Intra- and Interannual Dynamics of Remotely Sensed Functional Diversity in Temperate Forests from Sentinel-2 Time Series

Isabelle S. Helfenstein, Tiziana L. Koch, Meredith Christine Schuman , Felix Morsdorf

Monitoring biodiversity change requires approaches that capture ecological dynamics across space and time. Satellite remote sensing provides unique opportunities for such monitoring, but most studies of functional diversity rely on single-date imagery, typically at peak greenness, neglecting seasonal variability. Here, we used multi-year, dense Sentinel-2 time series (2017-2021) to assess seasonal and interannual dynamics of trait-related spectral indices (CIre, CCI, and NDMI) and associated functional diversity metrics (richness and divergence) of temperate mixed forests in northeastern Switzerland as a case study. Seven-day composites were analyzed to characterize intra- and interannual trajectories and to compare patterns across different forest communities. We show that all indices and metrics exhibit substantial seasonal and interannual variation, with important deviations from peak greenness values. Needle-dominated stands displayed higher functional diversity metrics than broadleaf stands, likely partly reflecting canopy structural effects, while functional divergence remained comparatively consistent over the years and communities. These results demonstrate that accounting for temporal dynamics is essential to accurately characterize forest functional diversity from space. Sentinel-2 time series offer a promising basis for biodiversity assessments and can complement field and airborne approaches in assessing forest dynamics.