Soil provides the foundation for plant and tree growth in a forested ecosystem. The use of forest-derived biomass has steadily increased in the Nordic countries during the past decades leading to more intensive forest management practices in the region, such as whole-tree harvesting, both above- and belowground. Stump harvesting causes severe soil disturbance, such as mixing and relocation of the organic material and the mineral soil within the soil profile potentially leading to decreases in site productivity. The objectives of this study were (i) to evaluate the spatial and temporal extent of soil surface disturbance caused by stump-root system harvesting, (ii) to quantify how much biomass and nitrogen is removed from the stand in stump and coarse root removal, and (iii) to study the effects of stump harvesting on soil carbon and nitrogen stocks and processes. The extent of surface disturbance was assessed in three clear-cut Norway spruce (Picea abies, (L.) Karst.) stands in southern and central Finland, differing in time since harvest. All the experimental stands had been clear-cut and planted with Norway spruce seedlings following site preparation in the form of mounding. To determine the biomass distribution of the stump-root system, stumps and coarse roots were excavated at one of the experimental stands. Soil sampling was performed 8–13 years after harvest and each disturbance class (undisturbed surface, mound, pit) was sampled separately.
Results/Conclusions
Across all age classes (time since harvest) less soil surface had remained undisturbed at the stump harvesting sites than at the sites where only mechanical site preparation had been carried out. Thus, the findings of this study indicate that soil disturbance caused by stump harvesting can exist on forest soil surface for more than a decade following harvest. The total biomass of the stump-root system in the stand was estimated to 39.3 Mg ha-1and 79 % of this biomass was removed during stump harvesting. The stump-root system accounted for 17 % of the whole-tree biomass, and coarse roots and fine coarse roots represented a significant portion of it (73 %). Thus, the stump-root system represents a large biomass component in boreal forest stands. There were no clear effects of stump harvesting on soil C and N dynamics 11–12 years after harvesting. Stump harvesting increased soil mixing, down-mixing of organic layers and up-mixing of mineral soil from deeper layers, as indicated by significant effects on C concentrations and near-significant effects on soil pH and C/N-ratios.