Ground-based skidder traffic changes chemical soil properties in a mountainous Oriental beech (Fagus orientalis Lipsky) forest in Iran

Increased soil compaction following harvest traffic is an inroad for adverse changes in soil biological properties and processes, soil microbial activity, bacterial communities, and growth and development of plant roots. This study investigated the impacts of three levels of traffic intensity (3–6, 7–14, and 15+ passes) of a Timberjack 450C skidder on changes in several chemical soil properties at two levels of skid trail slope gradients (gentle =<20% and steep >20% inclination) in the Hyrcanian forest. Skidding increased soil bulk density between 19% and 39% and, averaged over both slope gradients, reduced amounts of soil organic carbon (33–67%), concentrations of nitrogen (51–80%), phosphorous (0–17%), potassium (11–36%), and hydrogen ions (78–98%) compared to undisturbed areas. Most soil damage occurred after a few skidder passes, particularly on steep slopes that generally experienced the highest level of soil deterioration. The primary mechanism that induced immediate chemical soil changes was an uplift and exposure of deeper soil layers in response to compression and displacement of the former soil surface. Skidding can jeopardize the sustainability of forest ecosystems by creating unfavorable changes in soil characteristics and nutrient status.