Tracking pre-event temperature anomalies and spring onset in silver birch in Poland

Spring phenology in temperate trees is strongly controlled by air temperature, yet the role of short-term thermal anomalies preceding phenological events remains insufficiently understood. In our previous analyses of silver birch (Betula pendula) phenology in Poland, we showed that the timing of both ground-observed leaf unfolding (LU) and satellite-derived start of season (SOS) was closely related to temperature anomalies in March and April, and that not only the magnitude but also the within-season timing of warming played an important role in shaping spring onset. Preliminary anomaly-based analyses further suggested that the period of approximately 20 days preceding leaf-out may be particularly important. However, contrasting early-spring seasons indicated that similar phenological outcomes may arise from different thermal trajectories, including both sustained warming and short but intense late-season warm pulses.

In this study, we revisit these relationships using an extended dataset comprising LU observations for 2007–2026 and SOS estimates for 2018–2026. LU data were obtained from the Institute of Meteorology and Water Management – National Research Institute, while SOS was derived from Sentinel-2 imagery using the Enhanced Vegetation Index. Temperature conditions were assessed from in-situ measurements and a high-resolution station-based gridded dataset. We focus specifically on pre-event temperature anomalies, examining their magnitude, persistence, and timing in relation to the observed dates of spring onset. Rather than relying solely on calendar-month means, we analyse temperature anomalies in moving windows preceding LU and SOS to identify the period of greatest phenological sensitivity and to test whether exceptionally early spring onset is more strongly associated with sustained positive anomalies or with brief but intense warm episodes occurring shortly before the event.

By combining long-term ground observations with satellite-based estimates, this study provides a more process-oriented perspective on spring phenology and its thermal controls. The results are expected to improve our understanding of how silver birch responds to anomalous thermal conditions and to help refine interpretations of phenological sensitivity under ongoing climate warming.