Fir (Abies sibirica Ledeb.) under the Combined Influence of Changing Climate and Bark Beetle Attack in the Siberian Mountains

Mesophytic conifers Abies sibirica and Pinus sibirica have responded both negatively and positively to the changing hydroclimatic regime. We studied the dependence of A. sibirica growth on the elevational climatic gradient and bark beetle Polygraphus proximus attacks in the Kuznetsk Ala‐Tau Mountains, Siberia. We applied in situ measurements, dendroecology, remote sensing, and GIS analysis. We found that fir growth was moderated by atmospheric (vapor pressure deficit, VPD) and soil (Palmer’s scPDSI index) dryness. Growth dependence on air temperature switched from negative at low elevations (below ca. 1000 m a.s.l.) to positive within the timberline and treeline ecotones (around 1300 m). At low elevations, fir has been experiencing chronic water stress together with heatwaves since the warming onset in the 1970s. Drought‐weakened trees became the food base for the P. proximus outbreaks. Fir responded to bark beetle attacks by xylem lignification and by creating a mechanic ‐ chemical barrier within its tree rings, consisting of rows of ducts filled with polyphenol‐containing resin. Fir mortality was observed mostly at low elevations and was located mostly on sunlit slopes with shallow rocky soils. Meanwhile, within the timberline and treeline ecotones, fir is increasing its growth and migrating uphill at a rate of ca. 2.3 m/year. The insect outbreak range is limited to ~1100 m a.s.l., whereas the insect species range coincides with that of A. sibirica (~1400 m). At high elevations, beetles feed on weakened and felled trees. Nowadays, P. proximus become the major threat to fir forests. Fir regeneration has mostly survived due to water stress mitigation by the overstory. Its vigor and abundance are sufficient for fir recovery within the majority of the damaged area. However, sparse fir stands, shrub and grass communities cover ca. 30% of the devastated stands. The predicted increase in water stress and heatwaves will trigger cycles of bark beetle outbreaks and fir mortality with a consequent shrinkage of the fir range in the lowlands.