# Net Ecosystem Exchange of CO\({}_{2}\) in Deciduous Pine Forest of Lower Western Himalaya, India
## 3 Results
### Seasonal Variations in NPP, Rs, and NEE
As described earlier, this was mostly attributed to the overcast condition. Outside the window of July3d-Aug3d, NEE continued as a carbon sink, with an average of \(-6.3\pm 1.3\)\(\upmu\)mol CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\). A peak in the carbon sink was observed during post-monsoon season (September-October) when the clear sky condition prevailed with abundant soil moisture. Due to the physiological dormancy of pine during winter 2010 when the understory was removed, NEE was  
Figure 1: Variations of carbon fluxes net primary productivity (NPP), soil respiration (\(R\)s), and net ecosystem carbon exchange (NEE; \(\upmu\)mol CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\)) at a 10-day (Dekads) interval over the period November 2010 to December 2011 (vertical bars as SE). Fluxes of the winter season 2010 and 2011 were without understory and with understory, respectively. Fluxes since April 2011 were aggregated of overstory and understory. NEE with a negative sign indicated a carbon sink. observed as a carbon source, with an average of 5.57 \(\pm\) 0.23 umol CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\). Whereas, during the winter 2011 (with the understory layer), the ecosystems became a net sink of CO\({}_{2}\), with an average of \(-\)0.66 \(\pm\) 1.72 umol CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\). Higher s.e. observed during winter 2011 might be due to less observations (\(n\) = 6, representing November-December), while winter 2010 had more observations (\(n\) = 12, representing November to February). Nevertheless, the substantial difference in the winter season NEE fluxes could be attributed to the presence of an understory (lantana). The effect of understory removal was exhibited until the spring-summer period of 2011. During this period both understory and overstory canopy was still developing and the ecosystem exhibited a carbon source (3.96 \(\pm\) 1.0 umol CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\)). These results underlined that the removal of the understory has disturbed an aspect of the carbon balance of this ecosystem. The ecosystem was found to be a source of carbon when understory was absent for about five months (November 2010-March 2011). The understory (_Lantana camera_: An invasive species) is extensively distributed in the lower Himalaya and elimination of understory is a prevalent forest management practice in this region. It appeared that understory is a significant component of the forest carbon balance that can be attributed to sustained NPP from understory during the lean season. Monthly variations of NEE and corresponding phenological stages were shown in Table 2. The results showed that NEE was consistently negative during the growing period (green vegetation fraction of 100%) except for the month of August as described earlier. During winter (physiologically dormant stage) and the water-limited summer season, NEE was consistently positive (carbon source). A pronounced seasonal pattern was observed in NEE. At the annual-time scale (January-December, 2011), the pine forests in conjunction with the understory (lantana), exhibited a net carbon sink (negative NEE), with an accumulated NEE of \(-\)99 \(\pm\) 9.9 g C m\({}^{-2}\) year\({}^{-1}\) (i.e., an equivalent of \(-\)9.55 \(\pm\) 0.95 umol CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\)). As represented by s.e., the magnitude of variation associated with carbon sink was about 10%.