Soil disturbance and early vegetation response to varying intensity of energy wood harvest

Forest biomass plays an important role in the production of renewable energy, particularly in northern Europe. During recent decades, tree stumps have also been increasingly used for energy production. The environmental impacts of stump removal on the functioning of forest ecosystems, for example on soil processes and biodiversity, are still inadequately understood. To investigate the separate impacts of tree stump and logging residue removal on soil and plant variables, we conducted a large-scale replicated experiment in clear-felled stands in three geographical areas. Four energy wood harvesting levels were combined with patch mounding (organic and mineral soil layer turned upside down) as a site preparation method. The harvesting treatments were: (1) no energy wood harvesting, (2) 70% logging residue removal, (3) 25 stumps per hectare left at site and 70% logging residue removal (maximum level in official recommendations), (4) complete stump and 100% logging residue removal. Soil disturbance in terms of mineral and organic soil cover increased with the increasing harvesting level, but complete stump removal did not increase the mineral soil cover more than partial stump removal (on average 95% removed). Despite the increase in soil disturbance, stump removal did not have an overall impact on plant species richness or vegetation composition in the second growing season after harvesting. However, an increasing trend in Betula spp. biomass with increasing harvesting intensity was seen in two of three study areas in four years. Our results underscored the high site-dependency of the effects of stump removal on ecological variables, thus the initial stand structure and vegetation type need to be considered when planning harvesting treatments.