Climate Effect on Carbon, Phytomass, and Litter Stocks in Forest Stands in the South of European Russia

The manifestation of modern global climate change is increasingly intensifying, affecting anthropogenic and natural systems. Bioclimatic models predict significant shifts in biome boundaries, including a reduction in the portion of forests in the forest-steppe areas. This process may also affect carbon stocks. The aim of this work is to assess the impact of climate and its changes on the values and variations of carbon stocks in the phytomass of forest stands by data of repeated surveys of permanent test plots. Eight forests have been selected in the southern regions of European Russia (ER), and test plots are laid in them. Repeated surveys were performed on the test plots in 2010-2011, 2014-2015, and 2019-2020. The carbon reserves of living and dead components of forest stands were calculated according to taxation characteristics. The litter carbon stocks were determined by the gravimetric method. Meteorological data from the meteorological stations closest to the study objects were analyzed. The mean annual temperature for 1991-2020 increased as compared to 1961-1990 by 1.13 degrees C, which was 2.5 times greater than the global mean. Annual precipitation decreased from 448.2 to 445.4 mm. The mean value of G.T. Selyaninov's hydrothermal coefficient (HTC) for May-September was 0.85 during the period of 1961-1990 and 0.79 in 1991-2020. The stock of the phytomass carbon in the studied forest stands varied from 38.5 +/- 7.4 t C ha-1 to 270.6 +/- 52.8 t C ha-1. Repeated surveys revealed both increases and decreases in phytomass carbon stocks, which ranged from -23.8 to 31.9 t C ha-1 over a 5-year period. The analysis of phytomass carbon stocks and climatic characteristics revealed a statistically significant correlation with the HTC for May-September. However, the comparison of changes in phytomass with variations in the mean annual temperature, annual precipitation, and the HTC for May-September did not reveal significant dependencies. The absence of significant correlations between changes in phytomass carbon and in climatic parameters may be explained by the stability of forest ecosystems, which ensures the preservation of their functions during several-year-long periods.