# Net Ecosystem Exchange of CO\({}_{2}\) in Deciduous Pine Forest of Lower Western Himalaya, India
## 4 Discussion  
### Seasonal Variability of Ecosystem CO\({}_{2}\) Fluxes  
Components of carbon balance, such as _Ac_ (\(\sim\)GPP), _R_s, and NPP exhibited a strong seasonality in this subtropical pine forest having an understory as lantana. Both _R_s and NPP peaked during the post-monsoon season and were lowest in the winter, which is typical for any temperate or subtropical coniferous forests [5; 32]. The daily minimum and maximum rates of NPP were well comparable with the existing literatures for any temperate and subtropical coniferous forests [5; 33; 34]. For instance, the NPP from mixed forest plantation was reported in the rage of 2 umol CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\) to 6 umol CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\) over Northern Himalaya [33; 35]. Notably, we observed that the NPP was almost 11 times higher during winter 2011 owing to the presence of an understory as compared to winter 2010. Soil respiration (_R_s) is an integral part of the ecosystem carbon cycle and coupled to various components of ecosystem production and decomposition processes. _R_s is directly related to ecosystem carbon balance, nutrient cycling, regional-global carbon cycling, and climate change. Moreover, it plays a critical role in regulating atmospheric CO\({}_{2}\) concentration and thus, climate dynamics. However, given its ecological significance and role in atmospheric CO\({}_{2}\) concentration, it is unknown how climate warming would affect the positive feedback loop between the global carbon cycle and climate system and what is its role of soil respiration in regulating this positive feedback. To understand these pertinent questions, we have tried to decipher the biotic and abiotic controls in a pine forest of lower Himalaya. Soil respiration (_R_s) and productivity (NPP) relations were prominent at a seasonal time-scale as evidenced by their tight-link during the growing period, while they decoupled during the summer-time water stress. Studies across a variety of biomes also revealed a strong correlation between _R_s and NPP [36; 37]. A tight-link between _R_s and NPP was obvious because primary production provides the organic substrate that drives soil metabolic activities [38]. However, a complete reversal in the _R_s-NPP relation was noticed (_r_ = -0.93), when winter-time temperature-limited conditions changed to the water-limitation period during summer (April-June). It was probably due to a reduction in photosynthetic availability (as the overstory and understory canopy was still developing) as well as moisture stress-induced reduction in soil metabolism. Anaplerotic uptake of CO\({}_{2}\) could be another reason. Emergence of new leaves and canopy development is an important phase in the annual phenological cycle. In this critical phase, in the absence of sufficient numbers of matured leaves, the central process in energy metabolism, i.e., the tricarboxylic acid (TCA) cycle, increasingly functions to biosynthesize essential, non-essential amino acids, fatty acids, glucose, etc. The anaplerotic uptake of CO\({}_{2}\) ensures continued function of TCA cycle even if anions are removed from the cycle for biosynthesis. Accordingly, during dark CO\({}_{2}\) fixation, an additional inorganic carbon is essential for amino acid synthesis during canopy development so, additional carbon can be sourced either through the scavenging of respired CO\({}_{2}\) or through the uptake of CO\({}_{2}\) from the surrounding atmosphere. It is reasonable to conclude that during summer, _R_s decouples from photosynthetic carbon input and it reduces due to a combined effect of photosynthetic availability and water deficit. Even during dormant winter, understory played a crucial role in determining the strength of the _R_s-NPP relation.