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Topographic Effects on CO2 Flux Measurements at the Chi-Lan Mountain Forest Site



本研究採用多套渦度相關儀器方法,以檢測地形效應對於棲蘭山森林樣區二氧化碳通量量測的影響。本研究結果顯示通量發散主要發生於早晨時段、冠層上不同高度的通量量測系統之間。此時段二號塔22公尺處(T2L2)的平均二氧化碳通量為-11.9 μmol m^(-2) s^(-1),二號塔26公尺處(T2L1)與一號塔24公尺處(T1)則分別為-14.8和-15.3 μmol m^(-2) s^(-1)。T2L2所量測的二氧化碳通量分別比T1與T2L1所量測的通量低22與20%。除此之外,透過垂直剖面、通量及氣象參數的量測,本研究發現棲蘭山樣區每日的起霧/晴天、山風/谷風及晝夜變化形塑了各時段不同的二氧化碳傳輸型態。下午至傍晚起霧時段,紊流發展較弱且主要為間歇性的渦流。渦流通量(-1.4 μmol m^(-2) s^(-1))與暫存量(0.4 μmol m^(-2) s^(-1))皆佔淨生態系二氧化碳交換(-1.0 μmol m^(-2) s^(-1))不可忽略的比例。入夜後隨著天氣型態由有霧轉晴,大氣趨於穩定,渦流通量與暫存量總和顯著地下降(3.5下降至2.3 μmol m^(-2) s^(-1))。此短時間內的下降現象反應出冠層下泄流現象的生成,將土壤及地表植被呼吸所釋放的二氧化碳向下坡方向帶離生態系,總計約造成了約34%的淨生態系二氧化碳交換低估。本研究闡述地形效性造成白天與夜晚淨生態系二氧化碳交換20~22與34%的偏差,與近期相關研究相近(20~80%)。此顯著的地形效應需要更多後續研究,以發展合適的通量資料校正或檢核方法。

Parallel abstracts

The multiple eddy covariance measurement approach was applied to discern topographic effects on CO2 flux measurements at the Chi-Lan Mountain (CLM) site, northern Taiwan. The results suggested that fluxes diverged between different heights above the canopy in the morning. Mean morning CO2 fluxes at 24 m in height on the main tower (T1) and 26 m in height on the second tower (T2L1) were respectively -15.3 and -14.8 μmol m^(-2) s^(-1) in summer of 2007, while the value was -11.9 μmol m^(-2) s^(-1) at 22 m in height on the second tower (T2L2). The measured fluxes of T2L2 were respectively 22 and 20% lower than those of T1 and T2L1. In addition, we propose that complex CO2 transport regimes evolve beneath the canopy during transitions of foggy/clear, day/night, and valley-wind/mountain-wind regimes. Under foggy conditions in the late afternoon and early evening, intermittent turbulence dominated and sporadically penetrated downward into the forest. Either vertical eddy flux (-1.4 μmol m^(-2) s^(-1)) or storage change (0.4 μmol m^(-2) s^(-1)) significantly contributed to net ecosystem exchange (NEE) of CO2 (-1.0 μmol m^(-2) s^(-1)). As the fog dissipated and the atmosphere became stable in the evening, significant decreases in eddy flux plus storage change (from 3.5 to 2.3 μmol m^(-2) s^(-1)) reflected that drainage flow was generated below the canopy and carried CO2 released from the soil and understory vegetation respiration downhill. The drainage effect consequently led to a 34% underestimation of the nighttime NEE. Our results revealed that topographic effects could respectively bias daytime and nighttime NEE estimations by 20~22 and 34%, which were previously reported at 20~80%. The topographic effects led to evident uncertainties in NEE estimates, and further research is urgently needed to develop adequate data-filtering or correction approaches.

Parallel keywords

eddy covariance advection storage cloud forest