# Net Ecosystem Exchange of CO\({}_{2}\) in Deciduous Pine Forest of Lower Western Himalaya, India
## 3 Results
### Seasonal Variations in Factors Controlling NPP and Rs  
The optimal temperature that favored maximum productivity was around 25 \({}^{\circ}\)C. NPP and EF were positively and significantly correlated (R\({}^{2}\) = 0.75, \(p\) < 0.001, excluding October), but showed remarkable decline at higher EF (\(>\)0.75; Figure 2a). It is notable that in this subtropical environment, reduction in light availability due to overcast conditions could reduce the productivity. Optimum EF that favored maximum productivity varied between 0.5 and 0.75. It means that if dryness (EF \(<\) 0.5) or  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Months**} & **NEE (Mean \(\pm\) SE),** & \multirow{2}{*}{**Seasons**} & \multirow{2}{*}{**Phenology**} \\  & **CV (\%)** & & \\ \hline January–February & 310.3 \(\pm\) 2.8, 2 & Winter & Physiologically dormant stage \\ \hline March–April & 247.0 \(\pm\) 10.5, 10 & Spring to Summer & 10\% to 63\% GVF \\ \hline May–July & \(-\)314.6 \(\pm\) 5.4, 4.8 & Summer to Monsoon & 100\% GVF \\ \hline August & 208.0 \(\pm\) 8.6, 4 & Monsoon & 100\% GVF \\ \hline September–October & \(-\)508.3 \(\pm\) 23, 11 & Post-monsoon & 100\% in September, 92\% GVF in October \\ \hline November & \(-\)129.2 \(\pm\) 9.7, 13 & Fall & 80\% GVF \\ \hline December & 87.8 \(\pm\) 14.4, 28 & Winter & Physiologically dormant stage \\ \hline Annual sum & \multicolumn{3}{c}{\(-\)99 \(\pm\) 9.9 g C m\({}^{-2}\) year\({}^{-1}\)} \\  & \multicolumn{3}{c}{(peak carbon sink reached in post-monsoon)} \\ \hline \hline \end{tabular}
\end{table}
Table 2: Monthly net ecosystem exchange (NEE, g C m\({}^{-2}\) month\({}^{-1}\)) during an annual cycle of 2011. The corresponding standard error (SE) and green vegetation fraction (GVF) have been shown. Fluxes were the sum over the months. Further, months were selected and aggregated based on source/sink of NEE (positive or negative).  
wetness (EF \(>\) 0.75) prevails for a longer period (at least over a month); then it may have an impact on the carbon processes. We also observed that an increase in NPP was related to the decrease in VPD (Figure 2b). During the monsoon and post-monsoon period of the growing season with VPD more than 2 kPa, the NPP remained almost constant. However, VPD between 0.6 kPa and 2.0 kPa was the most optimal condition for higher NPP.  
Typically, soil respiration (\(R\)s) rate was mostly controlled by temperature and EF during the growing season, while soil moisture had a larger role during the summer water-limited period. Therefore, the seasonal transition phases of \(R\)s fluxes in this region were critically associated with temperature, soil moisture, and EF. The night-time canopy respiration (\(R\)nc) variability up to 83% can be attributed to temperature, dewfall, VPD, and EF during the growing season. For detailed environmental factors controlling night-time canopy respiration (\(R\)nc) and soil respiration (\(R\)s), one can refer to [PERSON] and [PERSON] (2017).  
Figure 2: Relationship between evaporative fraction (EF) and NPP at a 10-day (Dekads) interval over the growing period April to October 2011 (**a**). Solid circles indicate the growing period (April–June), while open circles indicate the peak growing period (July–September). In subplot (**b**), the relationship between the vapor pressure deficit (VPD) and NPP over the growing period April to October 2011 (solid circles) was shown (excluding August).