Can mixed species stands enhance arthropod diversity in plantation forests?

Tree species composition is a key driver of forest biodiversity, influencing structural components of the environment from soil and litter to vegetation layers and the canopy, and ecosystem processes, such as nutrient cycling. Single species stands, particularly intensively managed monoculture plantations, are typically more homogenous in habitat structure and the biotic communities supported, than mixed stands. Thus, international forest policy increasingly promotes the establishment of mixed stands as an alternative to enhance biodiversity in plantations. Forests represent around 10% of the land area of Ireland, with most being monocultures of non-native conifers. By contrast, natural forest cover, primarily comprised of deciduous species, is just 1%. In recent years there has been an increase in mixed plantations; however, optimum tree species combinations, which aim to promote biodiversity under sustainable forest management, have yet to be established. Arthropods (ground-dwelling spiders and Carabid beetles, and night-flying macrolepidoptera) were examined in twenty mixed and monoculture plantations in Ireland (Norway spruce–oak mix, Norway spruce–Scots pine mix, Norway spruce monoculture). Both oak and Scots pine were secondary mix components, comprising between 15% and 40% of stems. Spiders and Carabid beetles were sampled using pitfall traps during summer 2008 and moths using light traps during summer and autumn 2008 and spring 2009. There was no evidence for an influence of oak or Scots pine on the arthropod fauna when they were a secondary component in a mix. Overall, arthropod communities were similar in species richness, assemblage structure and habitat specialists among the forest types. Furthermore, the mixed stands exhibited similar environmental conditions to monocultures in terms of stand structure, vegetation and litter cover. This suggests that there is limited biodiversity value from an additional canopy species comprising 15–40% of the mix, at least for the taxa and tree species studied here. This has implications for forest policy, where recommendations are often based on the proportion of each mix component, at least in terms of the potential biodiversity value of additional canopy species. Further research is required to determine the proportion at which oak or Scots pine begin to influence the arthropod fauna.