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Climate change increases the frequency of forest fires throughout the entire boreal zone. This paper examines the long-term wildfire dynamics in pine forests of Central Siberia, relationships between environmental and climatic variables on the one hand and the occurrence frequency of fires and size of burnt forest areas on the other, and the postfire dynamics of vegetation cover productivity. A coupled analysis of ground survey data, remote sensing data (spectroradiometric and gravimetric information collected by the Terra/MODIS and GRACE satellites), and dendroecological data is performed. In the period from the 18th to the 20th century, fire return intervals decreased from 33 to 20–25 years. No statistically significant trends in fire occurrence frequency were identified in the current century; however, catastrophic (i.e., affecting more than 1 million ha) fires were observed in its second decade, and both the number of fires and size of burnt areas have significantly increased (by 3.5 and 3.0 times, respectively). The frequency of fires and size of burnt areas closely correlate with wetting and temperature conditions in the prefire period. Furthermore, fire statistics parameters correlate with wetting conditions (precipitation amount, moisture content in the ground cover and soil, and the Self-Calibrated Palmer Drought Severity Index (sc-PDSI)) stronger than with air temperature. It is shown that equivalent water thickness values obtained using gravimetric methods can be used in fire risk assessments. High correlation levels were identified between the growth index of pine trees and vegetation cover productivity indices (i.e., gross primary productivity (GPP) and net primary productivity (NPP)) generated based on remote sensing data. The results indicate that these indices can be used to estimate forest stand productivity dynamics. The vegetation cover productivity and the radial growth index of pine trees in burnt areas quickly (within a decade) restore to prefire values, which indicates that northern pine forests retain their carbon sequestration function despite climate change and the increasing frequency of fires. [ABSTRACT FROM AUTHOR]

The radial increment of Abies sibirica Ledeb. and mortality dynamics of fir stands in the mountain forests of the Eastern Sayan (Stolby State Nature Reserve) have been analyzed. The unprecedented decline in fir stands is caused by water stress due to an increase in air temperature and synergy with the impact of the Polygraphus proximus Blandford. This xylophage was not previously observed in the Abies sibirica range. In the initial phase of climate warming, an increase in radial increment was observed, which was replaced by a depression in 1985–2017. The declining of fir trees was preceded by an increase in the relationship between growth index and SPEI, as well as between the growth index and root-zone moisture content. At the same time, the growth index of the declining cohort was more closely associated with the root zone moisture content (r2 = 0.56) and SPEI (r2 = 0.74) than the growth index of the surviving cohort (r2 = 0.15 and r2 = 0.39, respectively). The decline in fir began in the 2000s, when Polygraphus proximus Blandford attacked trees Abies sibirica. During this period, dead stands were localized mainly on the relief elements with the highest probability of water stress (steep slopes located in a "rain shadow"). By 2017, the decline spread throughout the entire territory of fir stands, which led to the mortality of ~75% of fir stands. During the period of tree decline, a close relationship was observed between growth index and fir mortality (r = –0.79). The surviving trees grew under conditions of a higher root-zone humidity (compared to declining trees). The proximity of growth-index trajectories of cohorts of declining and surviving trees, which indicates the probability of mortality of surviving trees in the context of a predicted climate aridity increase, is noteworthy. [ABSTRACT FROM AUTHOR]