Satellite-based springtime phenology of silver birch (Betula pendula) in Poland and its relations to air temperature

Recent advancements in satellite phenology research demonstrate its effectiveness as a robust tool for monitoring vegetation and its responses to climate drivers. Satellite data provides insights distinct from ground-based observations, yet both sources complement each other well. Utilizing new-generation satellites, with high spatial and temporal resolution, facilitates a deeper understanding of climate-phenology relationships. These satellite-derived indices prove especially valuable in regions with scarce ground-based observations like Poland.

Air temperature plays a crucial role in driving various plant developmental processes, among which the initiation of spring activity is the most prominent. However, drawing ultimate conclusions regarding this relationship remains challenging as quantitative estimates strongly diverge. Recent observations indicate that climate warming has altered plant phenology across many European regions. Investigating these complex connections is crucial, given that changes in plant phenology affect fundamental ecosystem functions, including water, carbon, and energy fluxes, as well as interactions between plants and animals, ultimately shaping ecosystem productivity.

The main aim of this study is to evaluate the impact of air temperature conditions on the onset of spring activity of silver birch (Betula pendula) in Poland over the years 2007-2024. The phenology data used in the study include both ground observations of leaf unfolding and satellite-derived estimates of start-of-season (SOS) for silver birch, which is among one of the most abundant deciduous tree species in Poland. Ground-based data were sourced from the Institute of Meteorology and Water Management – National Research Institute and spans years 2007-2024, while the satellite-based SOS metric was derived from Sentinel-2 imagery and covers the 2018-2024 period. The SOS metric was based on EVI (Enhanced Vegetation Index) derivatives. Temperature conditions during individual seasons were assessed using in-situ measurements as well as the E-OBS (v28.0e) gridded dataset, and the relationship between air temperature and the start of the season was studied using temperature anomalies and the Growing Degree Days (GDD) index for 5°C base temperature.

The study showed large inter-annual variability in silver birch phenology – the estimated SOS varied between studied seasons by more than 20 days. The results also proved the usefulness of Sentinel-2 data in monitoring the phenology of deciduous tree species, while indicating that satellite-based estimators pertain to a slightly different phenological phase than ground-based observations. The study provided insights into the links between temperature conditions and the spring phenology of birch, showing that responses to air temperature are complex. The relationship remains evident, as, for instance, in 2018, characterized by extremely warm April and May, an advanced start of the season was observed. Conversely, in 2021, a delayed start of the season was noted as a consequence of cold temperatures during these months. However, quantitatively describing these relationships remains a challenge and further research is necessary.