## 3 Results
### GHG Record for Howland Forest Tower (US-Ho1) and CARB-CAR (CAR 681, CAR1161)  
Figure 2 shows NEE results for US-Ho1, 2, 3, and CARB-CAR values for the combined period of 1996 to 2017; absolute error results for the period 2008 to 2017 are shown (see also Supplement Table S6 for corresponding percentage error data). (A) NEE gC m\({}^{-2}\) y\({}^{-1}\)  
Figure 5: Greenhouse gas (GHG) social cost forest (GHG-SCF) yearly means (bars) for CO\({}_{2}\) (blue) and CH\({}_{4}\) (orange) fluxes measured at the top of the tower, N\({}_{2}\)O (green) fluxes measured at chambers no. 2 (CO\({}_{2}\) and CH\({}_{4}\)) and no. 10 (N\({}_{2}\)O), and the total sum of the three (GWG economic value; gray). Soil flux averages correspond only to the period May–November, while the tower means to the full year. For illustration purposes, results were extrapolated to the Howland research area (557 acres; panels (**A,C**) and to 40,000 acres (panels (**B,D**). Social cost values (USD) used were (**A,B**) $52, $1500, and $19,000 per GHG metric ton of CO\({}_{2}\), CH\({}_{4}\), and N\({}_{2}\)O, respectively (values estimated for the year 2021). (**C,D**) $73, $2500, and $28,000 per GHG metric ton of CO\({}_{2}\), CH\({}_{4}\), and N\({}_{2}\)O, respectively (values estimated for the year 2040) [33]. In each panel, GHG-SCF Economic values and CO\({}_{2}\) NEE are represented by left Y-axes, while CH\({}_{4}\) and N\({}_{2}\)O fluxes by right Y-axes. In panel (**A**), yearly means capture is annotated for each gas in g m\({}^{-2}\) y\({}^{-1}\) above each bar. B) NEE tCO\({}_{2}\) acres y\({}^{-1}\) (ac\({}^{-1}\) y\({}^{-1}\)); shown (A,B) are annual and cumulative values (years 2008 to 2013 represent CAR 681, years 2015 to 2017 represent CAR1161), (C) absolute and percent errors for CAR annual project data relative to NEE overlapping data. Net carbon sequestration values are shown as positive. The mean and standard deviation for the aggregated CAR time-series (2008-2013, 2015-2017) of \(-\)768.1 \(\pm\) 1716 gC m\({}^{-2}\) y\({}^{-1}\) is \(\sim\)2.7x the mean of \(-\)288.3 and \(\sim\)17x the standard deviation of 95.5 gC m\({}^{-2}\) y\({}^{-1}\), for Howland NEE Ho1 and Ho2 towers, respectively, consistent with exceeding the CARB 5% invalidation threshold [52] and natural ranges for reported interannual NEE variance [1,23]. CAR681 reports a 2008 value of \(-\)43,687.0 tCO\({}_{2}\)e, or \(-\)5334.7 gC m\({}^{-2}\) y\({}^{-1}\), -19x larger than the Ho-1 NEE 2008 value of \(-\)287.1 gC m\({}^{-2}\) y\({}^{-1}\), and \(\sim\)32x the mean value of global forest NEE [53], establishing the CAR681 2008 value as exceeding the known natural ranges for NEE [23,53]. Likewise, CAR681 reports an exact value of 834 tCO\({}_{2}\) (e.g., \(-\)198.0 gC m\({}^{-2}\) y\({}^{-1}\)) for the years 2009 to 2013 in contrast to the absence of repeating values for the 20-year US-Ho1 record, documenting incapability of CAR model annual resolution. The CARB-CAR NEE trend is not observed in the natural variance of Ho1. US-Ho1 data are augmented with US-Ho2 NEE data, approximately 775 m apart, representing \(\sim\)95% of the project footprint area shared by NEE and CARB-CAR data source areas, as shown in Figure 1. A two-tailed t-test comparing US-Ho1 and US-Ho2 returned a \(t\)-statistic of \(-\)0.22 with an associated p-value of 0.83. Thus the difference is not significant. The overlapping years are from 1999 to 2009 (10 years; at 95%, the approximate \(t\)-threshold is 2.2), consistent with [41]. US-Ho3 documents the recovery of NEE after a shelterwood harvest. The absolute and percentage error of CARB-CAR data relative to NEE data ranges from 0.65 tCO\({}_{2}\) ac\({}^{-1}\) y\({}^{-1}\), 25.7% to 75.9 tCO\({}_{2}\) ac\({}^{-1}\) y\({}^{-1}\), and 2258% for the years 2011 and 2008, respectively, if US-Ho1 yearly values are taken as the representative values.