Leaf-out in a warming climate: linking air temperature anomalies and silver birch phenology in Poland using ground and satellite data

Plant phenology is primarily driven by air temperature, although this relationship is complex and not yet fully understood – particularly regarding the influence of temperature extremes on the timing of individual phenophases. In recent decades, observed springtime warming has led to significantly earlier green-up in temperate deciduous forests. Furthermore, spring temperatures in Central Europe are projected to rise by 2–5°C by the end of the century, depending on the scenario. Such changes are expected to profoundly affect forest structure and functioning, including ecosystem processes such as water, carbon, and nutrient cycling. These shifts may, in turn, have far-reaching consequences for ecosystem productivity, food webs, and species interactions.

To improve our understanding of deciduous trees responses to temperature changes – and how such shifts may affect ecosystems – we examined the relationship between the onset of spring activity in silver birch (Betula pendula) and recent springtime temperature anomalies in Poland (2018–2024). This period includes several extreme spring seasons (e.g. 2018, one of the warmest on record), offering a unique empirical basis for evaluating how future climate conditions may influence temperate forest phenology.

The phenological data used in this study included both ground-based observations of leaf unfolding and satellite-derived estimates of the start of season (SOS) for silver birch, one of the most abundant deciduous tree species in Poland. Ground-based data were obtained from the Institute of Meteorology and Water Management – National Research Institute. The SOS metric was derived from Sentinel-2 imagery (10 × 10 m resolution), based on derivatives of the Enhanced Vegetation Index. Temperature conditions for individual seasons were assessed using in-situ measurements and a newly developed high-resolution gridded dataset (1 × 1 km) for Central Europe. The relationship between air temperature and the onset of the season was explored using both temperature anomalies and the Growing Degree Days index.

The study provides empirical evidence of the sensitivity of spring phenology in deciduous trees to temperature conditions in Central Europe, offering valuable insights into potential future shifts under ongoing climate change. Our findings show a significant advancement in silver birch phenology during exceptionally warm springs and notable delays during colder ones. Most importantly, the results suggest that not only the magnitude, but also the timing of temperature anomalies within the season plays a crucial role in shaping spring phenological responses.