Assessment of Natural Regeneration Rates in Logged Dried-Up Spruce Forests Using Remote Sensing Data

Due to the widespread dieback of spruce forests, reforestation following sanitary measures has become an urgent priority. When surveying large areas, remote sensing methods are more practical. Interannual linear trends of the Normalized Difference Vegetation Index (NDVI) and spectral brightness coefficients (SBC) in the red band were calculated for regenerating clear-cut areas of dried-up spruce forests affected by the bark beetle (Ips typographus L.) using Landsat satellite imagery for the period 2014-2025 within the coniferous-broadleaf forest zone of Perm Krai. The study included 26 logged areas where clear sanitary cuts were carried out in 2014. Three control sites, subjected to clear cutting of mature and overmature stands, were selected for comparison. Time-series analysis of NDVI using the Mann-Kendall non-parametric test revealed a predominance of statistically significant positive trends in the recovery of deciduous species, both on cleared dried-up spruce plots (77% of sites) and control sites (100% of sites). On half of the logged dried-up spruce areas, post-sanitary cutting regeneration involved a shift to less valuable deciduous species. Forest regeneration dominated by young spruce trees was observed in only 12% of the dried-up spruce clear-cuts, which is corroborated by statistically significant decreasing trends in SBC values in the red band. Analysis of red-band SBC values by forest type indicated more successful spruce regeneration in green-moss forests (mean SBC – 0.2741) and less successful regeneration in lime-dominated forests (mean SBC – 0.4388). Conversely, deciduous species regenerated more successfully in lime forests: the rate of increase in mean NDVI values reached 249.2% on experimental plots and 225.9% on control plots. In green-moss and oxalis forest types, logged dried-up spruce plots regenerated more actively than areas previously occupied by healthy stands: in green-moss forests, the NDVI increase was 200.6% on experimental plots versus 57.9% on controls; in oxalis forests, the increase was 194.8% versus 26.8%, respectively. According to the authors, this pattern can be explained by lower initial NDVI values in the time series for regenerating logged dried-up spruce forests, as well as by more favorable growing conditions associated with increased soil nutrient availability due to the decomposition of fallen spruce needles resulting from stand drying.