Slash and stump harvest – effects on site C and N, and productivity of the subsequent forest stand

Abstract

The EU renewable energy policies will likely be dependent on all renewable energy sources to meet set climate goals. In forest-rich countries like Sweden, primary residues following harvest operations, such as slash and stumps, could serve as additional biomass sources since secondary residues from the industry are already used. However, harvesting these residues might harm site productivity and stand production due to nutrient loss. This thesis investigates the effects of slash and stump harvest on carbon (C) and nitrogen (N) pools in soil and forest stands across Sweden. Results show no negative impact on site C and N pools or stand production, even with more intense soil disturbance levels. Although deep soil cultivation, not used in practical forestry, had a negative impact on the soil C at one site out of two, it did not affect total C and N pools due to increased tree growth. There was no stump harvest effect in another field experiment designed with a lower soil disturbance more similar to practical forestry, underlining the importance of taking differences between the experimental design and practical operations into account when results are interpreted and used to guide practical forestry. Initial height growth matched stand production trends, seedling survival remained stable, and natural regeneration significantly increased with slash harvest. The findings emphasize the importance of analyzing the total C pool rather than soil and tree C-pools separately when making claims about climate impacts, and advocate for more practical experimental designs in future research. Long-term studies should include real-world factors like site preparation, timing of planting, supplementary planting, and pre-commercial thinning to better reflect forest management practices. This thesis suggests that logging residues like slash and stumps can provide a renewable energy source without depleting future forest C and N pools, and stand production, aiding climate goal.