Vertical Variation of the Caterpillar Community in Oak ( Quercus robur ) Canopies

Background Understanding how caterpillar communities vary within tree canopies is key to interpreting forest trophic dynamics and responses to environmental change, yet such variation remains poorly quantified due to the challenges of sampling in three dimensions. Aims We quantified within-canopy heterogeneity in caterpillar densities, diversity, and herbivory and explored relationships with host tree phenology and commonly used ground-based monitoring approaches.

Using direct canopy access, we sampled branches from lower, middle, and upper canopy strata of 34 mature pedunculate oaks (Quercus robur) in Wytham Woods, UK, during the spring abundance peak over three consecutive years (2023-2025). We tested for vertical stratification in caterpillar community metrics, examined patterns in early instar distributions at emergence, assessed associations with host tree phenology across spatiotemporal scales, and evaluated how well ground-based methods (water and frass traps) reflect canopy communities.

Vertical stratification was modest but varied among years: densities and species richness increased with canopy height in 2023, decreased in 2024, and were uniformly low across strata in 2025. Although within-crown budburst timing varied systematically, with upper branches bursting approximately two days earlier than lower branches, tree phenology did not explain within- or between-year variation in caterpillar communities. Frass trap data correlated moderately well with canopy caterpillar densities, whereas water traps showed weaker and less consistent relationships, reflecting behavioural and methodological biases.

Caterpillar communities showed no consistent patterns of vertical stratification across years, instead they are shaped more strongly by inter-annual and tree-level variation. Integrating targeted canopy sampling with scalable ground-based proxies could greatly improve monitoring of arboreal Lepidoptera and inform studies of trophic synchrony and wood-land resilience under environmental change. Competing Interest Statement The authors have declared no competing interest.