Anouchka Perret-Gentil

1. Artificial light at night (ALAN) is a recognized threat not only to urban areas but also to more remote habitats including forests. Advancements in LED technology offer potential for streetlight infrastructure with minimized disruptive impact on ecosystems. Light dimming ranges among the most promising ALAN-mitigation measures. Despite being essential for understanding ALAN effects on ecosystems, little is known about how dimming may affect bottom-up interactions, such as those between plants and herbivores.
2. We conducted a forest-located experiment on potted European beech saplings (Fagus sylvatica L.) that lasted 43 nights. We studied the impact of different LED streetlight intensities i.e. 70 lx (100% light intensity), 35 lx (50% light intensity), 21 lx (30% light intensity) and a dark control on leaf morphological and chemical traits, insect herbivory and the links between light-induced changes in leaf traits and herbivory.
3. The 100% and 50% light intensity treatments caused an increase in average leaf area but reduced specific leaf area. ALAN also induced changes in the specialised leaf metabolome, mainly affecting flavonols. Responses of individual metabolites to different ALAN intensities were highly variable, with many metabolites already responding to 30% light intensity. Insect herbivory was only reduced under 100% light intensity. The reduction in herbivory correlated with ALAN-related changes in phytochemistry.
4. Synthesis and applications: Our findings indicate that reducing LED light intensities (dimming) is a promising step towards sustainable outdoor lighting to mitigate ALAN effects on tree-herbivore interactions. However, the effects of dimming on leaf physiology, phytochemistry and herbivory were variable, with detectable effects on phytochemistry even at high dimming levels. The varying light responses across trophic levels highlight the challenge of optimizing light parameters in order to minimize ALAN impacts on species interactions. To address this challenge, our study advocates for increased collaboration between ecologists and lighting engineers.

• Extreme droughts can have long-lasting effects on forest community dynamics and species interactions. Yet, our understanding of how drought legacy modulates ecological relationships is just unfolding. We tested the hypothesis that leaf chemistry and herbivory show long-term responses to premature defoliation caused by an extreme drought event in European beech (Fagus sylvatica L.).
• For two consecutive years after the extreme European summer drought in 2018, we collected leaves from the upper and lower canopy of adjacently growing drought-stressed and unstressed trees. Leaf chemistry was analyzed and leaf damage by different herbivore-feeding guilds was quantified.
• We found that drought had lasting impacts on leaf nutrients and on specialized metabolomic profiles. However, drought did not affect the primary metabolome. Drought-related phytochemical changes affected damage of leaf-chewing herbivores whereas damage caused by other herbivore-feeding guilds was largely unaffected. Drought legacy effects on phytochemistry and herbivory were often weaker than between-year or between-canopy strata variability.
• Our findings suggest that a single extreme drought event bears the potential to long-lastingly affect tree–herbivore interactions. Drought legacy effects likely become more important in modulating tree–herbivore interactions since drought frequency and severity are projected to globally increase in the coming decades.

Sommerdürren können die Interaktionen zwischen Bäumen und Insekten kurzfristig stark verändern. Wie Dürreeffekte aber auch Jahre später noch auf Baum-Insekten-Interaktionen nachwirken können, zeigen Analysen, die am SwissForestLab Science Day 2023 präsentiert wurden.

Pinus radiata (radiata pine or Monterey pine) is threatened in its native range in California and, at the same time, one of the most widely-planted tree species worldwide, especially in the southern hemisphere. It is affected by a wide range of plant-feeding insects both in its native range and in regions where it is planted as an introduced tree. In addition, there are many invasive insects that have colonised P. radiata, in some cases causing major damage. Here, our objectives were to provide a complete and up-to-date overview of all insect species recorded from P. radiata worldwide, to summarise where these insects are native and which countries or regions they have invaded, to categorise them according to their impacts as damaging species or as vectors of plant pathogens, and to examine border interceptions to determine whether pathways exist that would allow these species to enter and potentially invade additional regions. Our compilation of insects feeding on P. radiata provides a list of 649 species (and an additional 11 species identified at the genus level only). Coleoptera is the most represented order in the list (299 species), followed by Lepidoptera (224 species) and Hemiptera (65 species). We classified 28 species as high-impact, including 12 true bark beetles (Coleoptera: Curculionidae: Scolytinae), eight Lepidoptera, five other Coleoptera, two Hymenoptera and one Hemiptera. These species can cause substantial direct damage or act as vectors of highly-damaging plant pathogens. Other species cause only occasional damage, rarely requiring management (classified as ‘low-medium impact’) or they are generally benign (‘negligible impact’). Hemiptera and Scolytinae have a high proportion of species established outside their native range. The Nearctic and Neotropic regions have been invaded by the most high-impact species, mainly by species native to Europe. Border interceptions of 185 species (29% of those on our list) were recorded during import inspections between 1995-2021, indicating considerable potential for further invasions. The findings of our study can be used to identify potential high-impact invaders and the pathways that may require more phytosanitary attention. Furthermore, our analyses provide useful insights into the insect-plant interactions resulting from the global distribution of a tree species and the native and non-native insects feeding on it.

