## 4 Discussion
The US-Ho1,2 outliers (Figure 3A) emphasize the need for high-frequency monitoring as anomalous years can have disruptive impacts on project revenue [32]. US-Ho3 confirms the sensitivity of eddy covariance NEE to timber harvest and regrowth (Figure 2A,B), a trend not detected by CARB-CAR methods but a requirement to test CARB-CAR modeled harvest and growth simulations [17]. Eddy covariance data provide insights into carbon dynamics and related economics not possible with biometric surveys conducted every 6- to 12-years, typical for the Howland CARB-CAR protocol [17]. Considering CO\({}_{2}\) alone, Howland Forest NEE tower data, US-Ho2, in conjunction with US-Ho1, covers \(\sim\)95% of the shared project footprint area with CARB-CAR forest plots (Figure 1). NEE values for US-Ho1 and US-Ho2 are comparable, lacking significant differences between the towers. The Howland two-tower NEE data confirms irreconcilable differences for carbon accounting relative to CARB-CAR data consistent with previous results [1] of offset over-crediiting and overpayment by \(\sim\)4x relative to NEE values [1]. The aggregate CAR681 and CAR1161 time series (2008-2103, 2015-2017) was \(\sim\)2.7x the mean and \(\sim\)17x the standard deviation for Howland NEE over the same period, exceeding the 5% invalidation threshold cited by CARB [52] and lying outside of the natural range for 20 years of measured interannual Howland [58] and NEE forest values [23; 53]. The exclusion of ecosystem respiration terms for CO\({}_{2}\) within the CARB-CAR protocols, critical for calculation of net forest carbon sequestration, confirm incomplete carbon accounting and likely erroneous, invalid offsets for the CARB compliance process for CAR681 and CAR1161. Absent ecosystem respiration, errors of up to 2258% per year were calculated (Figure 2C; Supplement Table S6), emphasizing the requirement for complete carbon accounting, consistent with the well-characterized relationship between \(\mathrm{R_{eco}}\) and GPP (Figure 2A,B), and soil chamber measurements for CO\({}_{2}\) efflux (Figure 5). The three-gas forest eddy-covariance systems employed at Howland are comprised of commercially available single and multi-gas analyzers for eddy covariance (e.g., CO\({}_{2}\) and CH\({}_{4}\), N\({}_{2}\)O) [38; 59; 60], also applicable to soil chamber gas analyses [38]. A combination of three-gas eddy covariance tower networks of varying heights and soil chamber measurement campaigns can be scaled up across specific ecosystem landscapes by employing expanded ground networks, increasingly inclusive of CH\({}_{4}\) and N\({}_{2}\)O monitoring [61; 62; 63; 64], scale-aware models [65], and remote sensing data [66; 67] available for the US and increasingly across the planet [68]. In contrast to the diversity of GHG direct measurements and applications for Howland, the CARB-CAR protocols identify and list CH\({}_{4}\) and N\({}_{2}\)O only as sources [20], precluding determination of net budgets for these gases, and emphasizinglimitations of estimation protocols that exclude direct measurement of GHGs. Equations for net GHG reductions and removal enhancements cited in [20] may apply to any GHG but are uniquely denominated for CO\({}_{2}\), as source or sink, linked to carbon and tree growth equations and models to satisfy the 100-year carbon baseline and tree harvest scenarios required for CARB-CAR products. Accordingly, CARB-CAR protocol uncertainties, if employed to determine offsets for non-CO\({}_{2}\) GHGs, are likely higher than for CO\({}_{2}\) and limited to source emissions rather than net emissions for these gases. Independent verification of emission reduction claims is critical to the integrity of GHG offset supply chains.