Fir (Abies sibirica Ledeb.) in a Changing Climate Hydrothermal Regime and Effects of the Ussuri Polygraph (Polygraphus proximus Blandford)

The changing hydrothermal regime, together with dendrophage outbreaks, is the main factor determining the vitality, growth dynamics, and changes in the ranges of conifers. The causes of mass dieback of Abies sibirica Ledeb. in the taiga of the Eastern Sayan Mountains (Krasnoyarsk Stolby National Park) and fir tree vitality within the area of fir dieback are studied. The mortality of fir is preceded by a chronic decrease in the tree radial growth index (GI) associated with water stress, which has been observed since the start of warming in the 1970s. The unprecedented dieback of the fir forest was triggered by the abnormal drought in 2012, which led to an abrupt increase in water stress and a strong decrease in the GI of trees. The extreme deterioration of the fir forests vitality stimulated the outbreak of the bark beetle Polygraphus proximus Blandford. The tree dieback, the maximum of which was observed in 2013–2018, resulted in the mortality of up to 75% of fir-dominant forests. The reaction of fir to P. proximus attacks included the formation of a physico–chemical barrier in the tree rings, consisting of closed rows of traumatic resin ducts and xylem lignification. Currently, the number of viable fir regeneration (3000–10 000 stems/ha) is potentially sufficient for fir restoration in most of the zone of fir mortality. However, the predicted worsening of hydrothermal regime in fir habitat in synergy with P. proximus outbreaks suggested future cycles of fir mortality and partial restoration in the 21st century. The unprecedented mortality of fir forest is part of the worsening of the health status of conifer mesophytes in the southern parts of their range.