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Research article2018Peer reviewed

Impact of Canopy Decoupling and Subcanopy Advection on the Annual Carbon Balance of a Boreal Scots Pine Forest as Derived From Eddy Covariance

Jocher, Georg; Marshall, John; Nilsson, Mats B.; Linder, Sune; De Simon, Giuseppe; Hornlund, Thomas; Lundmark, Tomas; Nasholm, Torgny; Lofvenius, Mikaell Ottosson; Tarvainen, Lasse; Wallin, Goran; Peichl, Matthias

Abstract

Apparent net uptake of carbon dioxide (CO2) during wintertime by an approximate to 90year old Scots pine stand in northern Sweden led us to conduct canopy decoupling and subcanopy advection investigations over an entire year. Eddy covariance (EC) measurements ran simultaneously above and within the forest canopy for that purpose. We used the correlation of above- and below-canopy standard deviation of vertical wind speed (sigma(w)) as decoupling indicator. We identified 0.33ms(-1) and 0.06ms(-1) as site-specific sigma(w) thresholds for above- and below-canopy coupling during nighttime (global radiation <20Wm(-2)) and 0.23ms(-1) and 0.06ms(-1) as daytime (global radiation >20Wm(-2)) sigma(w) thresholds. Decoupling occurred in 53% of the annual nighttime and 14% of the annual daytime. The annual net ecosystem exchange (NEE), gross ecosystem exchange (GEE), and ecosystem respiration (R-eco) derived via two-level filtered EC data were -357gCm(-2), -1,138gCm(-2), and 781gCm(-2), respectively. In comparison, both single-level friction velocity (u(*)) and quality filtering resulted in similar to 22% higher NEE, mainly caused by similar to 16% lower R-eco. GEE remained similar among filtering regimes. Accounting for changes of CO2 storage across the canopy in the single-level filtered data could only marginally decrease these discrepancies. Consequently, advection appears to be responsible for the major part of this divergence. We conclude that the two-level filter is necessary to adequately address decoupling and subcanopy advection at our site, and we recommend this filter for all forested EC sites.

Keywords

ecosystem respiration (R-eco); flux partitioning; gap filling; gross primary production (GPP); net ecosystem CO2 exchange (NEE); Pinus sylvestris

Published in

Journal of Geophysical Research: Biogeosciences
2018, Volume: 123, number: 2, pages: 303-325 Publisher: AMER GEOPHYSICAL UNION