Impacts of forestry operations on soil physical properties, water and temperature dynamics

Sammanfattning: Soil physical disturbances caused by forestry operations can be both intentional, e.g., soil scarification, and unintentional, e.g., soil compaction and rutting as a consequence of off-road traffic. All disturbances may alter the soil, and thus change its water and temperature dynamics, with consequences for both forest production and the surrounding ecosystems. The overall aim of this thesis was to summarize how forestry operations – here: off-road traffic and soil scarification – affect the soil, its water content and tem-perature at clearcuts, as well as what implications this may have for, e.g., tree seed-ling establishment, field vegetation, and the surrounding ecosystems. The sum-mary is based on four studies, three on off-road traffic at two clearcuts in northern Sweden and one on soil scarification (disc trenching) at a clearcut in central Sweden. The methods used were soil sampling, X-ray tomography, image and soil physical analyses, in situ measurements of soil temperature and water content, vegetation survey, evaluation of abiotic growing conditions, and finally, hydrolog-ical modelling. The most notable results were that even stony till soils in recharge areas were compacted by off-road traffic, which especially affected hydraulic conductivity, reducing it by 70%. Changes in soil physical properties caused by traffic may lead to longer periods of high water content, increased risk of surface runoff and insufficient root aeration. Five years after the operation, volumetric water content was higher in wheel tracks, and this was corroborated by the species composition of the vegetation. Simulations demonstrated how the changed soil hydrological properties influenced the water dynamics. Restricted aeration was more frequent in wheel tracks and could explain patches of bare soil in the lower parts of the slopes of the experimental plots. Logging mats and residues prevented creation of deep wheel tracks by off-road traffic, despite the additional passes necessary to apply (and remove) them. However, the logging residues were pressed into the soil, with potential soil compaction and element concentration beneath them as a result. To con-clude, care should be taken when planning off-road traffic, even on sandy till soils with high stone content. After soil scarification, the soil temperature during growing season was higher in the furrows than in the area without disc trenching; the temperature effect lasted for at least six years, but decreased over time. No microsite was wet enough to hamper soil aeration after disc trenching. In conclusion, furrows offered a suitable microclimate for planting at the dry, boreal site.