Estimating rutting and soil displacement in skid trails by soil sampling and 3D Structure for Motion (SfM) photogrammetry modelling: first trial in Vallombrosa forest (Italy)

Mechanization and skid trails network play a key role in the fast extraction of timber from areas affected by windthrown, especially for timber quality and worker health and safety. Nevertheless, heavy vehicle trafficking may have a strong impact on soil, due to soil compaction and rutting. Different methods have been applied for determining the effect of forest logging on soil. The most common are based on sample collection and/or field measurements. Recently, other methods such as terrestrial laser scanner (TLS) have been applied to construct 3D model of terrain (Digital Terrain Model) to assessing rutting and compaction of soil. However, TLS techniques require relatively expensive technologies and specialized users. With the evolution of Structure from Motion (SfM) photogrammetric technology, that reduces constraints by allowing the use of consumer grade digital cameras and highly automated, data processing acquisition of 3-D data has become easy, fast, automated and low-cost.The aim of this study was to test the use of SfM to assess soil compaction and rutting during logging operations in a windthrown area. The study area was a silver fir forest situated in Vallombrosa forest (central Italy) damaged by a windstorm in March 2015. For timber extraction both a forwarder (John Deere JD1110 D) and a skidder (John Deere 548H) were applied. In the logging area trails trafficked by both forwarder and skidder were selected for the study. On these trails two different methods were applied for determining the impact on soil: (i) soil sampling for determining soil physical parameters (bulk density, shear and penetration resistance) and (ii) temporal analysis of high resolution Digital Terrain Model (DTM) generated by SfM photogrammetric technology. Machine trafficking on the selected trail was carried out for 10 days and the data for assessing the impact on soil were acquired before, during (after five days from the beginning) and 5 days after the end of timber extraction. The two different approaches were compared for determining the difference in the assessment of the impacts on soil of the two vehicles used in logging operation. The two approaches are comparable and highlighted a significant difference between the trails trafficked by forwarder and skidder. Our findings showed that SfM may be easily used to produce 3D terrain model, thus allowing to assess soil compaction and rutting in a faster and cheaper way.