Persistence of soil compaction and effects on seedling growth in northwestern Quebec

Compaction is perceived as one of the leading causes of soil degradation resulting from forest operations. The objectives of this study were, for two contrasting soil textural groups (i) to assess persistence of compaction; (ii) to compare growth patterns of seedlings submitted to different levels of soil physical disturbance induced by harvesting, and (iii) to assess the relative importance of soil physical properties on these patterns. Six years following harvesting, penetration resistance showed significant relationships with traffic patterns while relationships between traffic patterns, soil macroporosity and bulk density were less significant. On fine-textured soils, five years after plantation, white spruce [Picea glauca (Moench) Voss] height was 28% higher in the between track (BT) microsites and 25% higher in the wheel track areas (WT) of skid trails than in the undisturbed areas (UD). Nonetheless, mortality rate was higher in the WT area. Competition severity explained over 40% of white spruce early height and radial growth while soil physical characteristics had little effect. On coarse-textured soils, competition severity in skid trails (WT and BT) decreased with the number of skidding cycles, resulting in significant increases in black spruce [Picea mariana (Mill.) B.S.P.] and jack pine (Pinus banksiana Lamb.) growth. Growth was negatively affected by high macroporosity values or positively so by high microporosity values, indicating that water retention may be limiting on these soils. On the coarse-textured soils, compaction was beneficial in the early establishment period. Further investigation is required to verify that these results also apply over the long term.