Spring frost and decay fungi are implicated in suppressing aspen re-growth following partial cleaning in juvenile stands
Aspen (Populus tremuloides Michx.) regenerates at high densities following manual cleaning. Ten-year-old stands located near Lac La Biche and Peace River, Alberta were manually cleaned to three densities (0, 500 or 1 500 stems ha−1 ) at three times (bud set, dormancy or bud flush) to test the hypothesis that maintaining residual aspen reduces regeneration. At Lac La Biche up to 98% of the aspen regeneration died in the partially-cleaned plots compared to 67% at Peace River five years post-treatment. A spring frost in the second growing season at Lac La Biche is hypothesized to be the inciting factor predisposing the stump sprouts to infection by decay fungi such as Armillaria root rot, resulting in reduced density and height of the aspen regeneration at Lac La Biche relative to Peace River. Drought and ungulate herbivory provided additional stresses. The high mortality of aspen regeneration at Lac La Biche shifted the understory regeneration from aspen to balsam poplar (Populus balsamifera L.). These results indicate that maintaining 1 500 stems ha−1 of residual aspen will not effectively control the re-sprouting of aspen; however, the vulnerability of aspen regeneration to spring frost and other stressors can nearly eradicate the re-growth of aspen.