Influence of skid trails and haul roads on understory plant richness and composition in managed forest landscapes in Upper Michigan, USA

We evaluated impacts of disturbance in interior haul roads and skid trails on understory vegetation by documenting the areal extent of these features and plant composition along 10m x 100m belt transects. Ten belt transects were sampled in each of three comparable northern hardwood forests under even-aged management. These forests were approximately 80 years old and were last entered for thinning 4–6 years prior to sampling. Soil compaction, canopy cover, and the richness and composition of trees, shrubs, and herbs were quantified within each feature (i.e. haul roads, skid trails, and forest without soil disturbance) encountered along transects. These variables were also quantified in unmanaged northern hardwood stands of comparable age. Differences in photosynthetically active radiation (PAR) and soil moisture between haul roads and adjacent patches of forest were measured along line transects placed across haul roads, perpendicular to their main axis. On average, haul roads and skid trails comprised 1 and 16% of total managed stand area, respectively. Compaction, PAR, and soil moisture were highest in haul roads. Understory plant richness was significantly greater in haul roads than in skid trails and forest, and resulted from significantly greater percentages of introduced species (13%) and wetland species that were native to the area, but not normally abundant in northern hardwood stands (23%). Skid trails had a greater percentage of wetland species (9%) than in forest, but differences in richness between skid trails and forest were not statistically significant. Up to the present time, the impact of haul roads on understory vegetation has received far less attention than impacts on soil properties and water quality. Although haul roads comprise a relatively small proportion of total stand area, they serve as primary conduits for the dispersal of introduced species into the interior of managed stands and contribute to significant shifts in plant richness and composition at the stand level.