Recovery of forest soil from compaction in skid tracks planted with black alder (Alnus glutinosa (L.) Gaertn.)

•Planting black alders promoted structural recovery of compacted forest soil.•Air permeability recovered faster than coarse porosity and bulk density.•Roots appeared to drive structural recovery by generating air-conducting porosity.•Treatment effects reached down to 0.7 m depth within seven years.

Although soil compaction has become a common problem in forests nowadays, little is still known on the possibilities to promote the structural recovery of compacted forest soils. Here we examined how planting black alder into the ruts of skid lanes with and without application of compost would affect the regeneration of soil structure in a gleyic Cambisol on the Swiss Plateau. Seven years after planting, soil samples taken at three different depths next to the trees were analyzed for bulk density, total and coarse porosity, air permeability, precompression stress and root mass density and compared to samples from unplanted skid lanes and planted but non-trafficked reference plots. All four physical parameters showed strong compaction effects in the skid lanes. Coarse porosity was reduced down to 0.5 m depth, and air permeability even down to 0.7 m depth. In combination with compost application, the planting of black alders resulted in a significant regeneration of structure, although recovery was still far from complete compared to the untrafficked reference soil. Without compost, there was no significant treatment effect compared to the unplanted skid lanes, but a clear trend, as the differences in coarse porosity and precompression stress between the two alder treatments with and without compost were not significant either. Air permeability showed a significant effect of the alder treatment with compost that extended down to 0.7 m depth, but also reached values of the same magnitude as in the non-trafficked reference soil. Air permeability furthermore showed a very similar pattern of treatment effects to rooting density in the skid lane soils. The correlation between these two parameters increased with depth. These results suggest that the roots of the planted alder trees initiated structure recovery by generating air-conducting porosity and thereby promoted other biotic structure-formation processes. Our study highlights the fact that tree root growth may significantly contribute to soil structure recovery after compaction.