  While single-tree transpiration is challenging to compare with earth
observation, canopy scale data are suitable for this purpose. To test the
potentialities of the second approach, we equipped the trees at two measurement
sites with sap flow sensors in spruce forests. The sites have contrasting
topography. The measurement period covered the months between June 2020 and
January 2021. To link plot scale transpiration with earth observations, we
utilized Sentinel-2 and local meteorological data. Within a machine learning
framework, we have tested the suitability of earth observations for modelling
canopy transpiration. The R2 of the cross-validated trained models at the
measurement sites was between 0.57 and 0.80. These results demonstrate the
relevance of Sentinel-2 data for the data-driven upscaling of ecosystem fluxes
from plot scale sap flow data. If applied to a broader network of sites and
climatic conditions, such an approach could offer unprecedented possibilities
for investigating our forests' resilience and resistance capacity to an
intensified hydrological cycle in the contest of a changing climate.
