The Influence of Amorpha fruticosa L. on Natural Recovery of Compacted Vertic Gleysol on Forwarder Trails after a Six-year Period

Sažetak

The study of the natural recovery of the compacted Vertic Gleysol after the period of six years was conducted in the narrow-leaved ash forest (subcompartment 137a, management unit Turopoljski lug, Croatia) by comparing the natural bulk density and porosity of the soil and the cone index, with measurements on and off forwarder trails. The last felling was conducted in the late summer (drier soil) in 2016 using a harvester–forwarder fully mechanised timber harvesting system. Parallel forwarder trails 3 m wide and 20 to 30 m apart were established, which subsequently led to a felling density of 43 m³/ha (~ 12.5 m³/100 m³) and a lower intensity of forwarder traffic per trail, all followed by an invasive false indigo bush (Amorpha fruticosa L.) growth. Samples of the surface soil layer in an undisturbed state (Kopecky cylinders), for laboratory analysis of soil physical properties, were taken on trails (8) and off forwarder trails during 2022. In addition to changes in soil physical properties, the determination of forest soil recovery was also investigated by measuring the cone index (penetration resistance) with a penetrometer, during January and September 2022 under wetter and drier conditions of the soil. The impact of false indigo bush on forest soil recovery was investigated on ten square sample plots with dimensions of 3 m × 3m, on which the following measurements were recorded: the number of plants, fresh mass of the aboveground part of the plants, root collar diameter and plant height. A one-way analysis of variance of bulk density and soil porosity for forty soil samples off trails (0.726 g/cm³, 68.4%) and on forwarder trails (0.699 g/cm³, 69.1%) indicated no statistically significant differences. Nevertheless, lower mean values of soil bulk density and higher mean values of soil porosity on trails, as well as the ranges of interval estimates of their values in relation to measurements outside the trails, indicate the natural recovery of compacted soil on forwarder trails over a period of six years. Similarly, a single analysis of variance of the average soil cone index at a depth of 5 to 25 cm, with 60 measurements taken at intervals across the winter and summer periods, indicated statistically significant differences, confirming the natural recovery of compacted soil on forwarder trails.

In addition to the passage of time, the obtained results of natural soil recovery were also influenced by the overgrowth of forwarder trails with false indigo, with an average of 833 plants per 100 m of trail and an average fresh mass of the above-ground part of false indigo plants of 208 kg per 100 m of trail, which, through the action of its branched root system, contributed to the improvement of the compacted Gleysol on forwarder trails.