## 2 Materials and Methods
### Site Descriptions
#### 2.2.1 Hf-EMS  
The HF-EMS EC flux tower is located within the Little Prospect Hill tract of Harvard Forest (42.538\({}^{\circ}\) N, 72.171\({}^{\circ}\) W, 340 m elevation) in Petersham, Massachusetts, approximately 100 km from the city of Boston [88]. A map of the site is shown in Figure 3. The tower has been recording NEE, heat, and meteorological measurements since 1989, with continuous measurements since 1991, making it the longest-running eddy covariance measurement system in the world. The site is currently predominantly deciduous broadleaf second-growth forests approximately 75-95 years in age, based on previous estimates [120]. Soils at Harvard Forest originate from sandy loam glacial till and are reported to be mildly acidic [88]. The site is dominated by red oak (_Quercus rubra_ L.) and red map (_Acer rubrum_ L.) stands, with sporadic stands of Eastern hemlock (_Tsuga canadensis_ (L.) Carriere), white pine (_Pinus strobus_ L.), and red pine (_Pinus resinosa_ Ait.). When the site was established, it contained 100 Mg C ha\({}^{-1}\) in live aboveground woody biomass [120]. As noted by [PERSON] et al. [88], approximately 33% of red oak stands were established prior to 1895, 33% prior to 1930, and 33% before 1940. A relatively hilly and undisturbed forest (since the 1930s) extends continuously for several km\({}^{2}\) around the tower. In 2000, harvest operations removed 22.5 Mg C ha\({}^{-1}\) of live aboveground woody biomass about 300 m S-SE from the tower, with little known effect on the flux tower measurements. The 40 biometric plots were designated via stratified random sampling within eight 500 m transects [PERSON] et al. [88]. The HF-EMS tower site currently  
Figure 2: Perfect Plasticity Approximation with Simple Biogeochemistry (PPA-SiBGC) model structure; [PERSON] et al. [116]; [PERSON] et al. [117]. contains 34 biometric plots at 10 m radius each, covering 10,681 m\({}^{2}\), or approximately one hectare, in area. Summary statistics for the EMS tower site for the year 2002 are provided in Table 1.  
A table of observed species abundances for the year 2002 are provided in Table 2, using tree species codes from the USDA PLANTS database ([[https://plants.usda.gov](https://plants.usda.gov)]([https://plants.usda.gov](https://plants.usda.gov))). Previous research at the EMS EC flux tower site found unusually high rates of ecosystem respiration in winter and low rates in mid-to-late summer compared to other temperate forests [122]. While the mechanisms behind these observed patterns remains poorly understood, this observation is outside the scope of the presented research. Between 1992 and 2004, the site acted as a net carbon sink, with a mean annual uptake rate of 2.5 Mg C ha\({}^{-1}\) year\({}^{-1}\). Aging dominated the site characteristics, with a 101-115 Mg C ha\({}^{-1}\) increase in biomass, comprised predominantly of growth of red oak (_Quercus rubra_). The year 1998 showed a sharp decline in net ecosystem exchange (NEE) and other metrics, recovering thereafter [88]. As [PERSON] et al. [88] note of the Integrated Biosphere Simulator 2 (IBIS2) and similar models at the time, \"the drivers of interannual and decadal changes in NEE are long-term increases in tree biomass, successional change in forest composition, and disturbance events, processes not well represented in current models.\" The two models used in the intercomparison study, \begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline
**Statistic** & **N** & **Mean** & **St. Dev.** & **Min** & **Pctl(25)** & **Pctl(75)** & **Max** \\ \hline DBH\({}_{Mean}\) & 34 & 24.64 & 3.50 & 17.32 & 22.39 & 27.13 & 31.97 \\ DBH\({}_{St.Dev.}\) & 34 & 10.92 & 2.74 & 6.11 & 8.60 & 12.51 & 16.88 \\ BA & 34 & 35.29 & 11.55 & 13.78 & 25.98 & 42.34 & 57.31 \\ Stocking & 34 & 639.43 & 232.38 & 318.31 & 421.76 & 787.82 & 1145.92 \\ QMD & 34 & 26.87 & 4.00 & 18.34 & 23.91 & 29.64 & 34.39 \\ SDI & 34 & 680.13 & 214.45 & 280.56 & 531.35 & 853.97 & 1071.37 \\ \hline \hline \end{tabular}
\end{table}