Ecosystem phenology, i.e. the timing of key biological events, is often considered as both a witness and an actor of climate change. Phenological interannual variations and decadal changes reflect climate variability and trends. Deciduous plant phenology also directly influences the carbon, water and energy exchanges of the ecosystem with the atmosphere. In the northern forests, a trend to earlier spring has been widely reported, often based on remote sensing methods. This trend is suggested to explain a part of the residual carbon sink. However methodological issues, especially related to the combined effects of the vegetation and of the snow cover seasonal changes on the remote sensing signal, were found to affect the results. This chapter describes a remote sensing green-up retrieval method designed to avoid signal contamination by snow. The result validation with ground observations showed that the method catches the interannual variations in phenology of the plant community. Changes in the 1998-2017 are analyzed and positioned in a longer term. This shows that the most persistent feature over the last decades is a large-scale shift in the green-up date at the end of the 1980's, and that the green-up date has not recovered yet to its status prior to 1987. Finally the green-up date maps were used to represent phenology in the northern ecosystem carbon budget simulations. No unidirectional effect of phenological changes in the annual carbon balance could be identified because of a complex interplay between vegetation, water resources and climate.