Drivers of belowground properties in fir-beech karst forests a decade after silvicultural treatment

Abstract Silvicultural treatments, such as thinning and clearcutting, significantly alter belowground forest processes, yet post-treatment forest ecology remains poorly understood, particularly within temperate mixed and broadleaved forests. Ten years after silvicultural treatments, we investigated fine root biomass (FRB), soil properties, microbial parameters (microbial biomass, mycelium production, potential decomposition) and vegetation cover (canopy, shrub, herb), in Dinaric fir-beech forests on high karst plateaus of Slovenia. We compared these parameters in 27 plots including control, thinning (50%), and clearcut sites. Tree FRB was significantly reduced in clearcuts (− 80.5%) but not in thinned plots, where high local variability was found. Despite reduced tree FRB, total FRB (trees, shrubs and herbs) remained comparable across treatments, indicating compensatory growth of shrubs and herbs. Clearcutting shifted fine root community composition toward beech dominance and increased cover and productivity of herb layer. Arbuscular fungal biomass and decomposition potential were significantly higher in clearcuts, while mycelium production correlated positively with tree FRB and negatively with nutrient-rich conditions. Our findings show that thinning was related to higher biomass of tree fine roots and associated microorganisms, while clearcutting corresponded to dominance of shrubs and herbs, increased decomposition rates and showed moderate positive association with increased soil nutrient levels. A suite of factors associated with the absence or reduction of tree cover may be driving altered nutrient cycling under herb dominated vegetation. Results also suggest a potential role of bacteria in the decomposition processes, particularly in sites with elevated atmospheric N inputs. These results highlight the importance of selecting appropriate intensity of silvicultural treatment and incorporating belowground processes in forest management of Dinaric fir-beech forests.