Impacts of timber forwarding on physical properties of forest soils in southern Finland

•Traffic by heavy machinery has significant impacts on soil properties.•First passes cause the strongest impacts.•Impacts depend highly on soil characteristics and actual site conditions.

Forest harvesting activities can cause soil damage and disturbance through soil compaction, rut formation and soil mixing. These affect the soil structure and functions and forest productivity. Soil compaction results for instance in increased bulk density and decreased porosity, affecting soil moisture, water infiltration and aeration. The effects of timber forwarding on soil physical properties have gained little attention in boreal forests. These issues will become more important in the future since harvesting operations on unfrozen soils are getting more common due to the anticipated climate warming.In this study, changes of forest soil physical properties (bulk density, moisture content and porosity) after 1-10 forwarder passes on two fine-grained mineral soil sites in southern Finland were analysed. Penetration resistance and rut formation were also measured. The measurements were performed in three periods with different soil moisture conditions. The test drives were carried out with a conventional 8-wheeled forwarder with total mass of 29.8 tons.Soil bulk density increased and porosity decreased after the machinery passes. However, soil moisture content increased on one site and mainly decreased on another. The first three passes caused the greatest compaction and rutting, the first pass having the strongest impact. After the first and third pass 34-55% and over 70% of the total mean rut depth was formed, respectively. Further passes caused only minor rutting. The compaction and changes of soil physical properties appeared to be greater in dry conditions. Rut formation and soil mixing were greater in moist conditions. The results are, however, site-specific, and more research is needed to achieve a better understanding of the relationships between different factors affecting impacts of timber forwarding on soil.