The phenology of European forests as seen by MODIS Leaf Area Index and GEDI Plant Area Index: toward an integrated approach

The timing of phenological events, such as the start of season (SOS) and end of the season (EOS), is critical to understand the response of terrestrial ecosystems to climate change. Phenology patterns in space are not easily detected in multi-layered canopy structures, such as broadleaved deciduous forests; discrepancies in measures from the ground and space are known. Lidar signals can penetrate canopy and is potentially useful to solve some of the challenges in remote sensing phenology. Here, phenology time series derived from LiDAR-based Plant Area Index (PAI) from the Global Ecosystem Dynamics Investigation (GEDI) were compared with passive optical Leaf Area Index (LAI) from the Moderate Resolution Imaging Spectroradiometer (MODIS), and further evaluated using high-resolution Sentinel-2–derived phenological metrics Results evidence clear differences in the detection of the senescence phase in broadleaved European forests at different latitudes, with GEDI-PAI estimating EOS up to 49 days later than MODIS-LAI. Sentinel-2–derived EOS dates were intermediate between MODIS and GEDI estimates (~35 days), supporting the interpretation that GEDI captures structural signals persisting beyond optical senescence. GEDI-PAI consistently retrieved later EOS dates and longer growing season length, reflecting its sensitivity to canopy structural changes during leaf fall. Robust phenological signals were detected in broadleaved forests, whereas needleleaved forests showed limited seasonal GEDI-PAI variability.. In contrast, MODIS-LAI better captures changes in leaf color and greenness and better represents fine-scale variations during the active growing season. Overall, these findings demonstrate that optical and spaceborne LiDAR sensors capture complementary aspects of forest phenology, and that their integration improves phenological characterization relevant to ecological and climate change research.