Seasonal dynamics and impact factors of atmospheric CO2 concentration over subtropical forest canopies: observation from eddy covariance tower and OCO-2 satellite in Northwest Himalaya, India.

Abstract

Carbon dioxide (CO2) is the key atmospheric gas that controls the earth's greenhouse effect, and forests play a major role in abating the atmospheric CO2 by storing carbon as biomass. Therefore, it is vital to understand the role of different forests in regulating the spatiotemporal dynamics of atmospheric CO2 concentration. In this study, we have used eddy covariance (EC) tower-based atmospheric CO2 concentration measurements and satellite-retrieved column average CO2 concentration of 2018 to understand the diurnal and seasonal dynamics of atmospheric CO2 concentration over the sub-tropical forest in the foothills of northwest Himalaya, Uttarakhand, India. EC study revealed that the CO2 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 CO2 concentration was observed during July in both the sites, which was a result of more release of CO2 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 CO2 concentration over the forest ecosystem. Among the meteorological variables, rainfall was found to have the highest control over the seasonal variability of the atmospheric CO2 concentration. Orbiting Carbon Observatory-2 (OCO-2) satellite-retrieved column average CO2 (XCO2) was also examined to comprehend its reliability on an ecosystem scale. The OCO-2 retrieved XCO2 value was higher than the EC carbon flux tower-measured atmospheric CO2 concentration, which might be due to differences in the vertical resolution of the CO2 column and scale difference. However, the monthly atmospheric XCO2 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 CO2 concentration.