Figure 2. Standardized parameter estimates averaged across the best competing models testing the effects of percentage of urbanization and local canopy cover, their interaction, year, mean spring temperature and/or mean spring precipitation (n = 298) on leaf damage (A) and the incidence of gall-inducing (B) and leaf-mining (C) herbivores. Circles and error bars represent standardized parameter estimates and corresponding 95% CI. The vertical dashed line centered on zero indicate the null hypothesis. Black and grey circles indicate significant and non-significant effect sizes, respectively. The year 2018 is the intercept and was contrasted with the years 2019 and 2020.
Insect galls were present on 6.34 ± 0.01% of the inspected leaves. Model selection retained the percentage of urbanization and local canopy cover, their interaction, year, spring temperature, and spring precipitation as important predictors explaining variability in leaf-gall incidence (Figure 2B, Table A ). In particular, gall-inducer incidence significantly decreased with increasing urbanization (from 6.30 to 5.00 % along the urbanization range,Figure 2B , Figure BB ) and with increasing local canopy cover (from 8.00 to 0.06 % along the range of local canopy cover,Figure 2B, Figure BE ). The effect of urbanization on gall-inducer incidence was, however, contingent on local canopy cover (significant urbanization × local canopy cover interaction;Figure 2B, Figure 3A ): the negative effect of urbanization on gall-inducer incidence was more pronounced when there was a greater canopy cover around focal oaks. The incidence of gall-inducers was significantly lower in 2019 and 2020 as compared to 2018, and significantly increased with increasing spring temperature (Figure 2B ). Spring precipitation had no consistent effect on gall-inducer incidence (Figure 2B ).