دانلود رایگان مقاله جذب CO2 زمستانی جنگل شمالی ناشی از تجزیه

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 0.33 m s−1 and 0.06 m s−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 −0.52 μmol m−2 s−1 to a more reasonable positive value of 0.31 μmol m−2 s−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 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, 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.

4. Conclusions

Apparent CO2 uptake in above-canopy winter EC CO2 fluxes led us to investigate the decoupling between above- and belowcanopy air masses and potential accompanying below-canopy horizontal advection. We found that decoupling occurs frequently in this boreal forest despite a low leaf area index and that towersurrounding topographicalfeatures are able to foster below-canopy drainage flow even when the nearest vicinity around the measurement tower is rather flat. Both decoupling and below-canopy horizontal drainage flow may promote below-canopy advective CO2 loss. Even in wintertime when CO2 fluxes are very low at these high latitudes, this potential advective carbon loss may account for a substantial part of the whole forest seasonal and, therefore, annual carbon budget. We evaluated three different parameters derived from abovecanopy measurements for their usefulness for identifying periods with decoupling. Neither a filtering with an above-canopy derived u* threshold, nor a horizontal wind speed threshold, nor a filtering with an above-canopy derived w threshold could reproduce the above-canopy CO2 flux data filtered with w thresholds for both above- and below-canopy data. Based on these findings we conclude that: (i) decoupling may occur even in quite open forest stands; (ii) filtering flux data with single-level above-canopy derived parameters is not a sufficient alternative for two-level investigations to address decoupling; (iii) topography beyond the nearest vicinity to the flux tower might have a profound influence on the EC measurements.