Effects of skid trails on understory vegetation in forests: A case study from Northern Bavaria (Germany)

Abstract

While it is well-known that skid trails in forests locally increase soil compaction and illumination, the effect on vegetation and its spatial and temporal variation have hardly been studied in temperate forests. We surveyed Norway Spruce (Picea abies), Scots Pine (Pinus sylvestris) and European Beech (Fagus sylvatica) dominated forests to reflect the range of the prevalent forests types and ecological gradients of Northern Bavaria, Germany. In each forest type, we chose skid trails with different times since last use (0–2 years ago, 3–6 years ago, ≥7 years ago) to address the temporal aspect and sampled 4-m2 plots on the trail itself, adjacent to the trail and in the forest interior to address the spatial aspect. We analysed the response of soil compaction, canopy cover, vascular plant and bryophyte species richness, vegetation cover and height, both in total and differentiated by layer, and mean ecological indicator values to the use of skid trails. Skid trails exhibited significantly higher soil compaction together with a more diverse, more light and moisture demanding vegetation compared to the forest interior. On the trails, graminoids were favoured over woody perennials. Vegetation adjacent to the skid trail resembled the forest interior, yet soil compaction exceeded that of the forest interior. Irrespective of time since last use, the soils on skid trails remained more compacted and likewise ecological response of understory vegetation did not change with time. Canopy cover and species richness responded strongly site-specific, and effects lasted the longest in the surveyed Scots Pine dominated forests but were diminished in European Beech dominated forests. Our results indicate a shift to a more ruderal and wetland-like vegetation on skid trails, which, however, hardly extended into the adjacent forest stand. Overall, plot-scale richness was higher on skid trails than in adjacent the adjacent forest, suggesting that skid trails could contribute to greater habitat diversity in managed forests. Since soils remained compacted even for several decades without use, a designated skid trail system is essential. There is a lack of long-term studies on the understory response to the use of skid trails and further research focussed on the persistence in time is needed.