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

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