Spring phenology of oak stands in the Western Carpathians: validation of satellite metrics from MODIS using ground-based observations

The study focuses on the validation of the leaf unfolding (LU) onset of oak stands in the Western Carpathians in 2000–2021 derived from MODIS satellite data. LU onset was derived from the annual trajectories of the Normalised difference vegetation index (NDVI) fitted with a double sigmoid logistic function. The satellite metric Growing speed day (GSD) corresponding to the LU onset is represented by the first derivative of the sigmoid function. Ground-based observations from 22 phenological stations of the Slovak Hydrometeorological Institute (SHMI) were used to validate the date of GSD. The results showed a good agreement between the medians of ground- and satellite-based LU onset dates. In addition to the median, the LU onset at the 5th and 95th percentiles were compared. For both percentiles, we found differences in the onset from MODIS and SHMI. The 5th percentile of the LU onset derived from MODIS was determined later than the one from SHMI data. With the 95th percentile, it was the opposite. As a result, the range determining the duration of LU onset from MODIS was significantly shorter than from SHMI observations. The trend analyses over the period 2000–2021 revealed a shift to the earlier onset of LU ~ 0.33 day.year⁻¹ (p = 0.13) from satellite and ~ 0.32 day.year⁻¹ from ground-based observations (p = 0.08). The validated LU onset and trends using the median allow analysing of the oak stands response to changing environmental conditions. However, the differences at the 5th and 95th percentiles, i.e. at the beginning and the end of the LU onset duration, remained unexplained.