Planting of alder trees at the edge of skid trails helps to stabilize forest topsoil structure against damage caused by heavy forestry machines

Abstract

The regeneration and soil-mechanical reinforcement effect by the rooting system of grey alder (Alnus incana) were tested in an annually driven forest skid trail on a silty clay loam soil located in Southern Germany. One side of the skid trail was bordered by an eight-year-old alder plantation, on the other side, there were no trees. Soil physical properties (relative apparent gas diffusion coefficient (Ds/D0), bulk density and the coarse porosity) were measured in order to detect the effects on soil structure as well as on fine root densities, taken as a biological target variable.Values of soil physical parameters linked to the soil aeration status did not show significant differences between planted and unplanted wheel track; however, low values of bulk density measured on the laterally planted wheel track indicated significant improvements in the topsoil compared to the unplanted one. Also, values for fine root density were higher in these plots. Strong relationships between Ds/D0, bulk density and the coarse porosity with fine root densities were also found, which give evidence of their mutual effects, either amplifying each other or reciprocal. In the top soil, root density positively affects soil gas permeability, while in the deeper horizons fewer macropores are found, which are partially blocked by fine roots for gas exchange.In summary, the wheel track which is laterally planted with alder trees showed signs of an initial regeneration of soil structure as well as a reinforcement against further soil damage caused by annual wheeling.