Response of forest vegetation and foliar δ13C and δ15N to soil compaction and forest floor removal in a boreal aspen forest

The effects of severe soil compaction and whole tree harvesting plus forest floor removal (referred to below as forest floor removal) on understory cover and species composition, and trembling aspen (Populus tremuloides Michx.) and white spruce (Picea glauca [Moench] Voss) growth and foliar δ13C and δ15N were investigated in a boreal aspen forest near Dawson Creek, BC, Canada. Soil compaction and forest floor removal shifted the understory community composition and understory total cover was highest in the compacted plus forest floor removed treatment, due to increased moss and shrub cover. Four years after the treatments were applied, soil compaction reduced aspen average height by 20% regardless of forest floor removal, while forest floor removal reduced aspen average height by 60% and unit leaf area by 18% regardless of soil compaction. Soil compaction reduced white spruce unit leaf area, unit leaf weight, and height increment by 29, 33, and 58%, respectively, when the forest floor was intact. In the non-compacted soil, forest floor removal reduced white spruce unit leaf area, unit leaf weight, and height increment by 29, 32, and 35%, respectively. Soil compaction or forest floor removal did not affect aspen or white spruce foliar δ13C, which can be affected by water availability and reflects plant water use efficiency. Aspen foliar δ15N was reduced by soil compaction when the forest floor was intact. Forest floor removal reduced aspen foliar N concentration but increased foliar δ15N. Our results indicate that soil compaction and forest floor removal changed understory community structure but did not affect water availability for trees in the study year, while soil N dynamics or N acquisition by aspen was affected by soil compaction and forest floor removal.