Examining Tundra Greening from Ground-based to Satellite Observations

Abstract:

Multiple studies have shown positive multiyear trends, often referred to as “greening,” in satellite measured Normalized Difference Vegetation Index (NDVI) in tundra ecosystems. However, the ecological meaning of these trends often is not clear. Ongoing ground data collections of tundra spectral reflectance and plant type cover begun in 2010 near Utqiagvik (Barrow) and Atqasuk, on the Alaskan North Slope provide a unique opportunity to link ground measurements to moderate-resolution satellite observations, such as MODIS, illuminating how surface characteristics relate to arctic greening inferred from satellite data. Visible-near infrared spectral reflectance and plant coverage data were collected at plots in the Circumpolar Active Layer Monitoring (CALM) grids. The CALM grid measurements were 30 plots arranged in a 5 by 6 matrix with plots separated by 100 m along each row and column providing a framework for repeatable measurements that sample areas approximately the size of MODIS pixels. For the combined CALM plot data, decadal multiyear trends in summertime MODIS NDVI successfully track change in total green vegetation cover, mainly related to increasing graminoid coverage. The ground measurement sampling provided a description of the spatial heterogeneity within a MODIS pixel and showed that for a pixel with a greening trend, there were plots that were browning, not changing, or greening at a greater rate. Browning plots tended to have low shrub and high graminoid cover, while strongly greening plots tended to have relatively high shrub and low graminoid cover. Further, the rates of springtime NDVI increase with growing degree days were found to be related to plant type coverage. Plots with low responses to springtime warming had relatively high evergreen shrub and lichen cover, while the highly responsive plots had relatively high forb and graminoid covers.