Impact of mechanized harvesting on compaction of sandy and clayey forest soils: results of a meta-analysis

Nowadays, harvest operations are predominantly performed fully mechanized using heavy tractors or forestry machines. The resulting soil compaction may negatively affect the soil ecosystem. We wanted to draw general conclusions concerning the impact of mechanized harvesting on forest soil bulk density and the influencing factors. Therefore, we combined the data of several studies using a meta-analysis approach. The impact decreased from the surface towards deeper soil layers. At 0-10 cm depth, the impact on clayey soils was highest although not significantly different from the impact on sandy soils. Higher initial bulk densities, i.e., on already compacted forest soils, generally led to smaller extra increases of bulk density after machine traffic. For sandy soils, the impact was also significantly smaller when machines were lighter. No significant relationship was observed between the compaction degree and traffic intensity. We observed clear compaction on both clayey and sandy soils, especially in case of low initial soil compaction degrees and heavy machines. The compacted initial state of many forest soils, the long recovery period, and the generally high impact of the first passes that is frequently mentioned in literature all count in favour of designated skid trails and an adjustment of the machine type to the job.