Extreme Climatic Events in the Altai-Sayan Region as an Indicator of Powerful Volcanic Eruptions

Analytic data on anomalies of the tree-ring structure of Siberian larch on the transect, passing through the Russian part of the Altai-Sayan Mountain Region from the west to the east along the top forest limit, are given for the first time. Four extreme events (1783–1785, 1788–1789, 1812–1814, and 1884) have been determined by a microscopic analysis of the anatomic structure of tree rings (identification of frost, light-colored, and missing rings and fluctuations in wood density). These were periods of strong cooling in almost the entire area of the Altai-Sayan Mountain Region. The data correspond to archived historical materials. A comparison of dates of extreme cold periods with data on volcanic eruptions, the emissions of which reached the stratosphere (the Volcanic Explosivity Index VEI ≥ 4), has shown that they coincide with periods of the awakening of volcanoes such as Laki (1783, VEI = 4), Asama (1783, VEI = 4), Etna (1787, VEI = 4), Soufriere (1812, VEI = 4), Awu (1812, VEI = 4), Suvanosedzima (1813, VEI = 4), and Krakatau (1883, VEI = 6). Nevertheless, the trees in the studied area did not respond to the large eruptions of the Tambora (1815, VEI = 7), Novarupta (1912, VEI = 6), and Pinatubo (1991, VEI = 6) volcanoes. This difference in the reaction of forest vegetation to strong volcanic eruptions of the 19th–20th centuries may be explained by changes in the direction and speed of atmospheric streams, the mosaic pattern of stratospheric aerosols in the north of Central Asia after the eruptions of some volcanoes, and the warmer climate in the 20th century (which reduced the sensitivity of trees at the top forest line in the Altai-Sayan Mountain Region to volcanic eruptions due to the upward shift of the temperature limit of forest development).