Resilience of Scots Pine Trees Against Droughts on Different Reproductive and Life Stages

The article presents data on the study of the drought resilience limits of Scots pine trees (Pinus sylvestris L.) in the south of the East European Plain in connection with changes in the forest-steppe climate and an increase in the number and intensity of droughts. It was shown that the rate of the global warming in the 20th century was 0.11°C/10 years, while in the 21st century it became 0.35°C/10 years. The objective of the study was to determine the drought resilience limits of the Scots pine trees on different life cycle stages in the forest-steppe zone of the East European Plain. The object of the study was the northern Kaluga and Ryazan, central Voronezh and southern Belgorod pine populations growing in the ecologically favourable territory of the forest-steppe zone of the Russian Plain. As a result of studying the pine response to 6 severe droughts (early 1991 and 2007, summer 2010 and 2020, late 2014 and 2020) and an 8-year heat wave (2007—2014), the threshold of resistance of the stages was determined, which, according to the descending vector, represent the following series: ontogenesis, sporogenesis, embryogenesis, gametogenesis. The cause for the poor harvest in 2012 was the cumulative effect of two droughts on the megasporogenesis processes (2010 — in the year of the megasporogenesis initiation, 2012 — in the year of fertilisation). The impact of the 1991 drought on early embryogenesis led to a 5-fold decrease in pine yield and overall lower seed viability. The weather stress of 2010 during late embryogenesis affected such vital indicators of the future plants as growth rate, phenotypic variability, and seasonal development rhythms. It caused mass death of 1-year-old seedlings and the appearance of many morphoses and developmental deviations in 2-year-old plants. Severe droughts in 2007, 2010, 2012, 2014 during the heat wave of 2007—2014 caused destabilization and transition of pine forests to a state of semi-nonequilibrium systems. The return of pine stands to a state of equilibrium continued for three optimal years.