Conifers and non-native tree species shift trophic niches of generalist arthropod predators in Central European beech forests

Abstract

Background Functional diversity is vital for forest ecosystem resilience in times of climate-induced forest diebacks. Admixing drought resistant non-native Douglas fir, as a partial replacement of climate-sensitive Norway spruce, to native beech forests in Europe appears promising for forest management, but possible consequences for associated biota and ecosystem functioning are poorly understood. To better link forest management and functional diversity of associated biota, we investigated the trophic niches (∆13C, ∆15N) of epigeic generalist predators (spiders and ground beetles) in mixed and pure stands of European beech, Norway spruce and non-native Douglas fir in north-west Germany. We assessed the multidimensional niche structure of arthropod predator communities using community-based isotopic metrics. Results Whilst arthropod ∆13C differed most between beech (high ∆13C) and coniferous stands (low ∆13C), ∆15N was lowest in non-native Douglas fir. Tree mixtures mitigated these effects. Further, conifers increased isotopic ranges and isotopic richness, which is linked to higher canopy openness and herb complexity. Isotopic divergence of ground beetles decreased with Douglas fir presence, and isotopic evenness of spiders in Douglas fir stands was lower in loamy sites with higher precipitation than in sandy, drier sites. Conclusions We conclude that tree species and particularly non-native trees alter the trophic niche structure of generalist arthropod predators. Resource use and feeding niche breadth in non-native Douglas fir and native spruce differed significantly from native beech, with more decomposer-fueled and narrower feeding niches in beech stands (∆13C, isotopic ranges and richness). Arthropod predators in non-native Douglas fir, however, had shorter (∆15N) and simplified (isotopic divergence) food chains compared to native forest stands; especially under beneficial abiotic conditions (isotopic evenness). These findings indicate potential adverse effects of Douglas fir on functional diversity of generalist arthropod predators. As tree mixtures mitigated differences between beech and conifers, mixed stands including (non-native) conifers constitute a promising compromise between economic and conservational interests.

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

“…This positive effect of openness in conifers on taxon richness and the above‐reported negative effect of lacking microhabitats in Douglas fir on taxon richness support our fourth hypothesis that structural habitat conditions imposed by the tree species contribute to explaining observed diversity patterns. However, we did not observe the expected pronounced positive effect on predators and omnivores, as previously reported for ground‐dwelling arthropod generalists (Kriegel et al, 2021; Wildermuth, Fardiansah, et al, 2023; Ziesche & Roth, 2008). This, once again, underlines that consideration of different forest strata is needed to better understand the effects of forest management on associated arthropod diversity.…”

“…In forests as well as in many other ecosystems, arthropods play essential roles in trophic interactions such as herbivory, decomposition, or predation (Wildermuth, Dönges, et al, 2023; Wildermuth, Fardiansah, et al, 2023; Yang & Gratton, 2014). Arthropod composition (taxonomic and functional) therefore strongly influences forest functioning and integrity (Schowalter, 2017).…”

Tree species identity, canopy structure and prey availability differentially affect canopy spider diversity and trophic composition

“…It is expected that non-native trees host no or few specialist arthropods in their new range (Roques et al 2006 ), and as generalists are less efficient in their use of resources compared to specialists, the same resources may sustain less individuals (García et al 2018 ). Yet, negative effects of non-native trees were less evident for predatory arthropods when only considering abundances and taxonomic diversity (Oxbrough et al 2016 ; Matevski and Schuldt 2023 ), whereas expanding the scope to functional divergence and trophic complexity unraveled negative effects (Wildermuth et al 2023 ; Matevski and Schuldt 2023 ). However, most studies to date have focused on lower forest strata, especially the forest floor, and the effects of non-native trees on canopy fauna are still little understood (Gossner and Ammer 2006 ).…”

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

“…Arthropod diversity is an important driver of forest functioning and resilience via provisioning of crucial ecosystem functions such as decomposition, predation, pollination and food resources for higher trophic levels (Leidinger et al., 2021; Maleque et al., 2009; Wildermuth, Fardiansah, et al., 2023). Particularly, beetles (Coleoptera) are hyperdiverse and highly abundant in forest ecosystems (Leidinger et al., 2021).…”