European beech controls biodiversity in mixed forests – Mixed versus pure forests of beech and conifers

Monocultures of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) have been promoted in Central Europe since the early 19th century due to economical, ecological and management constraints on sites naturally dominated by broadleaved species. Catastrophic damages and the call for multifunctional forest ecosystems with a high degree of naturalness initiated a large-scale conversion of these monocultures into mixed forests often with the naturally dominant European beech (Fagus sylvatica). Beside an increased stability, mixed forests are assumed to facilitate forest biodiversity conservation due to an increased habitat heterogeneity compared to the respective monocultures. Scientific evidence for this assumption is, however, scarce [1]. Here we investigated data on 15 organismic groups from plants to bacteria sampled in two regions of Germany to reveal the effect of mixed versus pure forests of beech and conifers on biodiversity. Although species composition of mixed stands was intermediate between beech and conifer stands for most organismic groups, they never exceeded gamma-diversity of either pure stand. To identify a landscape composition of pure and mixed stands that maximizes biodiversity, we generated virtual forest landscapes by resampling plots of pure and mixed stands in a way that all compositional combinations were represented in steps of 10 % with 1000 replications. Landscapes were created containing beech and pine stands and its mixtures for 15 taxonomic groups and containing beech and spruce stands and its mixtures for three groups. Most groups as well as multidiversity benefited from mixing different pure stands at the landscape scale and not from local within-stand mixtures. Except deadwood fungi that prefer a high percentage of pure beech stands, most species groups need a share of at least 30 % of conifer stands for a maximum biodiversity. For a landscape with beech, pine and its mixtures, 97 % of multidiversity was achieved with a combination of 70 % pure pine and 30 % pure beech stands. Within-stand mixtures supported only 77 % of multidiversity as the competitive beech determined resource availability. Thus, the habitat heterogeneity provided by different pure stands at the landscape scale is more effective for biodiversity than the heterogeneity by mixing species at the stand scale. Our findings indicate that the ongoing large-scale conversion of pure conifer into mixed stands with beech will lead to biodiversity loss. An optimized forest landscape for regional biodiversity contains pure beech and pure conifer stands that show complementarity in species composition. We can, however, not yet say if pure forest stands should be larger than the here investigated 1 ha to be effective. [1] Cavard X et al. (2011) Importance of mixedwoods for biodiversity conservation: Evidence for understory plants, songbirds, soil fauna, and ectomycorrhizae in northern forests. Environ Rev 19: 142-161