Quantifying phenology in the deciduous tree and phytophagous insect system: a methodological comparison

The extent to which phenological synchrony between trophic levels may be disrupted by environmental change has been a topic of increased focus in recent years. Phenological associations between deciduous trees, phytophagous insects and their consumers (passerine birds) have become one of the model systems for understanding this process. However, most existing research reports population-level associations rather than examining the smaller spatial scales at which these trophic interactions occur. Furthermore, a variety of methods have been used to measure phenology, particularly on producers and primary consumers, with little formal comparison. To investigate how different methods of measuring producer and primary consumer phenology influence our understanding of these biological relationships at the appropriate scale, we quantified phenological metrics for individual host trees and the phytophagous insects that depend on them in a deciduous woodland during spring 2023. We sampled 170 trees from six deciduous species in Wytham Woods, UK, deriving nine metrics of phenology from five distinct field methods: multispectral drone imaging (NDVI), hemispherical canopy photography, and bud-scoring observations to track tree phenology, as well as water traps and frass traps to monitor insect herbivore phenology. We assessed the reliability of these methods within both trophic levels and across tree species. We further evaluated the extent to which tree phenology metrics best correlated with herbivore phenology, at the level of individual trees and links to variation in subsequent herbivory rates across a subsample of 72 oak ( Quercus robur ) trees. Our results illustrate how methodological choices can affect our ability to study the timing of trophic interactions and reveal fine-scale spatiotemporal variation in phenology across both trophic levels. We discuss the implications of these results for considering how the scale-dependence of trophic interactions may stabilise populations and shape broader-scale responses to environmental change.