Can mixed stands of native and non-native tree species enhance diversity of epigaeic arthropods in plantation forests?

In regions with low cover of natural forests and high cover of plantations predominately comprised of non-native species, inclusion of a native tree species with a more productive non-native species has the potential to enhance biodiversity and meet production goals. In this context, we tested the alternative hypotheses that: i) equitable mixes of a non-native and a native tree species support greater diversity of ground-dwelling arthropods than single species stands; or, ii) native ash stands support greater diversity of ground-dwelling arthropods than mixed or single species stands that include a non-native conifer species. Active epigaeic spiders (Araneae) and beetles (Coleoptera: Carabidae, Staphylinidae) were sampled using pitfall traps in three forest types in Ireland: single species stands of non-native Norway spruce (Picea abies) or native ash (Fraxinus excelsior), and mixed stands of these species. Stands of Norway spruce did not negatively influence spider and staphylinid diversity, suggesting that they maintain a similar range of biodiversity to mixed plantations or stands of native ash. However, carabid beetle richness (but not abundance) was negatively affected by the presence of spruce suggesting caution when drawing conclusions about biodiversity impacts from single taxon studies. We found that equitable mixes of spruce and ash supported many species associated with native ash stands. Thus, we recommend that mixes with an equitable species ratio (e.g. 50:50) and containing a native species will enhance epigaeic arthropod diversity and heterogeneity in plantations. Furthermore, our finding that ash stands supported greater beta diversity than spruce stands supports current guidelines that recommend a range of stand types, including native species, to enhance diversity within and between stands.