Importance of mixedwoods for biodiversity conservation: Evidence for understory plants, songbirds, soil fauna, and ectomycorrhizae in northern forests

Increasing environmental concerns with forestry have led to mounting interest in mixedwood management as a possible strategy to achieve a diversity of ecological and productivity goals within the context of sustainable forest management. This review focuses on the effects of mixedwoods on biodiversity including understory plants, birds, soil fauna, and ectomycorrhizae (ECM). It examines stand-level species diversity and composition but particularly focuses on the regional scale (gamma diversity) by examining evidence for the occurrence of mixedwood-indicator species. The main conclusions are as follows: (i) The existence of different tree species in the canopy is associated with provision of a greater diversity of microhabitats allowing for the addition of understory plant species associated with each canopy species. However, there is little evidence for the existence of understory plant species uniquely associated with mixedwoods. (ii) Some bird species require or prefer the presence of different tree species within a landscape or within a stand. Mixed forest stands and landscapes are thus critical to support populations of these species. (iii) A few studies showed a positive effect of mixed stands or mixed litters on some groups of soil organisms, but high variability makes it difficult to draw firm conclusions about any potential benefits of mixedwoods for biodiversity of soil fauna. (iv) Some ECM taxa are associated with multiple hosts, and could thus benefit from availability of multiple possible host species found in mixed forest stands. Several studies confirmed that there was a greater abundance of multiple-host ECM taxa in mixedwoods.