ESA's Sentinel missions provide new opportunities for Earth Observation with spatial and temporal resolutions not seen so far. Especially Sentinel-2 (S2) Multispectral Imager (MSI) allows retrievals of variables that are of core interest for modelling the global carbon cycle, monitoring forest change and establishing carbon inventories. For the retrospective production of long term time series only the Landsat archive and Landsat 8 (L8) Operational Land Imager (OLI) become pertinent. Both product lines require intensive calibration and validation based on ground truth data especially for dynamic and complex forest ecosystems that have largely not been used for time series calibration. Now new technologies such as terrestrial laser scanning and hyperspectral UAVs/drones provide novel data sources to address this systematically for Sentinel 2 and the integration and inter-calibration with Landsat 8.