Controls over intra-seasonal dynamics of AVHRR NDVI for the Arctic tundra in northern Alaska

We analysed the Normalized Difference Vegetation Index (NDVI), calculated from biweekly NOAA Advanced Very High Resolution Radiometer (AVHRR) images for northern Alaska at both regional (latitudinal gradients) and site scales. Our objectives were to determine if tundra types and arctic subzones could be differentiated in terms of intra-seasonal patterns of greenness, and to construct the relationships between NDVI and air and soil temperatures. There were common intra-seasonal patterns of NDVI along two latitudinal transects, and a general latitudinal gradient of time of greenness onset and length of growing season was observed. At the site scale, in most cases, wet tundra (WT) had the lowest NDVI values throughout the year, while shrub tundra (ST) had the highest values. The peak NDVI appeared in the period of 22 July to 4 August, with mean values of 0.552 for ST, 0.495 for moist acidic tundra (MAT), 0.434 for sandy tundra (Sandy), 0.426 for moist non-acidic tundra (MNT) and 0.343 for WT. The earliest onset of greenness occurred in ST, followed by MAT, Sandy and MNT, while WT had the latest onset. There were positive linear relationships between bi-weekly NDVI anomalies and air temperature, soil surface temperature, and 20 cm depth soil temperature anomalies in the region. Plant functional type abundances, tundra type, air and soil temperatures all appeared to influence the seasonal dynamics of NDVI.