Scots Pine (Pinus sylvestris L.) on the Southern Border of Its Range in Siberia: Growth Dynamics under Changing Climate Conditions

Under climate warming conditions, changes in the growth, vital state, and habitats of woody plants are predicted mainly on the border of their ranges. We have studied the effect of the changing hydrothermal regime on the state of pine forests growing on the southern border of the range of Pinus sylvestris L. (Tuva Depression). During the “warming restart” in the 21st century, the values of the pine growth index (GI) exceeded the previously observed GI maxima as a result of a relative improvement in humidification conditions in the last decade. At the same time, the frequency and area of fires has increased during this period (the fire return interval has decreased to 5 ± 1 years in fragments and 3 ± 2 years in the whole area). The abundance of viable undergrowth in the burnt areas (on average, up to 10 thousand/ha) is potentially sufficient for successful post-fire regeneration of pine forests. However, repeated grass-root fires periodically destroy the undergrowth. The increase in air temperature combined with satisfactory humidification currently contributes to the growth of pine trees. Improvement of humidification is predicted under all climate scenarios (SSP4.5, SSP7.0, and SSP8.5) in the studied pine habitats. However, the current fire frequency (which leads to the destruction of undergrowth in areas covered by fire) threatens the preservation of relict stands of pine at the southern limit of its range. Under warming conditions, the preservation of pine in the south of its range is determined by a combination of two factors, the dynamics of burning and humidification conditions.