Harvesting impacts on soil and understory vegetation: the influence of season of harvest and within-site disturbance patterns on clear-cut aspen stands in Minnesota

We investigated impacts of harvesting on soil disturbance and vegetation in the aspen cover type of northern Minnesota, United States. The soil disturbance (resistance to penetration) and understory vegetation were characterized for 19 sites on five 60-m 2 plots placed along a disturbance gradient: landings (high harvesting traffic), skid trails (inter- mediate harvesting traffic), and areas off skid trails (low to no harvesting traffic). Penetration levels were quite variable, but they still indicated that within-site responses to disturbance patterns created by clear-cut harvesting were not uniform. In general, soil disturbance and understory species composition within landings were similar to those with skid trails. The soil disturbance and vegetation composition of these two levels differed from those of the low-disturbance plots (off skid trails), indicating that removing trees alone did not affect vegetation composition as much as creating an es- tablished skid trail, regardless of harvest timing. However, sites with more variable species composition (win- ter-harvested sites) and lower disturbance levels were less altered than sites with likely lower initial diversity (summer-harvested sites). The results suggest that it is important for recovery of understory plant communities to not only limit the amount and level of disturbances but also consider the spatial layout of harvesting, thus maintaining a spatially connected network of remnant forest patches large enough to contain interior forest species. Resume : Les auteurs etudie l'impact des activites de recolte sur la perturbation du sol et la vegetation dans les forets de peuplier faux-tremble du nord du Minnesota aux Etats-Unis. La perturbation du sol (resistance a la penetration) et la vegetation de sous-bois ont ete caracterisees pour 19 sites dans cinq places-echantillons de 60 m 2 etablies de long d'un