Photosynthesis dynamics of different ecosystems in the south of Western Siberia according to TROPOMI data for the period 2018-2024

Photosynthesis is a fundamental biological process on Earth that uses energy from the Sun to synthesize organic compounds from inorganic substances with the release of oxygen as a by-product. Currently, one of the most promising approaches to monitoring photosynthetic activity of ecosystems on regional and global scales is to detect the fluorescence emission of chlorophyll induced by sunlight (SIF) by aerospace.

Plants assimilate only part of the incoming solar radiation, called photosynthetically active radiation (400-700 nm). Absorbed by chlorophyll, it is consumed by several processes: photosynthesis (80-90%), dissipation as heat (5-10%), and fluorescence (1-2%). SIF is generated within a few nanoseconds of light absorption in the wavelength range from 600 to 800 nm with peaks at 680 and 740 nm. Weak signal registration by satellite instruments is possible because the peaks of spectral SIF are located near the Fraunhofer lines (687 and 760 nm). However, in practice, the 680 nm band is not used in remote sensing due to attenuation of radiation as a result of its re-absorption by chlorophyll at the exit and high background reflected sunlight, while operational SIF data (740 nm) are applied to assess the intensity of photosynthetic activity of ecosystems in dynamics.

This paper considers SIF variability for the most widespread ecosystems in the south of Western Siberia, a region characterized by high natural heterogeneity and pronounced seasonality. The results of the estimation of photosynthetic activity dynamics are presented both for this territory as a whole and for its administrative-territorial units.

The SIF data used in this study are derived from TROPOMI sensor observations (Sentinel-5P satellite) over the south of Western Siberia, obtained for the period 2018-2024 from May to September. Taken from the VEGA-Science service developed at the IKI RAS, the vegetation type distribution map is reduced to a spatial resolution of 0.05° to obtain statistically supported monthly SIF data. Daily values were aggregated in grid nodes with a radius of 4.5 km to obtain monthly SIF data because TROPOMI scans the Earth surface with a spatial resolution of 3.5×5.5 km (3.5×7 km until August 2019) with an orbital periodicity of 16 days. Based on these values, the seasonal variation of this parameter was plotted for different ecosystems of the target region. Deciduous forests and grasslands have the highest and similar photosynthetic activity, while wetlands have the lowest. Photosynthesis intensity of most ecosystems in the south of Western Siberia reaches its peak in June (more rarely in July) and its minimum in September. Significant deviations of photosynthetic activity from the mean are of particular interest. In particular, in 2020, SIF values were the lowest for the entire observation period, presumably due to a cold spring and a precipitation deficit at the beginning of the growing season. The analysis of data for individual administrative-territorial units revealed several notable findings, such as higher SIF values for croplands compared to deciduous forests in the Kemerovo region, a decline in the photosynthetic activity of all major ecosystems in the Tomsk region over the study period, and others.

The work was carried out as part of the state assignment of the IAO SB RAS.