Interannual Variability of Ecosystem Exchange of CO2 and H2O in the Middle Taiga Cowberry–Lichen Pine Forest in the European Northeast of Russia

Extreme weather events change the diurnal and seasonal variations in CO2 fluxes between the forest and the above-surface atmosphere. An analysis of interannual variability of net CO2 exchange (NEE), total ecosystem respiration (TER), gross ecosystem photosynthesis (GEP), and evapotranspiration (ET) in a cowberry–lichen pine forest of the middle taiga subzone is performed under different weather conditions during the 2021–2023 growing season. According to measurements by eddy covariance system, in the pine forest over three years of observations, the total NEE in May–August varied from –653 to –980 g CO2/m2, GEP varied from 1711 to 2360 g CO2/m2, and TER from 913 to 1380 g CO2/m2. The total evapotranspiration corresponds to 137–262 mm, and the GEP/ET ratio varies within 1.8–3.9 mgC/g H2O. As a result of multilevel statistical analysis, a correlation of GEP with gross radiation, temperature, and humidity in the growing season is revealed. A relationship between GEP and air temperature and humidity characteristics of the growing season is revealed. GEP and NEE decrease at abnormally high temperatures and a long absence of precipitation in the first half of the 2021 growing season. The maximal effect of air temperature on ecosystem CO2 exchange is observed in 2023 upon an increased supply of soil moisture. It is concluded that the water reserve in the soil mitigates the negative effect of atmospheric moisture deficit in the summer on photosynthesis and evapotranspiration and thereby ensures a stable seasonal net sink of CO2 in the ecosystem of the cowberry–lichen pine forest of the middle taiga subzone.