Window-aware chill and heat requirements in olive: Partial least squares-derived thermal windows validated using Sentinel-2 NDVI

Olive flowering depends on two sequential thermal phases: winter chilling for endodormancy release, followed by spring heat accumulation for pre-anthesis development. Under increasing inter-annual variability, fixed calendar sums may misrepresent the true periods of thermal control. This study proposes a window-aware framework that objectively identifies the most influential chilling and forcing windows and cross-validates their phenological coherence using satellite greenness dynamics. Five seasons (2019–2020 to 2023–2024) were analysed for three olive cultivars (Picholine Marocaine, Haouzia, Menara) in the Middle Atlas (Morocco) with field observations of full bloom (BBCH scale stage 65; BBCH 65). Daily minimum/maximum temperatures (Tmin/Tmax) were reconstructed to hourly series to compute Chill Portions (Dynamic Model) and Growing Degree Hours (GDH; base 4.5 °C) for quantifying chill and heat requirements. Partial least squares (PLS) regression with leave-one-year-out stability screening isolated two robust windows: late November–mid-January (chilling) and late February–late March (forcing). Mean chilling requirements were similar across cultivars (~69 Chill Portions (CP); Coefficient of Variation (CV) ~10–13 %), whereas forcing demand differed (mean GDH ≈ 7864 for Haouzia, 8524 for Menara, 8967 for Picholine Marocaine). Sentinel-2 normalised difference vegetation index (NDVI) (moderate resolution imaging spectroradiometer (MODIS), fallback) showed that the spring NDVI peak occurs ~1 month before BBCH 65 and therefore does not directly indicate full bloom timing in evergreen olive canopies. In contrast, BBCH 65 consistently fell within the start/end of season (SOS–EOS) envelope, providing a robust coherence check. This workflow provides objective thermal windows, cultivar-specific forcing fingerprints and an earth-observation (EO)-based coherence check for climate-ready cultivar selection in Mediterranean conditions for phenological forecasting and varietal adaptation.