## Appendix A  
The seasonal time series of NEE, GPP, R\({}_{\text{eco}}\), and key meteorological variables are shown in Figure 11. Seasonal patterns of NEE (Figure 11a) are similar to those usually observed in northern deciduous forests, resulting from the known phenolic cycle. During spring and fall dormancy, NEE values are in the positive range, suggesting a higher proportion of respiration than photosynthesis that leads to a net carbon source. As leaf flush occurred, the opposite happens with productivity outpacing respiration, leading to increasingly negative NEE and net carbon sequestration. NEE and GPP progressively increased throughout the season (Figure 11a), reaching peak productivity during July (1.4 \(\pm\) 0.02 mg m\({}^{-2}\) s\({}^{-1}\) and 1.8 \(\pm\) 0.03 mg m\({}^{-2}\) s\({}^{-1}\), respectively). Peak respiration (0.80 \(\pm\) 0.03 mg m\({}^{-2}\) s\({}^{-1}\)) also occurred during July but slightly offset from peak productivity. NEE, GPP, and R\({}_{\text{eco}}\) showed a gradual decline into leaf senescence. The length of the growing season was approximately 170 days (\(\pm\) 7 days), spanning from 15 April to 1 October, 2015.  
Seasonal trends in temperature (Figure 14c) show a continuous increase from the beginning of the season to late June, after which temperature progressively decreases until the end of the season (October). Overall, relative humidity (Figure 14d) increases until mid-June and then remains stable during the remainder of the growing season. VPD seasonal changes (Figure 14e) showed an initial increase during the start of the season, followed by a sharp decrease during mid-June and then remaining low and stable from July until end of the season. Beyond these general seasonal changes, short-term changes in climatic status directed intra-seasonal variability in productivity. Throughout the season productivity was linked to air temperature (Figure 14c), particularly during spring and fall warming/cooling events, where warming was associated with increased GPP and cooling with seasonal decreases. The largest drops in GPP and R\({}_{\text{eco}}\) values occurred during periods were temperature reached approached \(<\)5 \({}^{\circ}\)C. VPD (Figure 14e) was principally influenced by temperature and shared similar patterns and associations to GPP. PPFD appeared to be the principal driver of productivity throughout large parts of the season (Figures 14b and 5).  
A combination of factors appears to affect apparent quantum yield (\(\alpha\)) over the course of the growing season. Periods of low quantum yield occurred during high temperatures (Figure 13a), which we would expect from a heat-stressed ecosystem. The opposite relationship is observed when comparing RH and \(\alpha\) (Figure 13b). In general, high \(\alpha\) values coincide with periods of high relative humidity, which represent low evaporative demand. Peak \(\alpha\) values correspond closely with soil water moisture (Figure 13b). An exception to this connection was observed during 31 August when a soil moisture peak is observed but \(\alpha\) seems to decrease slightly. This decrease in \(\alpha\) is associated with low PPFD (61.49 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\)) and T\({}_{\text{air}}\) (3.7 \({}^{\circ}\)C), indicating that these variables are limiting during this period.