LAUSR

The water availability in dry steppes during various active growing seasons is analyzed for the first time. Climatic fluctuations are examined, and their specific features are determined. Contrary to the popular opinion that the maximum precipitation in the steppe zone occurs during the summer period and that this is a distinctive feature of the steppe zone that is applicable to all its subzones, analysis of dry steppes in the Lower Volga Region over a period of 64 years shows that the maximum precipitation is attributed to summer only in 39% of the studied years, to spring in 27% of the years, and to fall in the remaining 34% of the years. Analysis of the water availability time series produced for various seasons covering more than a semicentennial period shows a general trend: the summer precipitation is decreasing, while the precipitation in spring and fall is increasing. The application of the exponential smoothing method to the time series and regression analysis indicate a growth dynamics of water availability in the spring period after the watershed years of 1989 and 1990 (the data validity is confirmed by the Student’s t-test; the probability value is 0.0005) amid a decrease in the fall precipitation (the probability value is 0.01). These trends characterize the demutation processes ongoing in the secondary fallow phytocoenoses, which are manifested by the predomination of loose- and firm-bunch grasses: water availability in late spring (May) and early summer (June) plays a critical role in the generative process of these plants. In total, 28 species belonging to the Poaceae family have been identified in the studied phytocoenoses; their share in the total species richness is 24.5%. The water availability patterns indicate aridization of the climate in dry steppes. The presence of ephemeral and ephemeroid plants in the communities additionally confirms this: the spring precipitation is their key life support factor. The ephemeral and ephemeroid species constitute 8% of the total species richness.