Apparent CO2 uptake of a boreal forest due to decoupling

Apparent net uptake of carbon dioxide (CO₂) during winter periods with presumable photosynthetic dormancy has been observed in studies using the eddy covariance (EC) technique above a ~90-year-old Scots pine (Pinus sylvestris L.) stand in northern Sweden. The overall experiment was initiated in 2006 to study the effect of nitrogen (N) availability on stand-scale carbon cycling and consists of two stands with different N addition rates and a non-fertilized reference stand. The current study was conducted at the stand with the high N addition rate, which has received 100 and 50 kg N ha^-1 yr^-1 since 2006 and 2012, respectively, applied over an area of 15 ha around the EC tower. The observed winter carbon uptake led us to investigate the potential impact of decoupling of below- and above-canopy air mass flow and accompanying below-canopy horizontal advection on the CO₂ net ecosystem exchange (NEE) measurements.

We used the correlation of above- and below-canopy standard deviation of vertical wind (σ_w) as main mixing criterion, derived from EC measurements above and below the canopy. We identified 0.33 m s^-1 and 0.06 m s^-1 as site-specific σ_w thresholds for above and below canopy, respectively, to reach the fully coupled state. Decoupling was observed in 45% of all cases during the winter measurement period (5.11.2014 – 25.2.2015). After filtering out decoupled periods the above-canopy mean winter NEE shifted from -0.52 µmol m^-2 s^-1 to a positive value of 0.31 µmol m^-2 s^-1. The same two-layer approach was applied to the consecutive growing season (10.5.2015 – 9.10.2015) at the same site. This analysis revealed that decoupling is strongly dependent on the incoming solar radiation and consequently thermal turbulence. While decoupling occurred only in 18% of all data during daytime (global radiation > 20 W m^-2) it was observed more frequently at night (53% of all data with global radiation < 20 W m^-2). The growing season NEE estimate declined from -380 g C m^-2 to a less negative value of -350 g C m^-2 after filtering out and gapfilling the decoupled periods.

Meso-scale topographical influences induced a predominant below-canopy wind direction and consequently frequent wind shear between below- and above-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 300 m and 1000 m around 110 FLUXNET forest EC towers were 24 m and 66 m, respectively, compared to 24 m and 114 m, respectively, at our site. Consequently, a profound influence of topography-induced below-canopy flow on above canopy NEE may be common.

Based on our findings we suggest including below-canopy EC measurements as standard procedure in sites measuring forest CO₂ budgets.