## 4 Discussion
### GPP and Its Driving Factors under Different Sky Conditions  
Water vapor pressure is an objective factor to represent the dynamic exchange and balance (or vapor pressure deficit, VPD) of water and water vapor among the atmosphere, plants and soil, as well as PAR use by GLPs, i.e., water vapor plays vital roles in not only water circulation but also PAR energy use/transfer (through OH radicals) in atmosphere-plant-land interaction. It should be mentioned that the responses of GPP to the changes in air temperature and relative humidity are extrapolated from and similar to (i.e., positive or negative) those in water vapor, as there is an inherent relation between water vapor and air temperature and relative humidity (T and RH) [97]. Water vapor pressure acts not only as a surrogate for T and RH, but in more significant and objective roles than T and RH in the processes in the atmosphere, vegetation and land. The dominant roles of temperature in GPP or BVOCs can be represented by/reflected in the water vapor factor (such as in the sensitivity test) to some extent. In more detail, water supply and drought play different roles under different sky conditions (e.g., high or low S/Q). For example, precipitation can accelerate GPP in this subtropical _Pinus_ forest [72; 98]. Further investigations about water supply (precipitation, drought, etc.) are still needed.  
At the canopy interface, chloroplasts absorb the incoming PAR to convert CO\({}_{2}\) and H\({}_{2}\)O and decompose water into [H] and O\({}_{2}\). The attenuated PAR by CO\({}_{2}\) (equivalent to GPP and described by GPP term) in the atmosphere was utilized by plants to produce GPP, which is described by the GPP term. The GPP increase/decrease with the decrease/increase in PAR clearly indicates this intrinsic relationship of PAR transfer, use and conversion between the atmosphere and plants (sensitivity analysis). GPP increases with the decrease in S/Q (corresponding to the increase in scattered PAR, i.e., the decrease in S/Q leads to the increase in scattering \(e^{-\text{S/Q}}\)), and GPP increases with the increase in water vapor/water supply. These results were in good agreement with several model studies and observations, e.g., the increased diffuse radiation results in the enhancement of GPP [80; 81; 83; 84; 85; 86; 87; 89; 99; 100], indicating that similar mechanisms exist in the ecosystem in different regions and that the EMGPP empirical model can reasonably describe some mechanisms in GPP. The PAR energy point of view deserves further investigation to study complicated GPP processes and multiple interactions in PAR-atmosphere-vegetation-land. In more detail, there are dynamic exchanges of chemical components (e.g., CO\({}_{2}\), BVOCs, water, particles) in the atmosphere-vegetation (by stoma)-soil. It should be also emphasized that there are multiple interactions in GPP and its influencing factors (PAR, E, S/Q) as well as their related processes (absorption and scattering in the atmosphere), not one-direction interactions acting as assumed in the sensitivity test. It is important and beneficial to develop an energy method to reveal the inherent mechanisms/interactions associated with GPP and capture/quantify its main processes. GPP and all its associated factors/processes change synchronously, and PAR-CO\({}_{2}\)-other GLPs-biosphere-land is suggested to be studied as a whole system.