Vicarious calibration and validation (Cal/Val) of spaceborne imaging spectrometers rely on stable, uniform, and homogeneous pseudo-invariant calibration targets. While this improves the quality of the match between satellite and field properties, the evaluation does not necessarily guarantee the quality of heterogeneous pixels, where the assumptions underlying the atmospheric correction are less likely to be met. Despite these greater uncertainties, it is still necessary to assess the similarity between field and remote-sensing observations across a broad range of conditions for diverse remote sensing applications. In this study we showcase the potential of the Integrated Carbon Observation System (ICOS) Ecosystem network, providing such a wide range of cover types, to support Cal/Val of medium-resolution imaging spectrometers. Las Majadas de Tiétar ICOS station (ES-LMa) was used as a challenging test site to obtain reliable spectral in-situ measurements over vegetation targets for cross-comparison with the Environmental Mapping and Analysis Program (EnMAP) images. In-situ spectral data were acquired within ±1 hour of two different EnMAP acquisitions on July 2025 over a selected calibration area dominated by senescent grass and surrounded by evergreen holm oaks, located 300 m south of the ES-LMa ICOS tower. Comparison between ASD and EnMAP reflectance factors demonstrated the impact of image off-nadir acquisitions in Cal/Val activities on vegetation targets as well as the susceptibility to atmospheric disturbances of the shortwave infrared bands typically used for the estimation of non-photosynthetic vegetation. Overall, the results prove the potential of ICOS ecosystem sites to complement traditional high-reflectance Cal/Val locations increasing the representativeness of terrestrial targets.