Relationships between Overstory and Understory Components of Young Natural and Reconstructed Boreal Aspen Stands

A clonal tree species, Populus tremuloides (trembling aspen) typically regenerates from root suckers following forest fire but is also capable of reproducing via small, wind-dispersed seeds. On land reclaimed following oil sands mining in northeastern Alberta, aspen forests are naturally regenerating from seed in contrast to nearby fire-origin stands that are regenerating, predominantly, from root suckers. This study compares relationships between developing canopies and understory vegetation communities for aspen stands originating from forest fire and from land reclamation activities utilizing forest floor—mineral mix (FFMM) and peat—mineral mix (PMM) cover soils. Across all stand types, understory biomass was negatively related to canopy stem density, indicating competitive interactions are beginning to structure the vegetation six years following disturbance. However, several understory components exhibited differential responses to developing forest canopies in natural and reclaimed sites. Positive correlations between overstory and understory components in fire-origin stands were driven by native flora while negative correlations on PMM reclamation soils were driven by non-native species. On FFMM reclamation soils both positive correlations with native species and negative correlations with non-native species were seen. The negative relationships point to non-native forbs inhibiting the establishment or growth of aspen seedlings on reclamation sites. In contrast, native species were associated with greater canopy densities in fire-origin stands where an open seed bed was not required for establishment. Although precise mechanisms are difficult to determine, it is clear that after six years of growth, different processes are influencing plant community development in reclaimed versus fire-origin boreal aspen stands.