Contrasting Sentinel-2 NDRE and NDMI anomalies during the 2024–2025 hot–dry sequence in the Kazdağları forests, Türkiye
Keywords:
Sentinel-2, NDRE, NDMI, Hot-dry Sequence, Spatial block cross-validationAbstract
Eight growing seasons of harmonised Sentinel-2 surface reflectance (2018–2025) were analysed on a 20 m target grid across 969.1 km² of screened forest in the Kazdağları massif, north-western Türkiye. Pixel- and period-specific 2018–2023 references compared NDRE (B8A–B5), NDMI (B8A–B11) and a B8A-based NIRv implementation during the 2024–2025 hot–dry sequence. June 2024 had the highest June air-temperature and monthly mean-state vapour-pressure-deficit proxy anomalies in the analysed 1991–2025 ERA5-Land series, yet no index showed a region-wide negative seasonal departure beyond available reference-year variability. In 2025, when the six-month precipitation-minus-potential-evaporation proxy reached −2.41 standard deviations, the mean pixel-level NDRE departure was −2.15 between-year standard deviations whereas NDMI was −0.15; block bootstrapping placed the NDRE−NDMI contrast at −1.05 (95% interval −1.22 to −0.87). The contrast persisted across variations in the robust-scale floor, seasonal window, cloud-screening threshold and reference-baseline length. The raw NDRE anomaly became less negative from −1.65 below 600 m to −0.60 above 1300 m. Pixel-level prediction failed: across 50 repeated spatial-block allocations the median out-of-fold R² was −0.005, against +0.039 for random splits. The results show contrasting NDRE and NDMI behaviour in this single sequence; they do not establish a causal delay, soil-water depletion or a physiological mechanism.
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