A warming Arctic has been associated with increases in aboveground plant biomass, specifically shrubs, and changes in vegetation cover. However, the magnitude and direction of changes in NDVI have not been consistent across different tundra types. Here we examine the responsiveness of fine-scale NDVI values to experimental warming at eight sites in northern Alaska, United States. Warming in our eight sites ranged in duration from 2‑23 seasons. Dry, wet and moist tundra communities were monitored for canopy surface temperatures and NDVI in ambient and experimentally-warmed plots at near-daily frequencies during the summer of 2017 to assess the impact of the warming treatment on the magnitude and timing of greening. Experimental warming increased canopy-level surface temperatures across all sites (+0.47 to +3.14˚C), with the strongest warming effect occurring during June and July and for the southernmost sites. Green-up was accelerated by warming at six sites, and autumn senescence was delayed at five sites. Warming increased the magnitude of peak NDVI values at five sites, decreased it at one site, and at two sites it did not change. Warming resulted in earlier peak NDVI at three sites and no significant change in the other sites. Shrub and graminoid cover was positively correlated with the magnitude of peak NDVI (r=0.37 to 0.60) while cryptogam influence was mixed. The magnitude and timing of peak NDVI showed considerable variability across sites. Warming extended the duration of the summer green season at most sites due to accelerated greening in the spring and delayed senescence in the autumn. We show that in a warmer Arctic (as simulated by our experiment) the timing and total period of carbon gain may change. Our results suggest these changes are dependent on community composition and abundance of specific growth forms and therefore will likely impact net primary productivity and trophic interactions.

北极变暖与地上植物生物量（特别是灌木）的增加和植被覆盖的变化有关。但是，NDVI的变化幅度和方向在不同类型的苔原上并不一致。在这里，我们研究了美国北部阿拉斯加北部八个地点的细微NDVI值对实验升温的响应能力。我们八个地点的变暖持续时间为2-23个季节。在2017年夏季，以近日频率监测干燥，湿润和湿润的冻原苔原群落的冠层表面温度和NDVI，以评估周围环境和实验升温的地块的冠层表面温度和NDVI，以评估变暖处理对绿化程度和时间的影响。实验性升温使所有地点的树冠层表面温度升高（+0.47至+3.14˚C），在6月和7月以及最南端发生的变暖效应最强。六个地点变暖加速了绿化，五个地点的秋天衰老被延迟了。变暖增加了五个位置的峰值NDVI值，降低了一个位置的噪声，而在两个位置则没有变化。变暖导致三个地点的NDVI较早达到峰值，而其他地点则无明显变化。灌木和类虫盖度与NDVI峰值呈正相关（r = 0.37至0.60），而隐孢子虫的影响混合。NDVI峰值的幅度和时序在各个站点之间显示出相当大的可变性。由于春季的加速绿化和秋季的衰老延迟，大多数地区的变暖延长了夏季绿色季节的持续时间。我们表明，在北极地区（如我们的实验所模拟）中，碳吸收的时间和总周期可能会发生变化。我们的结果表明，这些变化取决于社区组成和特定增长形式的丰富程度，因此可能会影响净初级生产力和营养相互作用。