Plant nitrogen (N) use is an essential component of the N cycle in Arctic terrestrial ecosystems, and important processes include plant N uptake and reallocation during the growing season. While the availability of N to deciduous tundra plants in part relies on their internal reallocation of N from leaves to stems and roots during autumn senescence, the species-specific importance of reallocation timing and its community-wide implications on landscape- and regional-scales remains not well known. Here, we quantified leaf N contents and C:N ratios of four widespread shrub species in West Greenland from June through October and compared plot observations to landscape scale based on a new Sentinel-2-derived index. Our Sentinel-2 index captures overall N reallocation trends well across time and space at the plot level (R² = 0.81, p < 0.001). Using this novel approach is therefore relevant for upscaling of current and future changes in plant C:N dynamics. Using satellite data, we estimate the leaf N mass reallocated during senescence equaled about 0.8 g N m⁻² from leaves to stems and roots. We conclude that (1) in-situ data from the entire growing season is critical to quantify timing and contrasting strategies of N allocation at species level, (2) key species such as Salix glauca are capable of halving their leaf N content during senescence, (3) Sentinel-2 (S-2) satellite data is a strong candidate for quantifying these plant functional dynamics in space and time. Our study has implications for research on competition among Arctic plants, litter decomposition and consequently carbon accumulation in tundra soils from a climate change perspective. Further, it captures plant functional dynamics during critical parts of the growing season, including autumn senescence which is generally more complex than capturing spring greenup. Lastly, our new index has important implications for remotely mapping temporal and spatial variation in substrate quality for both wild and domesticated herbivores, as it is a first step towards a tool to assess the tundra vegetation and fodder quality for cold region animal husbandry.