Leaf herbivory
In early summer (about 10–12 weeks after oak budburst at each
location), scientists and partner schools haphazardly selected four low
hanging branches per tree facing opposite directions. They haphazardly
collected 30 leaves per branch (total: 120 leaves per tree). Then, 60
leaves were drawn blindly to reduce unconscious bias in leaf sampling.
Scientists oven-dried leaves for at least 48 h at 45°C right after
collection (n = 203 oaks). Leaves collected by partner schools (n = 98
oaks) were oven-dried when received by the project coordinators, to
warrant optimal preservation prior to herbivory assessment.
Three response variables were used to characterize leaf herbivory
(Valdés-Correcher et al., 2021): leaf damage (i.e., the percentage of
leaf area removed by insect herbivores), leaf-miner incidence (i.e., the
proportion of leaves with leaf-mines) and leaf-gall incidence (i.e., the
proportion of leaves with galls). Herbivory was visually scored by
assigning each to one of the following classes: 0, 0.1–5.0, 5.1–10.0,
10.1–15.0, 15.1–25.0, 25.1–50.0, 50.1–75.0 or >75%,
where the percentage represented the proportion of leaf surface removed
by chewing herbivores, or mined by leaf-miners. We then used the
midpoint of each class to average herbivory at the tree level (see
Valdés-Correcher et al., 2021 for details). We did not assess damage
caused by sucking insects because punctures vary widely among species
and for some species are not very visible (Schaefer and Panizzi, 2000).