Linking CO2-advection estimates to vegetation structure at a forest site
O 1.1 in Ecosystem Function
15.04.2010, 08:45-09:00, H13
Surface exchange flux measurements of Net Ecosystem Exchange NEE are incomplete if only the turbulent flux is considered and advection neglected. However, including advective terms in the budget has not proven to be a robust alternative since the uncertainties inherent in advection estimates are large und thus increase the uncertainty and scatter of NEE estimates. The current study investigates some of the processes generating measured horizontal CO2 concentration gradients, which are generally used to compute horizontal advective flux terms. In contrast to standard methodology where gradients are computed over 30 minute time frames and a spatial extend in the order of tens to hundreds of meters the focus of this study is on short and small events such as coherent structures. Moreover, the effect of vegetation structure on concentration gradients is considered. The results suggest that coherent structures act as a transfer function between vegetation structure such as Plant Area Index PAI and subcanopy CO2 concentration. Very local mixing of a CO2 concentration distribution with strong vertical gradients by coherent structures is an alternative explanation for horizontal variability of subcanopy CO2 concentration as opposed to consistent larger scale motion representative for the whole area under study, which is an often made assumption. The small scale variability of vegetation structure leads to high local variability of concentration gradients. Thus gradients are not representative for the scale they need to be to complement above canopy turbulent flux measurement with advective flux terms. The findings do not directly improve the NEE budget but help to understand the mechanisms generating an observed CO2 concentration signal.
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