Impact of Experimentally Induced Rainfall Reduction on Soil Properties and Tree Growth in a Norway Spruce Stand

The aim of this study was to evaluate the effect of experimentally induced drought on the physical and chemical properties of soils under a mature Norway spruce monoculture on a nutrient‑rich site at lower elevations. Since 2010, a 41‑year‑old stand has been monitored on two plots: (i) one with the soil surface covered by PE foil, eliminating 60% of rainfall (treated plot), and (ii) a control plot with an unaltered rainfall regime. On both plots, soil samples were regularly collected for chemical analyses, climatic and soil data were recorded, and biometric parameters of the aboveground parts of spruce trees were measured. The impact of drought differed between the organic O_H horizon and the upper mineral A horizon. In the O_H horizon, lower concentrations of available magnesium (Mg) and calcium (Ca) were found under the treated plot compared to the control plot (Mg: 150.53 ± 6.81 vs. 171.46 ± 12.48 mg kg⁻¹; difference −20.93; 95% CI: −40.00 to −1.85; p = 0.03; Ca: 571.94 ± 49.47 vs. 752.92 ± 75.04 mg kg⁻¹; difference −180.98; 95% CI: −301.80 to −60.10; p < 0.001). In the A horizon, lower concen-trations of aluminium and higher concentrations of magnesium were observed under the treated plot compared to the control plot (Al: 84.76 ± 3.24 vs. 94.08 ± 1.77 mmol kg⁻¹; difference −9.32; 95% CI: −17.51 to −1.13; p = 0.02; Mg: 96.18 ± 6.44 vs. 77.99 ± 5.14 mg kg⁻¹; difference +18.19; 95% CI: 7.26 to 29.12; p < 0.001). The reduction in rainfall did not affect overall height or diameter growth of spruce trees and did not cause a significant decrease in nutrient concentrations in spruce needles. Drought induced minor, but statistically significant, changes in the soil environment, indicating a lower intensity of leaching from the soil sorption complex.