Sentinel-2 Time Series: a Promising Tool in Monitoring Temperate Species Spring Phenology

The changes in vegetation phenology observed in recent decades may have significant implications for carbon, water, and energy fluxes as well as ecosystem productivity. Phenology is an important indicator of a changing environment, and studying species-specific phenology can help understand their responses to varying climate conditions. While satellite-based phenology provides long-term information for large areas, it describes different traits than ground-based phenology. Nonetheless, emerging research on remote sensing-based phenology has shown its usefulness in determining inter- and intra-specific differences, as well as interannual changes. In this study, we examined the potential of the Sentinel-2 time series from 2018 to 2022 in assessing the spring phenology of seven temperate tree species. Three vegetation indices were calculated, namely, the Medium Resolution Imaging Spectrometer (MERIS) Terrestrial Chlorophyll Index, the Normalized Difference Vegetation Index, and the Enhanced Vegetation Index. We examined interspecific variations in the start of the season (SOS) and interannual variability. Additionally, we explored intraspecific variations in spring phenology among common beech stands across an elevation gradient. Our findings reveal substantial differences in Sentinel-2-derived phenology across species, with larch, birch, and hornbeam displaying an earlier SOS compared with oak and alder. The latest SOS dates were observed in 2021, whereas the earliest occurred in 2018 and the difference exceeded 20 days. It corresponded to the diverse spring meteorological conditions in the examined years, with the highest heat accumulation in 2018 and the lowest in 2021. Additionally, the three examined indices showed different phenological curves, with MERIS Terrestrial Chlorophyll Index exhibiting distinct patterns with a delayed start of the spring increase compared with the other two indices. The relationship between elevation and SOS dates was generally positive, but there was high variability in some years. The earliest SOS dates occurred at similar to 600 m a.s.l. and larger interannual variations were observed for stands located on the highest elevations. These results underscore the potential of Sentinel-2 in monitoring the spring phenology of temperate species, offering insights into inter- and intra-specific variations, as well as year-to-year changes. In the future, longer Sentinel-2 time series will improve estimates and facilitate the tracking of long-term changes. Such advancements hold relevance for forest management, carbon budget predictions, and understanding ecosystem dynamics.