Air temperature impact on springtime tree phenology in Poland based on satellite data

Air temperature is a crucial driver of many plant developmental processes, among which the initiation of spring activity is the most prominent. Recent climate warming has been found to alter plant phenology in many European regions. However, ultimate conclusions regarding this relationship are still challenging as quantitative estimates are strongly diverging. Moreover, the results concerning the role of temperature extremes, i.e., unusually warm or cold periods, are equivocal. Studying these complex links is of utmost importance as changes in plant phenology affect basic ecosystem functions, including water, carbon, and energy fluxes, as well as plant-animal interactions and ecosystem productivity. Recent progress in research based on satellite phenology indicates that it is a powerful tool to monitor terrestrial vegetation and its responses to climate drivers. It provides qualitatively different traits than ground-based observations, but these two sources of information complement each other well. The use of new generations of satellites with high spatial and temporal resolution helps to gain a better insight into the climate-phenology relations.

The main aim of this study is to assess the effect of air temperature conditions on the start of the spring activity of deciduous trees in Poland during 2018-2023. Data used in the study include the E-OBS (v27.0e) gridded daily mean, maximum and minimum temperatures, and the Sentinel-2 imagery over March-June. The tree species chosen for the analysis are common beech (Fagus sylvatica), silver birch (Betula pendula), and two species of oak (Quercus), which are among the most abundant in Poland. Several hundred single species stands were delimited across the area of Poland to study inter- and intraspecific differences in temperature-phenology relations. General temperature conditions during six individual seasons were assessed using the growing degree days (GDD) for 0°C and 5°C base temperatures. Special attention was paid to extremely warm or cold events, i.e., warm and cold spells, defined using percentile-based thresholds. Their effects were evaluated regarding their intensity and timing within the season. Trees’ response to temperature conditions was assessed using satellite-derived start-of-season (SOS) metric based on derivatives calculated from two indices: MTCI (MERIS Terrestrial Chlorophyll Index) and EVI (Enhanced Vegetation Index).

This preliminary study provided insights into complex links between temperature conditions and spring phenology of deciduous trees, showing that responses to climate drivers, especially extreme events, are species-dependent. Large inter-annual variability in phenology has also been shown – estimated SOS varied between studied seasons by more than 20 days. The results also proved the Sentinel-2 data to be useful in the monitoring of individual tree species phenology.