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  • 學位論文

山坡地區森林次冠層通量特徵之研究

Investigating the Flux Patterns within Forest Subcanopy over a Hilly Terrain

指導教授 : 莊振義

摘要


架設通量觀測塔以「渦流相關法」(eddy-covariance method, EC)技術,針對生態系與大氣之間物質或能量交換進行長期且連續的直接量測,已經成為學界探討碳循環問題的重要方法。但實際的現地條件(如地形、冠層結構與氣象條件)通常難以符合EC的基本假設,進而造成通量量測的不確定性。已有許多研究指出,在高大森林樣區及山坡地形下,容易形成複雜的空氣流場而對通量量測造成影響。此外,特別在大氣較為穩定的夜晚時段,可能因平流效應或斜坡地形導致的泄流作用而造成通量量測的低估,而森林的冠層結構造成複雜的風場分布亦增加通量量測的困難。 棲蘭山通量研究站位於台灣東北部,為一典型的亞熱帶雲霧森林,目前有一通量觀測塔(離地24m高)於2006年起持續進行長期的通量觀測。本研究於2009年9月起,於棲蘭山森林樣區的森林次冠層架設兩座高約2公尺的小塔於斜坡地形上,試圖量化森林次冠層的通量傳輸特徵,以提供未來修正冠層上方通量數據的參考。研究結果發現,棲蘭山過去的通量計算中,關於平均垂直風速明顯系統性的正偏(約0.05m s-1),主要來自於平面擬合法中迴歸係數的計算偏差。使用特定月份的風速資料所計算的迴歸係數,無法用來計算全年的通量資料。棲蘭山森林次冠層的風場有其空間變異的特性,並明顯反應在通量特徵的差異。棲蘭山森林次冠層的可感熱通量因冠層結構影響熱力分布特性而與與冠層上方呈現相反趨勢。森林次冠層的動量水平平流通量與森林次冠層的垂直風速有關,在大氣狀況相對穩定的夜晚時段,呈現負值垂直風速時,同時出現較明顯往下坡處運動的動量水平流通量,顯示夜間重力泄流的確是影響棲蘭山樣區通量估算的不確定來源。

並列摘要


The tower-based eddy- covariance (EC) flux measurement could provide a reliable way to quantify the exchange of scalars, energy and momentum across the interface between the atmosphere and the terrestrial ecosystem. However, the ideal assumptions for EC measurement hardly meet the real situation, and it has been well documented that the complex terrain and tall forest canopy could significantly cause complicate situations to underestimate flux measurement. In addition, slope-induced drainage flow and horizontal/vertical mean transport have been suggested to misestimate nocturnal fluxes of the carbon dioxide and energy fluxes through deep forest canopy in hilly terrain due to the local complex wind field. In this study, the data collected from Chi-Lan Mountain flux site, a typical subtropical cloud forest located in north-eastern Taiwan, is used to characterize the flow pattern and its influences on the flux measurement. At this site, a long-term flux tower (height is 24 m) is operating under continuous basis. In this research, two 2-m-high towers were established since September of 2009 trying to characterize flux patterns below canopy and provide sufficient information to adjust the fluxes measurement over a complex terrain. The result showed that long term mean vertical wind speed obtained from planar-fit (PF) method may be very sensitive to the PF coefficients. The PF coefficients calculated from wind speed data from the shorter time period may not be applicable to estimate annual fluxes. The daily pattern of sensible heat flux is significantly different between the above- and below-canopy due to the influences of canopy structure. The horizontal momentum advection within the subcanopy directly related to the vertical mean wind speed. During the nighttime when the atmosphere is relatively stable with negative vertical mean wind speed, the downslope horizontal momentum advection becomes obvious. This strongly implies that the nighttime drainage flow within the subcanopy plays an important role to affect flux estimation in this site.

參考文獻


Meehl, G. A., Washington, W. M., Collins, W. D., Arblaster, J. M., Hu, A., Buja, L. E., Strand, W. G., and Teng, H. (2005) How much more global warming and sea level rise? Science, 307(5716): 1769-1172.
Aubinet, M. (2003) Horizontal and vertical CO2 advection in a sloping forest. Boundary-Layer Meteorology, 108(3): 397-417.
Aubinet, M. (2008) Eddy covariance CO2 flux measurements in nocturnal conditions: An analysis of the problem. Ecological Applications, 18(6): 1368-1378.
Aubinet, M., Chermanne, B., Vandenhaute, M., Longdoz, B., Yernaux, M., and Laitat, E. (2001) Long term carbon dioxide exchange above a mixed forest in the Belgian Ardennes. Agricultural and Forest Meteorology, 108(4): 293-315.
Aubinet, M., Feigenwinter, C., Heinesch, B., Bernhofer, C., Canepa, E., Lindroth, A., Montagnani, L., Rebmann, C., Sedlak, P., and Van Gorsel, E. (2010) Direct advection measurements do not help to solve the night-time CO2 closure problem: Evidence from three different forests. Agricultural and Forest Meteorology, 150(5): 655-664.

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林宜蓁(2015)。溪頭柳杉人工林微氣象特性與能量收支之研究〔碩士論文,國立臺灣大學〕。華藝線上圖書館。https://doi.org/10.6342%2fNTU.2015.00464
王嘉瑋(2011)。山谷邊界層之觀測與模擬〔碩士論文,國立臺灣大學〕。華藝線上圖書館。https://doi.org/10.6342%2fNTU.2011.01354

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