Resilience of Scotch Pine Trees to Drought at Different Reproductive and Life Stages

This article presents data on the study of the drought resilience limits of Scotch 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 is shown that the rate of global warming in the 20th century was 0.11°C/10 years, while in the 21st century it has become 0.35°C/10 years. The goal of this work is to determine the drought resilience limits of Scotch pine trees on different life cycle stages in the forest-steppe zone of the East European Plain. The object of study was the northern (Kaluga and Ryazan), central (Voronezh), and southern (Belgorod) pine populations growing in the ecologically favored territory of the forest-steppe zone of the Russian Plain. As a result of studying the pine response to six severe droughts (early 1991 and 2007, summer 2010 and 2020, and late 2014 and 2020) and an 8-year heat wave (2007–2014), the threshold of resistance of the stages is determined, which, according to the descending vector, represent the following series: ontogenesis, sporogenesis, embryogenesis, and 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 and 2012 in the year of fertilization). The impact of the 1991 drought on early embryogenesis led to a fivefold decrease in pine yield and overall lower seed viability. The weather stress of 2010 during late embryogenesis affected vital indicators of the future plants such as growth rate, phenotypic variability, and seasonal development rhythms. It caused the mass death of 1-year-old seedlings and the appearance of many morphoses and developmental deviations in 2-year-old plants. The severe droughts of 2007, 2010, 2012, and 2014 during the 2007–2014 heat wave triggered destabilization and a transition of Scots pine forests into a weakly non-equilibrated system. The return of Scots pine to equilibrium lasted for three optimal years.