Subtle changes in vegetation are sometimes difficult for us to see when just looking at the imagery. The Normalized Difference Vegetation Index (NDVI) uses the difference in Landsat’s red and near-infrared reflectivity to assess vegetation health.
The graph below shows how NDVI significantly decreases in two areas of disturbed vegetation (red symbols) relative to undisturbed sites (green symbols) within the oyamel forest. The site locations are shown on high-resolution imagery in the second figure. The NDVI data can help determine if a given area is suitable to monarch overwintering.
NDVI analysis done by Birgit Peterson, scientist with ASRC Federal InuTeq, contractor to the USGS EROS Center.