Figure 1. These two photographs illustrate the change in aspen canopy coverage for the period of 1954 to 1992 on one site in Yellowstone National Park's northern range. The site is located along the Lamar River a short distance upstream from the confluence of Soda Butte Creek. The left photograph shows the site in 1954 and the right photo shows the same site in 1992. The decline in aspen canopy is clearly visible and one can observe a large number of aspen boles on the ground in the 1992 photograph. This site shows the greatest decline of all sites analyzed in the 1997 field season and is included to illustrate the potential of detecting change with matched pairs of aerial photographs.