Eine neue Untersuchung an Schweizer und skandinavischen Eschen zeigt, dass Eschen, die eine erhöhte Resistenz gegen das Eschentriebsterben besitzen, besser gegen den Eschenprachtkäfer gewappnet sind. Eine gezielte Förderung könnte die Ausbreitung des Käfers verlangsamen.

Le frêne est menacé par deux espèces invasives différentes; un champignon asiatique, également connu sous le nom de dépérissement des pousses du frêne ou chalarose, ainsi que l'agrile du frêne qui, selon les modèles, devrait atteindre la Suisse dans quelques années.

• Plants rely on cross-resistance traits to defend against multiple, phylogenetically distinct enemies. These traits are often the result of long co-evolutionary histories. Biological invasions can force naïve plants to cope with novel, coincident pests, and pathogens. For example, European ash (Fraxinus excelsior) is substantially threatened by the emerald ash borer (EAB), Agrilus planipennis, a wood-boring beetle, and the ash dieback (ADB) pathogen, Hymenoscyphus fraxineus. Yet, plant cross-resistance traits against novel enemies are poorly explored and it is unknown whether naïve ash trees can defend against novel enemy complexes via cross-resistance mechanisms.
• To gain mechanistic insights, we quantified EAB performance on grafted replicates of ash genotypes varying in ADB resistance and characterized ash phloem chemistry with targeted and untargeted metabolomics.
• Emerald ash borer performed better on ADB-susceptible than on ADB-resistant genotypes. Moreover, changes in EAB performance aligned with differences in phloem chemical profiles between ADB-susceptible and ADB-resistant genotypes.
• We show that intraspecific variation in phloem chemistry in European ash can confer increased cross-resistance to invasive antagonists from different taxonomic kingdoms. Our study suggests that promotion of ADB-resistant ash genotypes may simultaneously help to control the ADB disease and reduce EAB-caused ash losses, which may be critical for the long-term stability of this keystone tree species.

1. Despite worldwide reports of high tree mortality, growing evidence indicates that many tree species are well adapted to survive repeated dry spells. The drought resilience of trees is related to their phenotypic plasticity and ecotypic differentiation. Whether these two mechanisms act at the same organisational level of a tree and involve similar plant traits is still unknown.
2. We assessed phenotypic plasticity and ecotypic differentiation across four populations of Pinus sylvestris and Pinus nigra seedlings grown for three years under a recurrent summer drought treatment or well-watered control conditions in a common garden. We measured the response to the summer drought treatment of a total of 26 traits including shoot and needle morphology, needle anatomy, and foliar macronutrients, and related the traits to the growing season water deficit (GSWD) at the location of the seed origin.
3. Foliar phenotypic plasticity in response to recurrent summer drought was surprisingly low, with the needle length and the fraction of mesophyll and phloem tissue adjusting to some extent. In comparison, shoot morphological traits were much more plastic in both species with predominant responses to the summer drought stress including shorter and less numerous apical and lateral shoots. These three traits were also correlated with GSWD at the seed origin, indicating local adaptation. In contrast, between-population variation of foliar morphological and anatomical traits, and macronutrients were mostly unrelated to the GSWD at the seed origin.
4. Consequently, phenotypic plasticity and ecotypic differentiation occurred at the same level of organisation and in the same plant traits, i.e. shoot morphology. This combination of plasticity and ecotypic differentiation allowed P. sylvestris and P. nigra seedlings to rapidly acclimate to recurrent and long-lasting dry-spells.
5. Synthesis: P. sylvestris and P. nigra seedlings showed considerable ecotypic differentiation and phenotypic plasticity of shoot morphological traits, and not foliar traits. Acclimation to recurrent severe summer drought was achieved by reducing shoot growth.

Durch das Eschentriebsterben und den sich vom Einschleppungsort Moskau weiter Richtung Westen ausbreitenden Eschenprachtkäfer ist die Esche als Baumart in Europa stark gefährdet. Die Forschung an Eschen und ihren Interaktionen mit Schadorganismen trägt dazu bei, den Eschenfortbestand in der Schweiz zu sichern.