A long road to recovery: Soil structure and earthworms show partial recovery in skid trails of a clayey temperate forest soil after 18 years.

Compaction may affect soil functions for decades. To minimize soil compaction, ground-based timber harvesting in Central European hardwood forests was traditionally practiced in the dormant season on frozen soils. However, due to rising winter temperatures, timber is now often harvested on wet, non-frozen soil, increasing the susceptibility to compaction. In a controlled experiment on clayey soils (60 % clay, 38 % silt) in the Vienna woods, we assessed the effects of ground-based timber harvesting on soil functions and their recovery. We compared skid trails established and trafficked 1 versus 18 years ago at the same site. Earthworm data was collected in the tracks and at adjacent undisturbed plots (paired sampling design) using mustard extraction and hand-sorting. To evaluate soil structure with X-ray imaging, we sampled undisturbed soil cores at 5 and 15 cm depths in a similar design as the earthworms. We identified five earthworm species: Aporrectodea rosea, Dendrobaena platyura, Dendrodrilus rubidus, Lumbricus rubellus, and Octolasion lacteum. Earthworm abundance was highest on trails trafficked 18 years ago, indicating earthworm population recovery, particularly among endogeic and juvenile anecic individuals, while adult anecic earthworm abundance did not fully recover. X-ray data showed that image-resolved porosity was significantly reduced directly after trafficking (from 14.4 ± 5.0 % at untrafficked positions (U) to 3.5 ± 1.6 % at 5 cm depth in the tracks (T) and from 13.5 ± 4.9 % to 2.0 ± 1.1 % at 15 cm depth) and recovered at 5 cm within 18 years (12.0 ± 3.4 % (U) to 12.2 ± 4.3 % (T)), but only partially at 15 cm (14.2 ± 2.6 % (U) versus 7.1 ± 2.5 % (T)). Other imaging-based parameters, such as bio-pores, pore anisotropy, connectivity measures (Euler number and gamma), and percolating porosity, reflected similar patterns. Bulk density increased significantly directly after timber harvesting (from 1.0 ± 0.1 g/cm³ (U) to 1.3 ± 0.1 g/cm³ (T) at 5 cm and from 1.1 ± 0.1 g/cm³ (U) to 1.4 ± 0.1 g/cm³ (T) at 15 cm) and showed partial recovery after 18 years (1.0 ± 0.1 g/cm³ (U) versus 1.1 ± 0.2 g/cm³ (T) at 5 cm and from 1.0 ± 0.1 g/cm³ (U) to 1.3 ± 0.1 g/cm³ (T) at 15 cm). However, the anisotropy of stones did not recover. Pressure and shearing forces obviously arranged the platy stones in the trails horizontally, which might be irreversible. Our data demonstrate that earthworms can recolonize heavily compacted forest soils. The recovery of soil structure follows a depth gradient and is only partial at 15 cm depth 18 years after trafficking.