Artificial night light intensity modulates herbivory and phytochemistry in European beech

Abstract 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. 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. 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. 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.