Data for: Tracking the temporal dynamics of insect defoliation by high-resolution radar satellite data

1. Quantifying tree defoliation by insects over large areas is a major
challenge in forest management, but it is essential in ecosystem
assessments of disturbance and resistance against herbivory. However, the
trajectory from leaf-flush to insect defoliation to refoliation in
broadleaf trees is highly variable. Its tracking requires high temporal-
and spatial-resolution data, particularly in fragmented forests. 2. In a
unique replicated field experiment manipulating gypsy moth Lymantria
dispar densities in mixed-oak forests, we examined the utility of publicly
accessible satellite-borne radar (Sentinel-1) to track the fine-scale
temporal trajectory of defoliation. The ratio of backscatter intensity
between two polarizations from radar data of the growing season
constituted a canopy development index (CDI) and a normalized CDI (NCDI),
which were validated by optical (Sentinel-2) and terrestrial laser
scanning (TLS) data as well by intensive caterpillar sampling from canopy
fogging. 3. The CDI and NCDI strongly correlated with optical and TLS data
(Spearman’s ρ=0.79 and 0.84, respectively). The ∆NCDIDefoliation
(A-C) significantly explained caterpillar abundance (R2=0.52).
The NCDI at critical time-steps and ΔNCDI related to defoliation and
refoliation well discriminated between heavily and lightly defoliated
forests. 4. We demonstrate that the high spatial and temporal resolution
and the cloud independence of Sentinel-1 radar potentially enable
spatially unrestricted measurements of the highly dynamic canopy
herbivory. This can help monitor insect pests, improve the prediction of
outbreaks, and facilitate the monitoring of forest disturbance, one of the
high priority Essential Biodiversity Variables, in the near future.