Apparent winter CO2 uptake by a boreal forest due to decoupling

Abstract

Net uptake of carbon dioxide (CO2) was observed during the winter when using the eddy covariance (EC) technique above a ∼90-year-old Scots pine (Pinus sylvestris L.) stand in northern Sweden. This uptake occurred despite photosynthetic dormancy. This discrepancy led us to investigate the potential impact of decoupling of below- and above-canopy air mass flow and accompanying below-canopy horizontal advection on these measurements. We used the correlation of above- and below-canopy standard deviation of vertical wind speed (σw), derived from EC measurements above and below the canopy, as the main mixing criterion. We identified 0.33ms−1 and 0.06ms−1 as site-specific σw thresholds for above and below canopy, respectively, to reach the fully coupled state. Decoupling was observed in 45% of all cases during the measurement period (5.11.2014–25.2.2015). After filtering out decoupled periods the above-canopy mean winter NEE shifted from −0.52μmolm−2s−1 to a more reasonable positive value of 0.31μmolm−2s−1. None of the above-canopy data filtering criteria we tested (i.e., friction velocity threshold; horizontal wind speed threshold; single-level σw threshold) ensured sufficient mixing. All missed critical periods that were detected only by the two-level filtering approach. Tower-surrounding topography induced a predominant below-canopy wind direction and consequent wind shear between above- and below-canopy air masses. These processes may foster decoupling and below-canopy removal of CO2 rich air. To determine how broadly such a topographical influence might apply, we compared the topography surrounding our tower to that surrounding other forest flux sites worldwide. Medians of maximum elevation differences within 300m and 1000m around 110 FLUXNET forest EC towers were 24m and 66m, respectively, compared to 24m and 114m, respectively, at our site. Consequently, below-canopy flow may influence above-canopy NEE detections at many forested EC sites. Based on our findings we suggest below-canopy measurements as standard procedure at sites evaluating forest CO2 budgets.