Expansion of Tree Vegetation in the Forest–Mountain Steppe Ecotone on the Southern Urals in Relation to Changes in Climate and Habitat Moisture

— As found by comparing historical topographic maps and present-day satellite images, steppificated areas in the southern part of the eastern macroslope of the Southern Urals have decreased in size by 17.6% between 1986 and 2015 due to forest expansion. Analysis of the age structure of tree stands in the forest–mountain steppe ecotone provided evidence for intense pine and larch regeneration in this region from 1915 to the 1960s, which has led to significant increase in stand density and upward shift of treelines along slopes and ravines by the end of the 20th century. Intense pine regeneration during the past 35–40 years has been observed at the boundary between closed and open forests and in sparse woodlands. Forest expansion to the mountain steppes throughout the ecotone proceeded against the background of increase in temperature and precipitation during the cold period of the year. The rate of forest expansion to particular areas varied depending on differences in moisture supply, which, in turn, depended on local microclimatic and edaphic conditions. This is confirmed by our measurements of snow depth and density and soil moisture and thickness, which show that forest has expanded at the highest rate in more snowy and moisture-abundant slope areas.