## 4 Discussion
For example, a project that started in 1992 and finished in 2020 (28-years) would have returned less money than one that finished in 2002 (10 years), when discounted at 15%. In practice this means that landowners and investors who value the present more than the future would select shorter projects (below 10 years) compared to a retirement fund with an investment outlook of decades. Referring to Figure 2A-C, 5- and 20-year project exits would have exhibited less yearly physical and financial carbon variance (and SD) than no exit investments. Exits at 5 and 20 years (Figure 2C) (40,468 ha, $10 tCO\({}_{2}\)) are characterized by \(\overline{x}\) of \(\sim\)$256,000 and $891,000, and SD of \(\pm\) $93,000 and \(\pm\) $98,000, respectively, compared to the no exit 28-year project with \(\overline{x}\) of \(\sim\)$3.9 million and SD of \(\pm\) $2.2 million. Thus, extending a project time frame does not necessarily diminish its financial volatility, because the accounting method would have assigned more credits to longer projects that stored carbon for more years, increasing its return and variance, as shown in Figure 2, whereas the yearly variance would have been higher for a 20-year project than for a five-year project. Investors and project landowners may also consider that if the mean CO\({}_{2}\) NEE value was a good estimation of a project expected income, then to earn one dollar at the end of the project an investor would buy 0.16 ha or 0.05 ha (5- or 20-years projects respectively; $10 tCO\({}_{2}\)). For these project parcels (e.g., 0.16 or 0.05 ha) they would expect to earn $[0.65;1.35] for five-year projects and $[0.89;1.11] for 20-year projects (66% confidence), respectively. The retrospective US-Ha1 NEE data implies that high annual variance, while unpredictable, can be managed by investment objectives such as time horizon, desired return on investment and reliance on standardized direct measurement methods. Although we have only analyzed the US-Ha1 EMS flux tower results in this study, a portfolio of diverse forest projects (e.g., location, ecosystem type, management objective, and vulnerability to climate change) could buffer volatility reducing the impact of the US-Ha1 NEE decline experienced in 2010 [5], an established benefit of portfolio diversification [83]. Likewise, additional US-Ha1 flux tower experiments documenting decline of NEE for a stand of hemlock forest (HEM) due to insect damage (e.g., hemlock woolly adelgid) and for a clear-cut experiment (CC) demonstrate that networks of EC platforms and NEE measurements can capture and integrate multiple changing NEE forest landscapes [6]. The HEM site transitioned from a net sink (-450 gC m\({}^{-2}\)year\({}^{-1}\)) to source over the period studied (2005-2012). The CC experimental site initial timber harvest (2009) [84] was a large net carbon source but transitioned to an annual net carbon sink after the fifth year of disturbance regaining -66% of the carbon lost since harvest [6]. The detection of forest project carbon source(sink) transitions is considered a key capability for a forest carbon trading protocol, a well-documented feature of EC NEE, but an insuperable omission for estimation-based protocols and analysis of bulk soil carbon pools. In the context of establishing an approach relying on GPP and \(R_{cco}\) with emphasis on commercial products, multiple measurement methods and innovative approaches, are required to both understand carbon dynamics and transact reliable global forest carbon and soil carbon sequestration markets. Monetary calculations for the US-Ha1 28-year NEE record analyzed and described in this study did not include results for soil carbon flux measurement, summarized in Figure 1C. However, the US-Ha1 coupled EC and soil respiration dynamics record establish an unprecedented experimental biosphere for soil model development and validation.