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.

植物氮的使用是北极陆地生态系统氮循环的重要组成部分，重要的过程包括植物N在生长季节的吸收和再分配。虽然落叶苔原植物中氮的可用性部分取决于其在秋季衰老过程中从叶到茎和根的内部内部重分配，但重新分配时间的物种特定重要性及其对景观和区域尺度的整个社区影响仍然存在不太知名。在这里，我们量化了西格陵兰岛从6月到10月四种常见灌木物种的叶片N含量和C：N比率，并基于新的Sentinel-2指数将样地观测结果与景观尺度进行了比较。我们的Sentinel-2指数可以很好地捕获情节级别在整个时间和空间上的总体N重新分配趋势（R ² = 0.81，p  <0.001）。因此，使用这种新颖的方法对于提高植物C：N动力学的当前和未来变化具有重要意义。使用卫星数据，我们估计从叶子到茎和根的衰老过程中重新分配的叶N量约为0.8 g N m ^-2。我们得出的结论是：（1）整个生长季节的现场数据对于量化物种水平上氮分配的时机和对比策略至关重要；（2）关键物种如柳柳（Salix glauca）能够在衰老期间将其叶片N含量减半。（3）Sentinel-2（S-2）卫星数据是量化这些植物在空间和时间上的动态变化的强大候选者。我们的研究对于从气候变化的角度研究北极植物之间的竞争，凋落物分解以及冻土土壤中的碳积累具有重要意义。此外，它捕获了生长季节关键部分的植物功能动力学，包括秋季衰老，这通常比捕获春季绿化更为复杂。最后，我们的新指数对于远程绘制野生和驯化草食动物的底物质量时空变化具有重要意义，因为这是朝着评估寒带地区畜牧业苔原植被和饲料质量的工具迈出的第一步。