Broad-scale satellite Normalized Difference Vegetation Index data predict plant biomass and peak date of nitrogen concentration in Arctic tundra vegetation

Questions Is the satellite-derived Normalized Difference Vegetation Index (NDVI) an adequate proxy for the timing of the peak in plant nitrogen concentration in an Arctic tundra system? Can NDVI be used to reliably assess seasonal changes in aboveground plant biomass? Location The south plain of Bylot Island, an Arctic tundra ecosystem north of Baffin Island, Nunavut, Canada (73°08′ N, 80°00′ W). Methods Using plant data collected every 2 wk throughout the summer in 1991, 1993–1996 and 2006–2008, we assessed the relationship between four NDVI indices (AVHRR satellite data at 1-km2 spatial resolution) and the date of peak nitrogen concentration in wetland graminoid plants, which represents seasonal variability in plant quality. We also examined the relationship between NDVI and the seasonal changes in aboveground live plant biomass. Results Three out of the four NDVI metrics that we tested were significantly related to date of peak nitrogen concentration. The strongest relationship was found with the date at which NDVI values reached 50% of their annual maximum (r2 = 0.87). We also found a positive exponential relationship between NDVI and aboveground biomass of plants (r2 = 0.58), though this relationship was strongest early in the growing season. Conclusions NDVI can be used as a proxy to determine date of peak nitrogen concentration in some tundra plants, and can thus be a reliable measure of the yearly changes in the timing of the availability of high quality food for herbivores. To a lesser extent, NDVI can also be used to assess seasonal change in plant biomass. This study provides additional support for the use of broad-scale satellite-derived NDVI to assess seasonal changes in habitat quality for herbivores.