Monitoring European beech phenology in two long-term ecological research sites by remote sensing

The impact of latitude and altitude on phenological rhythms was studied from 2017 to 2023 at two European beech forest sites in western Bulgaria, Petrohan and Belasitsa. These sites are part of the European Long-Term Ecological Research network. We used products from the Copernicus program’s High-Resolution Vegetation Plant Productivity to extract the main phenological events: start-of-season date, max-of-season date, end-of-season date, and season length. Our findings indicate that the spring phenology of European beech is closely linked to altitude, while autumn events are more significantly affected by latitude. Spring phenological events were delayed by 2.9 days per 100 m at Petrohan and 2.3 days per 100 m at Belasitsa. This relationship weakens in summer and almost disappears in autumn when latitude becomes a leading factor. The average difference in the end-of-season date between Belasitsa and Petrohan is 10.8 days, which means 5.4 days per degree of latitude. Although the end of the season has been occurring later each year, the relationship is still insignificant. The dynamics of individual phenological events in different years, at various altitudes and latitudes, show that European beech has good potential for acclimating to present climate conditions in the western Bulgarian mountains. Further research is needed on the influence of longitude, considering the uneven transition between Mediterranean and temperate-continental climates in the southeastern part of the species’ range.