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Abstract

Tree species diversity can positively affect the multifunctionality of forests. This is why
conifer monocultures of Scots pine and Norway spruce, widely promoted in Central Europe since the
18th and 19th century, are currently converted into mixed stands with naturally dominant European
beech. Biodiversity is expected to benefit from these mixtures compared to pure conifer stands due
to increased abiotic and biotic resource heterogeneity. Evidence for this assumption is, however,
largely lacking. Here, we investigated the diversity of vascular plants, bryophytes and lichens at
the plot (alpha diversity) and at the landscape (gamma diversity) level in pure and mixed stands
of European beech and conifer species (Scots pine, Norway spruce, Douglas fir) in four regions in
Germany. We aimed to identify compositions of pure and mixed stands in a hypothetical forest
landscape that can optimize gamma diversity of vascular plants, bryophytes and lichens within
regions. Results show that gamma diversity of the investigated groups is highest when a landscape
comprises different pure stands rather than tree species mixtures at the stand scale. Species mainly
associated with conifers rely on light regimes that are only provided in pure conifer forests, whereas
mixtures of beech and conifers are more similar to beech stands. Combining pure beech and pure
conifer stands at the landscape scale can increase landscape level biodiversity and conserve species
assemblages of both stand types, while landscapes solely composed of stand scale tree species
mixtures could lead to a biodiversity reduction of a combination of investigated groups of 7 up to 20%.