DOI: 10.5061/dryad.9w0vt4bgr
title: Data for: Tracking the temporal dynamics of insect defoliation by high-resolution radar satellite data

description: 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.
description: 1. Data acquisition Sentinel-1 C-band SAR data
 and Sentinel-2 optical data were obtained from the ESA Scientific Hub
 (https://scihub.copernicus.eu/).<br> Terrestrial laser scanning was
 conducted to track the forest canopy structure in three dimensions before
 and after insect defoliation.  2. Data
 processing For the Sentinel-1 data, all available
 level-1 GRDH products by the IW mode were pre-processed using the Sentinel
 Application Platforms (SNAP) Sentinel-1 Toolbox software and transformed
 to γ<sup>0</sup> (see the Supplementary Note S 3.2 in Bae et
 al. (2019) and the batch processing graph at
 https://github.com/So-YeonBae/Sentinel1-Biodiversity). The
 γ<sup>0</sup> values of the VV and VH polarizations were
 converted to dB as
 10×log<sub>10</sub>⁡γ<sup>0 </sup>and their
 time-series values were smoothed using the Gaussian window
 function.<br> The difference between the smoothed values of VV and
 VH, defined herein as the canopy development index (CDI, unit: dB), was
 computed.<br> The normalized CDI (NCDI) was calculated by dividing
 the time-series CDI values at each plot by the minimum CDI value during
 the leaf-off season.<br> Four critical time-steps (A, B, C, and D)
 indicating prominent phenological transitions were selected from the 18
 time-steps of the NCDI of the 12-day composite time-series data. These
 four critical time-steps were used to calculate two indices related to
 defoliation and two indices related to refoliation. 
 Please find the details from Bae et al. (2021)  "Tracking
 the temporal dynamics of insect defoliation by high-resolution radar
 satellite data"
description: Each tab contains the data for making each figure in
 "Tracking the temporal dynamics of insect defoliation by
 high-resolution radar satellite data" (Bae et al.).
 Original paper DOI: 10.1111/2041-210X.13726