Seasonal dynamics and impact factors of atmospheric CO 2 concentration over subtropical forest canopies: observation from eddy covariance tower and OCO-2 satellite in Northwest Himalaya, India.

Carbon dioxide (CO ₂ ) is the key atmospheric gas that controls the earth's greenhouse effect, and forests play a major role in abating the atmospheric CO ₂ by storing carbon as biomass. Therefore, it is vital to understand the role of different forests in regulating the spatiotemporal dynamics of atmospheric CO ₂ concentration. In this study, we have used eddy covariance (EC) tower-based atmospheric CO ₂ concentration measurements and satellite-retrieved column average CO ₂ concentration of 2018 to understand the diurnal and seasonal dynamics of atmospheric CO ₂ concentration over the sub-tropical forest in the foothills of northwest Himalaya, Uttarakhand, India. EC study revealed that the CO ₂ concentration over the forest canopy peaks during mid-night to early morning and drop to a minimum during the afternoon. On a monthly scale, peak atmospheric CO ₂ concentration was observed during July in both the sites, which was a result of more release of CO ₂ by the forest ecosystem through ecosystem respiration and microbial decomposition. Enhanced photosynthetic activities during the late monsoon and post-monsoon resulted in the decrease of atmospheric CO ₂ concentration over the forest ecosystem. Among the meteorological variables, rainfall was found to have the highest control over the seasonal variability of the atmospheric CO ₂ concentration. Orbiting Carbon Observatory-2 (OCO-2) satellite-retrieved column average CO ₂ (XCO ₂ ) was also examined to comprehend its reliability on an ecosystem scale. The OCO-2 retrieved XCO ₂ value was higher than the EC carbon flux tower-measured atmospheric CO ₂ concentration, which might be due to differences in the vertical resolution of the CO ₂ column and scale difference. However, the monthly atmospheric XCO ₂ retrieved from OCO-2 strongly adheres with the ground-measured monthly pattern. Our study highlights that forests with varying functional traits within the same climatic conditions show variability in the regulation of atmospheric CO ₂ concentration.