshan Reserve Area, which had the research potential of forest carbon balance, to conduct the studies of CWD. The purposes of this study are to estimate the stock, the seasonal input pattern and the decomposition rate of CWD. The volume and decay levels of CWD were measured on transects (about 680 m in total length). Samples of CWD at each decay level were collected from the field to assess their volume and mass. Wood density of CWD at each decay level was then derived from the ratio of volume and mass. Finally, necromass was obtained from the product of the corresponding density in each CWD decay class and its volume. The seasonal input of CWD was estimated by resurveying the line transects every three months. Annual decomposition rate was acquired by comparing the differences of amount of CWD stocks. The results showed that the stock in this plot was 10.48–12.68 Mg ha-1 (89.60–99.18% for fallen dead wood and 0.82–10.40% for standing dead wood). The k-value (decay rate constant) was 0.55–2.37 yr-1. The maximum input necromass occurred in October, which might be caused by typhoon disturbances. Furthermore, the amount of necromass of seasonal input ranged from 6.37 to 8.09 Mg ha-1 yr-1.Comparing with other tropical forests, the necromass input was similar but the decomposition rate was higher in Nanjenshan. Thus, the major cause of low CWD stock was attributed to a higher decomposition rate. Furthermore, most of dead wood in Nanjenshan with non-decay-resistant characteristics might promote faster decomposition process.