利用遙測技術探討台灣福山地區森林之動態 Assessment of forest dynamics through remote sensing in Fushan, Taiwan

Peereman, Jonathan Manuel Eric
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永久樣區與遙測為森林生態學研究提供新的可能性,使我們對森林的動態有更多的了解。頻繁而大規模的擾動透過對森林的結構與過程影響而成為森林動態變化的重要趨動力。然而現有森林樣區對其所在的地景系統的代表性、不同熱帶氣旋對森林影響是否可相類比、以及由遙測影像發展出的上些常用的植生指數的可對比性仍有很大不確定性。 本研究以遙測技術利用常用的植生指數探討五個颱風對台灣北部亞熱帶雨林--福山試驗林的影響。研究首先檢測25公頃的福山森林動態樣區對所在的地景系統的代表性,並檢驗涵蓋較廣海拔梯度以及不同形狀的替代樣區設計的代表性。其次本研究比較五個颱風對福山試驗林的影響及其與颱風風速風以及地形特徵(如坡度坡向以及海拔高度)的關係。研究亦分析颱風造成的植生損失的頻率和損失程度之間的關係,最後研究檢測五個颱風對四個植生指數造成的變動的一致性。 研究結果顥示,現有的永久樣區僅涵蓋現有植生覆蓋情形的一小部分。雖然颱風造成的植生指數變動在永久樣區和地景系統間不一致,但動態樣區並無高估或低估颱風對地景系統影響的趨勢。以最小歐幾里得距離(Euclidean distance)為代表性的指標的分析結果顯示,現有的樣區比其他替代的樣區設置更能代表颱風造成的影響,且分析亦發現不同颱風造成的植生損失的空間分布差異很大(即相關性低)。在福山試驗林地形和颱風造成的植生損害之間的關係相當複雜,即使路徑與強度相似的颱風所造成的損害的空間樣貌亦不相同。然而颱風的影響仍有一些相似性,如均增加植生覆蓋的異質性,颱風前的植生覆蓋度皆和颱風影響的程度有正相關,以及植生損失的頻率和損失的程度亦多有正相關。又研究結果顯示不同植生指數對颱風擾動偵測的敏感度在不同颱風間並不一致,故在探討颱風對植生造成的影響時,宜同時使用多個植生指標。 森林動態必然會受到擾動特性(如頻率、強度與發生時間)改變的影響,總結而言本研究發現相較於替代的樣區設計方式,現有的福山森林動態樣區對整個福山森林生態系有不錯的代表性,但要將樣區所得結果外推到整個地景系統仍應小心為之。由於熱帶氣旋對植生的影響在不同事件間並不一致,未來對擾動與森林動態的研究宜包括多個熱帶氣旋事件,才能對複雜的熱帶氣旋和地景系統的交互作用有較完整的了解。此外欲探討擾動對植生影響,最好要有定期的地面調查,以驗證利用遙測技術發展出來的植生指數所測得的結果。
Permanent plots and remote sensing have permitted studies of many aspects of forests ecology and led to numerous advancements in our understanding of forest dynamics. Frequent large scale disturbances are important drivers of forest dynamics through their effects on forest structure, and processes. However, the landscape-representation of the current forest plots, and the comparability of tropical cyclone effects, and the comparability of commonly used vegetation indices based on remote sensing in assessing disturbance effects remain unclear. This study used remote sensing techniques and common vegetation indices to assess the effects of five typhoons on the Fushan Experimental Forest (FEF), a subtropical rainforest in northern Taiwan. First, it examined the landscape-scale representativeness of the 25-ha Fushan Forest Dynamics Plot (FFDP) and alternative plot designs based on wider altitudinal gradient and different shape. Then, the effects of the five typhoons on the FEF were compared in relation with their characteristics (e.g., wind speed and direction) and topographical variables (e.g., slope, elevation). The relationship between typhoon damages frequency – as defined by two detection thresholds – and intensity was also assessed. Finally, the consistency of four vegetation indices responses was examined across the five typhoons. The current permanent plot represented a small part of the vegetation cover diversity across the FEF. Although vegetation indices did not change similarly in the FEF and the FFDP after typhoon passages, there was no trend for under- or over-exposure of the forest plot to typhoon damages across the five events. The use of minimal Euclidean distances as a representativeness metric demonstrated that the current plot was a better approach to landscape disturbances representation than alternative sampling designs. Furthermore, the comparison of the five typhoons indicated that spatial distributions of damages within the FEF varied greatly across events, as most correlations were low among events. Topography-damage relationships appear to be complex within the FEF as typhoons with similar tracks and strengths did not have comparable damage distributions. Nevertheless, typhoons also had consistent effects such as increasing vegetation heterogeneity, and positive relationships between pre-typhoon vegetation cover and disturbance severity, and between typhoon frequency and disturbance severity. Finally, this study showed that the use of multiple vegetation indices is necessary when assessing typhoons disturbances as sensitivity of individual indices to vegetation change did not remain consistent among events. Forest dynamics are bound to be affected by changes in disturbance regime (e.g., frequency, intensity and timing). This study concludes that the current plot in northern Taiwan is a good representation of typhoon disturbances in its immediate landscape but that findings from the plot should be upscaled with caution. However, further studies focused on multiple cyclones are necessary to understand the complexity of cyclone-landscape interactions that vary among events, even when they have comparable tracks. In addition, routine surveys for studies that aim to assess disturbance effects on vegetation whenever possible are strongly recommended for the validation of assessments made with vegetation indices via remote sensing techniques.
森林擾動, 代表性, 地景生態, 林冠覆蓋, 植生指數, 森林動態, 颱風, 熱帶氣旋, 森林永久樣區, forest disturbance, representativeness, landscape ecology, canopy cover, vegetation index, forest dynamics, forest permanent plots, tropical cyclone, typhoon