Arctic greening associated with lengthening growing seasons in Northern Alaska

Many studies have reported that the Arctic is greening; however, we lack an understanding of the detailed patterns and processes that are leading to this observed greening. The normalized difference vegetation index (NDVI) is used to quantify greening, which has had largely positive trends over the last few decades using low spatial resolution satellite imagery such as AVHRR or MODIS over the pan-Arctic region. However, substantial fine scale spatial heterogeneity in the Arctic makes this large-scale investigation hard to interpret in terms of vegetation and other environmental changes. Here we focus on one area of the northern Alaskan Arctic using high spatial resolution (4m) multispectral satellite imagery from DigitalGlobe (TM) to analyze the greening trend near Utqiagvik (formerly known as Barrow) over 14 years from 2002 to 2016. We found that tundra vegetation has been greening (tau = 0.65, p = 0.01, NDVI increase of 0.01 yr(-1)) despite no overall change in vegetation community composition. The greening is most closely correlated to the number of thawing degree days (R-2 = 0.77, F = 21.5, p < 0.001) which increased in a similar linear trend over the 14 year study period (1.79.+/-.0.50 days per year, p < 0.01, tau = -0.56). This suggests that in this Arctic ecosystem, greening is occurring due to a lengthening growing season that appears to stimulate plant productivity without any significant change in vegetation communities. We found that vegetation communities in wetter locations greened about twice as fast as those found in drier conditions supporting the hypothesis that these communities respond more strongly to warming. We suggest that in Arctic environments, vegetation productivity may continue to rise, particularly in wet areas.