Although it is much more diverse than the tree layer, understory vegetation remains an unrecognized compartment of the Canadian boreal forest. However, it plays an important role in forest ecosystems, affecting the succession of stands, nutrient cycling and faunal composition. This study aims to evaluate how the composition and diversity of vascular understory vegetation in aspen (Populus tremuloides Michx.) stands are influenced by local and regional variables, such as the fire regime, climatic conditions, availability of resources (light, water and nutrients), and environmental heterogeneity. To this end, vascular understory vegetation was sampled in 33 poplar stands along a transcontinental gradient of precipitation and fire regimes, extending from western to eastern Canada. After analyzing the plant composition and the diversity indices across the country, model selections were performed to identify the best local and regional predictors of species richness, total intra-site beta diversity and species evenness.
Results/Conclusions
Although the plant composition changes gradually along the longitudinal gradient, species richness and total intra-site beta diversity do not depend on the longitudinal and climatic gradient, but rather are under the influence of local variables, such as the site’s physical heterogeneity, the soil’s cation exchange capacity, and the canopy’s leaf area index. The evenness, in turn, decreases gradually from west to east and is almost 25% lower in the stands of the east. Evenness is mainly influenced by regional variables, such as rainfall and the annual total of degree days. Thus, environmental variables are involved in the establishment of vegetation as many filters acting on local assemblies by selecting or eliminating certain species from a regional pool in which historical legacy plays a preponderant role.