Introduction
Non-native tree species have been extensively planted during the last centuries and are nowadays of major economic importance and, thus, prominent in many forests throughout the world (Branco et al.2015; Campagnaro et al. 2023; Wohlgemuth et al. 2022). Non-native tree species dominate globally already 44% of plantation forests (FAO 2020). Reasons named for their intensive use include higher productivity compared to native trees and a better performance under changing climatic conditions accompanied with positive effects on wood-derived services such as provision of timber, pulp, firewood and fodder (Branco et al. 2015; Kärvemo et al. 2023; Pötzelsberger et al. 2020; Wohlgemuth et al. 2022).
Besides their economic importance, non-native tree species become increasingly relevant in the discussion on resilient future forestry in environments which become unsuitable for native tree species (Wohlgemuthet al. 2022). Climatic changes – especially climatic extreme events such as increasing drought – show large-scale effects on forested ecosystems with major implication for the management of these systems but also for the retrievable goods and services. Drastic forest diebacks have been observable in the last years, raising the question about tree species that will be able to cope with these novel, climatic conditions. Being better adapted to certain future climates, many foresters already select for non-native tree species to maintain timber-related services of forests under increasingly changing climates (Canadell & Raupach 2008; Ennos et al. 2019; Sims 2004).
Undesired effects on resident species and ecosystem functioning such as declines of biodiversity, increases of disturbance frequency or disruptions of native reference states of forests are, however, important arguments against non-native tree planting (Campagnaroet al. 2023; Pötzelsberger et al. 2020). Furthermore, massive declines of terrestrial insects, which have been observed in Europe in the last decades (Hallmann et al. 2017) are assumed to be linked to land use change and habitat loss (Eggleton 2020) among other factors (Müller et al. 2023; Seibold et al. 2019). Land use change in forests is directly related to changes in forest management including changes in native and non-native tree planting. All these concerns resulted in a large body of legislation that has been created in many European countries during the last years – all of which aim for the regulation of non-native tree species establishment in forests (Pötzelsberger et al. 2020). Furthermore, transnational legislative counter measures have been released which define alien species of concern for the European Union (EU Regulation No 1143/2014; Wohlgemuth et al. 2022).
Effects of non-native tree species on native biodiversity have been assessed by numerous studies and often come to controversial results. Negative effects on local biodiversity have been reported for understory vegetation (Lanta et al. 2021; Taylor et al. 2016), saproxylic insects (Vogel et al. 2021) and fungi (Buée et al. 2011; Schmid et al. 2014). Wohlgemuth et al. (2022) conclude that non-native trees most often decreased both, insect and arthropod diversity. Although these negative biodiversity effects of non-native tree species predominate in literature, positive effects on the diversity of microorganisms, insects and other arthropods as well as for bryophytes and lichens are reported by several studies (see Wohlgemuthet al. 2022 for a recent summary of studies). Spatial and taxonomic bias in the conducted studies (i.e. predominantly regional-scale case studies with single or few species being compared) has been named as one of the reasons for inconsistency in the observed effects (Branco et al. 2015; Kärvemo et al. 2023). This inconsistency challenges the implementation of general management strategies. Studies with extensive geographic and taxonomic coverage, which might help to partly resolve these inconsistencies are missing so far (Wohlgemuth et al. 2022).
Prominent theorical concepts such as the enemy release hypothesis predict non-native trees to be less relevant for resident insect herbivores the more distant their geographic and climatic occurrence or their phylogenetic relation to native congeners is. Abundances and diversity of resident insects inhabited by non-native trees should therefore be higher the closer a respective non-native species is phylogenetically related to a native congener or the more similar climatic and or geographic spaces occupied by the native and the non-native are. Several empirical studies confirm these theoretical assumptions. The presence of native, congeneric species and the geographical extent of non-native trees in Europe has been shown to be positively correlated with the number of recruited resident insect species and the herbivore damage on non-natives (Branco et al.2015). Performance of non-natives under different temperatures at the sites of introduction has been furthermore shown to be closely related to the climatic conditions under which the non-native species naturally occur (Bayón et al. 2021). Furthermore, numerous studies provide evidence that phytophagous insects are more likely to be shared between native and non-native plants when phylogenetic distance decreases (e.g. Novotny et al. 2006; Ødegaard et al. 2005; Weiblenet al. 2006). In a global meta-analyses, Vila et al. (2015) conclude similar impacts of closely related non-native plants on resident species richness. However, the phylogenetic impact of non-native host trees on diversity of inhabited species has not been explicitly tested so far (Kärvemo et al. 2023). Furthermore, no exhaustive testing on the importance of geographic vs. climatic vs. phylogenetic distance on the effects of non-native species on resident insect abundances and diversity have been conducted so far. Most previous studies lack the comparison of impacts of native vs. non-native organisms in a common habitat (i.e. common-garden setting) although such comparison is considered to be critical to control for environmental effects on insect diversity such as local differences in population dynamics or environmentally induced differences in host plant fitness (Agrawal & Kotanen 2003; Schierenbeck et al. 1994).
In this study we try to close this knowledge gap by quantifying the effects of non-native tree species for four dominant and economically important Central European tree genera (Acer , Betula ,Fraxinus and Quercus ) with a total of 77 globally occurring, congeneric species in a common garden experimental setting. With this geographically and taxonomically extensive dataset we aim to test the importance of geographic vs. climatic vs. phylogenetic distance between the native species and their non-native congeners regarding their effects on insect abundance and diversity. Phylogenetic distance reflects the distance between the non-native tree species and a dominant native congener. Geographic and climatic (i.e. mean annual temperature, annual precipitation sum and continentality) distance reflect the distance between the non-native species’ distribution area and the study area. All three drivers have been proposed to strongly affect plant-arthropod-interactions in non-native plant species (Keane & Crawley 2002; Parker et al. 2012; Torchin & Mitchell 2004).
Based on existing theory and empirical evidence we hypothesize, that abundance and diversity of insects inhabiting non-native tree species decreases with increasing geographic, climatic and phylogenetic distance between the non-native and the native congener. We furthermore tested the effect size of each of the three drivers in explaining insect abundance and diversity to evaluate each driver’s relative importance.