Evaluating small‐scale harvesting disturbance to the forest soil in Mediterranean beech high forests

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AbstractEuropean beech (Fagus sylvatica L.) forests in Central and Southern Italy are managed applying the shelterwood system. Prior to the regeneration cut, it is common to apply 2–3 thinning interventions, aiming to obtain mostly firewood, considering the low dimension and poor quality of the stems. These interventions are usually carried out by local forest enterprises relying on a low or medium level of mechanization (small‐scale forestry). In particular, the short wood system is applied, thus processing the logs to 1 m length and extracting them with forestry‐fitted farm tractors equipped with forwarding bins. Despite the large application of this harvesting system in the Mediterranean forestry, no information is available in the literature about its possible disturbance to the forest soil. To fulfill this knowledge gap, we developed the first assessment of soil physicochemical (bulk density, penetration resistance, shear resistance, organic matter content) and biological (soil microarthropods biodiversity evaluated with the QBS‐ar index, that is, an index based on the idea that high‐quality soils have more groups of microarthropods that are morphologically better adapted to the soil than low‐quality soils) properties for this kind of logging operation. In three case study areas in Central Italy, we applied an experimental design to evaluate separately the impacts related to the passage of the machine and that of the silvicultural treatment itself. We further applied linear mixed‐effect models to investigate the relationship between changes in soil physicochemical and biological properties. We found the effect of the silvicultural treatment to be negligible, but there was a significant alteration of the investigated parameters in the soil affected by the passage of the machine. Soil penetration and shear resistance doubled in the forwarding trails (0.25 MPa and 4.02 t m−2, respectively) in comparison with the other two experimental treatments (control area and soil not affected by the machine passage; about 0.12 MPa and 2.10 t m−2, respectively). Soil organic matter and soil microarthropod biodiversity (QBS‐ar index) were reduced by 25% in the forwarding trails (about 30% and 92 respectively) in comparison with the soil not affected by the machine passage (about 39% and about 130, respectively). Such significant disturbance, which occurred even if the applied machinery had lower weight in comparison to other alternatives (forwarders), is related to the high number of machine passes needed to extract the woody material with forwarding bins applying the short wood system. We found a significant relationship between soil compaction and soil organic matter removal and microarthropod biodiversity, with the latter which resulted significantly lower in more compacted and organic matter‐poor soils. We demonstrated that this small‐scale extraction system can have a negative effect on soil features in the short term. We therefore recommend the application of best management practices, for instance placing brush mats and logging residues, on the forwarding trails to limit the soil disturbance within the framework of small‐scale forestry. We further recommend a wider application of aerial harvesting systems, which generally have lower impacts to the soil. However, this could happen only after an extensive training program aimed to increase the knowledge and skills of local loggers about aerial harvesting systems.