Partitioning vascular understory diversity in mixedwood boreal forests: The importance of mixed canopies for diversity conservation

Progress in understanding the patterns of plant diversity requires a conceptualization and quantification of the hierarchy of patch configuration that exists in plant communities across levels of observation. In order to identify the scales at which vascular plant diversity is maximized, we investigated the hierarchical organization of understory vascular plant diversity in relation to canopy patch types in mature boreal mixedwood forests in western Canada. In each of two study areas within a 30km² landscape (55° N, 112° E) we sampled four canopy patch types: conifer, mixed conifer-broadleaf, broadleaf and canopy gaps. Understory diversity (richness and Shannon’s (H′) index) was additively partitioned in relation to these four canopy patch types across a hierarchy of four scales; α₋ᵢₙdᵢᵥᵢdᵤₐₗ ₚₐₜcₕ+β₁₋ₐₘₒₙg ₚₐₜcₕₑₛ wᵢₜₕᵢₙ cₐₙₒₚy ₚₐₜcₕ ₜyₚₑ+β₂₋ₐₘₒₙg cₐₙₒₚy ₚₐₜcₕ ₜyₚₑₛ wᵢₜₕᵢₙ ₐᵣₑₐ+β₃₋bₑₜwₑₑₙ ₐᵣₑₐₛ. We also examined understory species abundance patterns by means of rank-abundance plots fitted to relative abundance models. The largest partition of species richness was among patches within canopy patch type (β₁). For Shannon diversity index, the largest partition was at the within-individual patch level (α₁) indicating that evenness was high at the patch level but that dominance increased at higher levels in the hierarchy. The assessment of relative abundance by means of rank abundance plots suggested that the canopy patch types differed in terms of the ecological mechanisms influencing diversity patterns. Considering plant diversity within a hierarchical framework is critical for the understanding and management of biodiversity as maximum levels of plant richness and evenness do not necessarily occur at the same observational scales. Management practices which retain the natural hierarchies of vegetation patches will help conserve plant community richness and diversity.