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.