Extensive forest canopy discoloration during the 2024 heat-drought in southeastern Central Europe: The response of sessile oak (Quercus petraea) in Croatia was consistent with premature leaf senescence

Extreme heat, drought, and pests impose growing stress on forests, causing canopy discoloration that is often difficult to attribute using remote sensing. In August 2024, large-scale premature leaf discoloration (PLD) occurred in forests of Southeastern Central Europe (SECE) associated with a prolonged heat-drought event. We analysed its spatial extent and species-specific occurrence using reflectance time-series from Terra/MODIS (2000–2026) and Harmonized Landsat and Sentinel-2 (2017–2025). The Green Red Vegetation Index (GRVI) was used to identify the timing of PLD. Red reflectance ( r red ) and NDVI, combined with tree-species distribution maps for Croatia, were used to analyse abiotic and biotic stress signals. An estimated 297–645 kha (1.7–2.5%) of SECE forests showed PLD in 2024. Species-specific analyses in Croatia (Slavonia) revealed that Quercus petraea was the most affected (69.4% in high-dominance stands), whereas Fagus sylvatica showed negligible PLD. In Q. robur, PLD was observed consistently since 2016 and associated primarily with the oak lace bug (OLB) infestation. Heat-drought PLD was characterized by rapid NDVI decline and sharp r red increase, whereas the OLB-consistent PLD showed slow NDVI decline and modest r red increase. Modelling results indicate that PLD proportion was positively associated with cumulative nighttime heat load and global radiation, rather than with maximum temperature. A strong increase in r red , a senescence-like NDVI decline, negligible PLD in F. sylvatica , and near-full recovery of Q. petraea suggest that the observed PLD was more consistent with the premature leaf senescence (PLS) hypothesis than with widespread acute leaf-tissue injury, but further research is needed.