Extreme heat and drought induced large-scale leaf senescence in sessile oak in summer 2024 with near-full recovery in the following year

Abstract:

Increased frequency and duration of extreme heat and drought events threaten European forests. The mechanisms that trees, and oaks in particular, have evolved to cope with such events are not fully understood. In August 2024, forests across Central and Southeastern Europe experienced an unprecedented episode of premature leaf senescence (PLS). The most widespread PLS occurred in forests of northern Croatia, as well as in Hungary, Bosnia and Herzegovina, Serbia, Romania, and Bulgaria. The aim of our work was to investigate the extent, timing, and species-specific characteristics of PLS and, based on recovery in the following year, to which degree the observed senescence reflected a controlled physiological response or tree mortality.

We used meteorological data (FORESEE, ERA5-Land) in combination with satellite-derived spectral reflectances and vegetation indices (VI), to detect the event at the regional scale with Terra/MODIS (2000–2025) and place it in a multi-decadal context. At the local scale, we used 30 m resolution Harmonized Landsat–Sentinel-2 (HLS, 2017–2025) data for precise characterization of the onset and spectral signature of PLS. Forest management maps and soil data were used to investigate PLS occurrence and intensity with the respect to tree species and location in Croatia.

GRVI and red reflectance proved superior to NDVI for discriminating heat- and drought-induced PLS from the gradual damage of oak lace bug (Corythucha arcuata, Say), an invasive species affecting oaks and complicating remote-sensing monitoring of phenology events. Species-specific analyses revealed that although extreme meteorological conditions reduced photosynthetic activity across multiple forest types, premature leaf senescence predominantly affected sessile oak (Quercus petraea, (Matt.) Liebl.), while European beech (Fagus sylvatica) remained mostly unaffected. In central Croatia in 2024 approximately 67% of sessile oak experienced PLS, occurring on average 54 days earlier than the long-term mean timing of autumn senescence.

Spring 2025 green-up confirmed that PLS in sessile oak was a reversible stress response rather than widespread mortality. Our results highlight the capacity of sessile oak for controlled premature senescence as an adaptive strategy under compound climate extremes, with implications for forest resilience, carbon cycling, and management.

 

Keywords:

Premature leaf senescence, Sessile oak, Space-borne remote sensing, GRVI, extreme weather, HLS, MODIS.

 

Funding:

The study was supported by the EU NextGenerationEU through the Recovery and Resilience Plan for Croatia under the project Dendro-Carbon (No. 400-01/23-01/6-2), the Hungarian Scientific Research Fund (OTKA FK-146600), National Multidisciplinary Laboratory for Climate Change (RRF-2.3.1-21-2022-00014) project within the framework of Hungary's National Recovery and Resilience Plan, supported by the Recovery and Resilience Facility of the European Union.