Dendrochronological Reconstruction of Waterlogging of Birch Forests in Steppe Watersheds in Southern Omsk Oblast

The relevance of this work is determined by the increased forest waterlogging in forest–steppe watersheds of Western Siberia since the 1990s, which has reached the greatest pace in the southern part of Omsk oblast. The goal of this study is a dendrochronological reconstruction of the timing and causes of waterlogging in insular steppe birch forests, using the Odessa district of the oblast as an example. This study uses dendroclimatic analysis methods based on the chronologies of the standardized radial increment and its year-by-year variability as an indicator of ecological and physiological stress in trees. For the period 1950–2024, contour diagrams are used to analyze the sliding correlation (response function) and year-by-year association of stress with the monthly precipitation and air temperatures for the current and previous years. Based on a combination of responses to precipitation and temperature, the influence of the dendroclimatic patterns of the steppe (typical), tundra–steppe, taiga, and nemoral forests is differentiated. Extreme birch stress events were recorded in 1952, 2007, 2014, and 2020–2024. In the 21st century, these events coincided with extremely high groundwater levels, as well as warm, snowy winters, creating conditions conducive to anaerobic root rotting. With subsequent spring flooding and drought, this led to the partial or complete death of weakened trees. Based on the trends of rising groundwater levels and regional climate warming with episodic intensification of the dendroclimatic pattern of nemoral forests, three periods were identified based on the responses of birch trees: (1) period of responses typical for the steppe zone before 1983, (2) transitional period, and (3) period of extreme atypical responses since 2007.