Landscape-Scale Mixtures of Tree Species are More Effective than Stand-Scale Mixtures for Biodiversity of Vascular Plants, Bryophytes and Lichens

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%.

Can multi‐taxa diversity in European beech forest landscapes be increased by combining different management systems?

“…This fact may also result in lower biodiversity differences among forest management systems and combinations of management systems respectively. However, both for eutrophic and acidic beech forests similar trajectories in understorey development after different scales of natural disturbances (see Kompa & Schmidt, 2003 and in response to tree species composition (Heinrichs et al, 2019) have been observed.…”

Stand growth and structure of mixed-species and monospecific stands of Scots pine (Pinus sylvestris L.) and oak (Q. robur L., Quercus petraea (Matt.) Liebl.) analysed along a productivity gradient through Europe

“…It was shown that species associated with Scots pine decrease in mixture with European beech, since the latter strongly determines microclimate (Heinrichs et al 2019). As a result, for example the composition of herbaceous plant species in mixtures of beech and conifers was more similar to pure beech stands than to conifer stands (Heinrichs et al 2019).…”

“…It was shown that species associated with Scots pine decrease in mixture with European beech, since the latter strongly determines microclimate (Heinrichs et al 2019). As a result, for example the composition of herbaceous plant species in mixtures of beech and conifers was more similar to pure beech stands than to conifer stands (Heinrichs et al 2019). We assume that similar negative effects on species richness by mixing tree species are unlikely to occur in a similar way in Scots pine-oak mixtures since the light regimes in monocultures of the two species do not differ as much as they do between oak and beech or Scots pine and beech.…”

“…facilitation (Jactel et al 2018;Liang et al 2016;Pretzsch et al 2017), and may provide a broader supply of ecological and socio-economical services (Biber et al 2015;Felton et al 2016;Gamfeldt et al 2013;Griess and Knoke 2013;Heinrichs et al 2019). Selected tree species mixtures such as Norway spruce (Picea abies (L.) H. Karst)/European beech (Fagus sylvatica L.) or Scots pine (Pinus sylvestris L.)/European beech and other mixtures including European beech are very well analysed (Knoke et al 2008;Pretzsch and Schütze 2009); however, there is still a long way for forest science to establish a solid theory of mixing effects and general rules of species behaviour in mixture or even guidelines for combining and thinning tree species or functional groups of species (Forrester 2014).…”

“…Potential for synergy arises in particular when mixing species with complementary resource use and access aboveground (Ammer 2019;Forrester et al 2018;Pretzsch 2014) or below-ground (Augusto et al 2002;Caldwell et al 1998;Rothe and Binkley 2001); to what extent this potential for synergy can be exploited depends in addition on the respective site conditions. Greater crown canopy packing in mixed stands due to species differences in crown morphology and light ecology has been identified as an important cause of overyielding (i.e.…”