Fine-root biomass in relation to soil properties in regenerating Norway spruce stands following large-scale bark beetle outbreaks in mountain and lowland forests

Fine-root biomass may be a sensitive indicator of functional regeneration in forest ecosystems subjected to large-scale disturbances. The study evaluated fine-root biomass (diameter < 2 mm) in Norway spruce stands regenerating after a large-scale bark beetle ( Ips typographus ) outbreaks in the lowland (Białowieska Forests - BF) and the mountainous (Beskid Silesia - BS) sites. The study included two scenarios of stand regeneration at various age classes, i.e., (i) by natural self-seeding and succession, and (ii) regular forest management and silviculture by planting seedlings. Control plots were established at undisturbed sites on both lowland and mountain forests BF-C and BS-C). Fine-root biomass was examined in relation to selected physicochemical soil properties, including pH, organic carbon, nitrogen, macronutrient content, and moisture. Significant differences were found between mountain and lowland soils, expressed as lower pH, higher moisture, and higher macronutrient content (N, P, K, Mg) in the mountain site (BS). Fine-root biomass was significantly higher in lowland (BF) than in mountainous sites (BS), whereas no significant differences were found between the post-outbreak and control sites. A negative correlation between fine-root biomass and stand age was observed at the mountain forest (BS), while in the lowland forest (BF), these relationships were not significant. Analysis of the relationships between fine-root biomass and environmental factors indicated that soil moisture was a consistent driver of fine root development across both lowland and mountain habitats, whereas the effects of soil pH, nitrogen, and phosphorus content varied between locations.