## 2 Materials and Methods
### Climate Data  
Air temperature and precipitation were obtained from the Sosnovka weather station (WMO index 36,099, distance to test sites is 20-30 km) using the AISORI online database [30].  
Climate aridity was estimated based on Palmer's scPDSI (Self-Calibrated Palmer Drought Severity Index) and SPEI (Standardized Precipitation Evaporation Index) indexes [31]. The scPDSI is sensitive to soil and air moisture. Negative values of scPDSI indicate drought conditions, and positive values indicate wet conditions [32]. The scPDSI calculation was performed in the R-Studio v.2024.09.0 build 375 ([[https://posit.co/download/rstudio-desktop](https://posit.co/download/rstudio-desktop)]([https://posit.co/download/rstudio-desktop](https://posit.co/download/rstudio-desktop)) (accessed on 20 March 2025)) using the R v.4.1.3 programming language. ([[https://cran.rstudio.com](https://cran.rstudio.com)]([https://cran.rstudio.com](https://cran.rstudio.com)) (accessed on 20 March 2025)) and based on the scPDSI v0.1.3 library ([[https://github.com/Sibada/scPDSI](https://github.com/Sibada/scPDSI)]([https://github.com/Sibada/scPDSI](https://github.com/Sibada/scPDSI)) (accessed on 20 March 2025)). The input parameters were the data on total monthly precipitation and potential evapotranspiration for 1950-2024 extracted from the ERA5-Land database [33; 34]. SPEI estimated an atmospheric drought [35]. SPEI increase, similar to Palmer's, indicated an air drought decrease, and vice versa. The SPEI and potential evapotranspiration (PET) were calculated based on precipitation and evaporation data extracted from the ERA5-Land database and using the SPEI R library ([[https://cran.r-project.org/web/packages/SPEI](https://cran.r-project.org/web/packages/SPEI)]([https://cran.r-project.org/web/packages/SPEI](https://cran.r-project.org/web/packages/SPEI)) (accessed on 20 March 2025)) scripting in the R-Studio.  
An increase in summer air temperature (1985 until ca. 1998) is followed by fluctuations (Figure 2a). In the 21 st century, mean summer temperature surpassed former values by about +1.3\({}^{\circ}\)C. The annual air temperature increased from the 1970s until ca. 2010 (Figure 2a). Warming-driven summer temperature increase (1985) coincided with soil drought increase (indicated by the decrease in the Palmer index (Figure 2c,d)). The precipitation trend is difficult to understand due to the fluctuation, although significant local maximums of precipitation occurred in the 2010-2013 and 2020-2023 years (Figure 2b). Soil and air drought decrease was observed during 2010-2023 years (Figure 2b-d).  
Figure 2: Dynamics of the annual and summer temperatures (**a**) and precipitation (**b**), summer Palmer (scPDSI) (**c**), and SPEI (**d**) drought indexes. The beginning of the summer warming (1985, (**a**)) coincided with soil drought increase (**c**) (indicated by the arrows). Precipitation increases in 2010–2023 (**b**) coincided with soil drought decrease (**c**). The beginning of air drought (**SPEI) decrease (**d**) shifted to the early dates in comparison with soil drought. Note: an increase in the SPEI and scPDSI indicates a decrease in drought, and vice versa.