Patterns and causes of variation in understory plant diversity and composition in mature boreal mixedwood forest stands of western Canada

Boreal mixedwood forest stands are comprised of a mixture of small canopy patches of varying dominance by conifer (mostly white spruce (Picea glauca (Moench) Voss)) and broadleaf (mostly trembling aspen ( Populus tremuloides Michx.) trees. The purpose of this work was to extend our understanding of the patterns and causes of variation in understory vascular plant communities in unmanaged, mature boreal mixedwood forests. First, I assessed variation in understory community composition in relation to canopy patch type (conifer, mixed conifer-broadleaf, broadleaf, gaps) within mixedwood stands. The mosaic of canopy patches leads to different micro-habitat conditions for understory species, allowing for communities that include both early and late successional species and contributing to greater understory diversity. This study suggests that the mosaic of small canopy patches within mixed forest stands resembles a microcosm of the boreal mixedwood landscape, across which understory community composition varies with canopy composition at the stand scale. Second, I investigated the hierarchical organization of understory diversity in relation to the heterogeneous mosaic of canopy patch types through additive partitioning of diversity. The largest proportion of species richness was due to turnover among patches within patch type while individual patches had higher evenness. The mosaic of canopy patch types within mixedwood forests likely plays a crucial role in maintaining the hierarchical levels at which understory diversity is maximized. Third, I examined interactions among understory plant species by investigating the effect of shrub removal on biomass, composition and diversity of herbs using a 3-yr removal study in a natural understory community. There is asymmetric competition for light between erect shrub and herb species but herb response to erect shrub removal was species-specific. Plant interactions play an important role in structuring boreal understory communities. Finally, I explored the relative influence of space, environmental variables, and their joint effects, on understory composition and richness. The environmental variation caused by small canopy patches and biotic processes, such as species interactions, converge at the fine scale to create a spatially patchy structure in understory communities in boreal mixedwood forests. Modifications in the natural mixture of small canopy patches could disrupt the spatial and environmental structures that shape understory composition and diversity patterns.