Metabarcoding reveals that mixed forests mitigate negative effects of non‐native trees on canopy arthropod diversity

Abstract

Averting climate change‐induced forest diebacks increasingly relies on tree species planted outside of their natural range and on the addition of non‐native tree species to mixed‐species forests. However, the consequences of such changes for associated biodiversity remain poorly understood, especially for the forest canopy as a largely understudied forest stratum. Here, we used flight interception traps and a metabarcoding approach to study the taxonomic and functional (trophic guilds) composition and taxon richness of canopy arthropods. We sampled 15 monospecific and mixed stands of native European beech, native Norway spruce—planted outside its natural range—and non‐native Douglas fir in North‐West Germany. We found that diversity of arthropods was lower in non‐native Douglas fir compared to native beech stands. Taxon richness of herbivores was reduced by both conifer species. Other functional guilds, however, were not affected by stand type. Arthropod composition differed strongly between native broadleaved beech and monospecific coniferous (native spruce or non‐native Douglas fir) stands, with less pronounced differences between the native and non‐native conifers. Beech‐conifer mixtures consistently hosted intermediate arthropod diversity and community composition compared to the respective monospecific stands. Moreover, arthropod diversity had a positive relationship with the number of canopy microhabitats. Our study shows that considering arthropod taxa of multiple functional groups reveals the multifaceted impact of non‐native tree species on forest canopy arthropod communities. Contrasting to previous studies that primarily focused on the forest floor, we found that native beech hosts a rich diversity of arthropods, compared to lower diversity and distinct communities in economically attractive, and especially in non‐native, conifers with few canopy microhabitats. Broadleaf‐conifer mixtures did not perform better than native beech stands, but mitigated negative effects of conifers—making such mixtures a compromise to foster both forest‐associated diversity and economic yield.This article is protected by copyright. All rights reserved.

Beneficial effects of native broadleaved forests on canopy beetle diversity are not reduced by admixture of non‐native conifers

“…Moreover, monospecific Douglas fir stands reduced landscape‐scale beetle diversity, yet not more severely than Norway spruce. These results partly corroborate past studies reporting negative effects of non‐native conifers on forest biodiversity (Schuldt et al., 2022; Wildermuth, Seifert, et al., 2023). However, they also highlight that native conifers planted outside their natural range can have similarly detrimental effects (Staab et al., 2023; Wildermuth, Dönges, et al., 2023).…”

“…The latter, however, remains disputed, with multiple studies reporting rather averaging than promoting impacts of mixed forests on the local diversity and abundance of different animal groups (Oxbrough et al., 2016; Schuldt et al., 2022; Wang et al., 2019). Yet, tree species bearing low biodiversity in monoculture consistently performed better in mixtures (Wang et al., 2019; Wildermuth, Seifert, et al., 2023). Notably, previous research was often hampered by focusing on local forest stands, while studies expanding their scope to regional scales detected higher landscape‐scale diversity in mixed than in monospecific forests, resulting from a higher species turnover between local communities (Leidinger et al., 2021; Wildermuth, Dönges, et al., 2023).…”

“…While drought‐tolerant and fast‐growing non‐native tree species are economically a promising solution for climate change‐adapted forest management (Fuchs et al., 2022), they can negatively affect native biodiversity and ecosystem functioning when planted in monocultures (Staab et al., 2023; Wang et al., 2019). When admixed with native tree species, such negative effects may be reduced, making mixtures a possible compromise between biodiversity conservation and economic interests (Thomas et al., 2022; Wildermuth, Seifert, et al., 2023). Still, such compromises might pose a threat to rare native forest biota, as their habitat requirements might be too specialized to adapt to novel forest types (Sánchez‐Bayo & Wyckhuys, 2019).…”

“…Past research showed that responses of forest arthropods to admixing broadleaved forest with (non‐native) conifers depend strongly on the trophic guild and the investigated forest stratum (Glatthorn et al., 2023; Pedley et al., 2016; Wildermuth, Seifert, et al., 2023). On average, arthropods in the canopy are more dependent on tree species identity than ground‐dwelling species because they are in direct contact with the specific crown architecture, resources and defence properties (Pedley et al., 2016; Wildermuth, Dönges, et al., 2023).…”

“…On average, arthropods in the canopy are more dependent on tree species identity than ground‐dwelling species because they are in direct contact with the specific crown architecture, resources and defence properties (Pedley et al., 2016; Wildermuth, Dönges, et al., 2023). Herbivores in the canopy, for instance, were severely reduced in diversity and abundance by non‐native trees, while showing intermediate diversity in mixed forest (Berthelot et al., 2023; Wildermuth, Seifert, et al., 2023). Omnivores and predators, however, were less affected by non‐native tree presence—particularly on the forest floor (Glatthorn et al., 2023; Matevski & Schuldt, 2023; Oxbrough et al., 2016).…”