The variability of humus form in different forest types and different years after skidding operations

The type of litter produced from trees as an influencing factor in ecological processes and the formation of soil horizons plays a key role in the function of the forest ecosystem. In this study, the form of humus was investigated in different forest types on skid trails in different time intervals after skidding operation (6, 10 and 20 years) in Kheyrud forest of Nowshahr County, Iran. Each skid trail was identified with three replications in pure beech (Fagus orientalis Lipsky), beech-hornbeam (Carpinus betulus L.) and mixed beech stands. In total, 81 profiles with dimensions of 30×30 cm2 were drilled in them to be classified using the European Humus Group proposal (EHGP). The mull of the most dominant humus was under the mixed beech litter, while in the pure beech litter the amphi humus was the most abundant system of humus. Three types of humus mull, moder and amphi were observed in the skid trail under beech-hornbeam litter. Oligomull was the most abundant form of humus mull in the skid trail under mixed beech litter, while in the trails under pure beech Eumacroamphi, Eumesoamphi and Hemimoder were the most common forms of humus. The highest thickness of the organic horizons in the 20-year skid trail was under pure beech litter (10.1 cm), while the highest thickness of the organic-mineral horizon was under mixed beech litter (3.8 cm). After 20 years, the humus form in the mixed beech forest stand has become closer to its original form compared to the undisturbed area and has provided better conditions for improving the soil condition.