Within‐crown microclimatic variation and growth‐stage shape oak leaf chemistry with limited consequences for herbivore performance

Abstract Mature trees exhibit distinct vertical gradients in temperature and light. These within‐crown microclimatic gradients have been linked to spatial variation in leaf chemistry and herbivory. However, phytochemical variation and resulting herbivore responses are also strongly influenced by tree phenology and genetic differences among individuals. In this study, we tested whether crown position influences the chemistry of mature leaves and whether these effects vary between distinct phenological stages within a growth cycle. We also examined whether microclimatic or growth‐stage‐related changes in phytochemistry influence herbivore performance. We exposed clonal pedunculate oak ( Quercus robur ) saplings within three crown strata (ground, middle and upper canopy) of mature trees at two forest sites in Germany and Switzerland. We measured targeted and untargeted leaf chemistry across growth stages and conducted bioassays with generalist spongy moth caterpillars ( Lymantria dispar ). We found that vertical crown position was a major driver of mature leaf chemistry. Growth stage had a significant additive effect on phytochemistry but explained less variation than crown position. We did not detect any interaction between crown position and growth stage on phytochemistry. Targeted analysis of flavonoids revealed differences among crown positions and between the two growth stages, with compounds showing distinct yet additive responses to these factors. Untargeted metabolomics revealed similar additive patterns, largely reflecting shifts in phenolic and terpenoid‐related metabolite classes. The relative growth rate of caterpillars (RGR) was affected by growth stage, declining during shoot elongation compared to the steady growth stage, but not by crown position. Among the measured metabolites, quercetin 3‐O‐glucuronide was negatively associated with caterpillar RGR, whereas higher triterpenoid levels were positively associated with RGR. Our findings demonstrate that within‐crown microclimatic variation and oak growth stage jointly shape mature leaf phytochemistry, but have only limited consequences for the performance of the generalist herbivore Lymantria dispar . Rather than reflecting broad metabolomic shifts, herbivore performance was associated with only a limited subset of measured metabolites, highlighting that pronounced chemical variation within‐tree crowns does not necessarily translate into equally strong ecological effects. Read the free Plain Language Summary for this article on the Journal blog.