Monitoring Annual Aspen Defoliation Patterns by Detecting Changes in Leaf Area from Multitemporal Landsat TM Imagery

Trembling aspen is the most widely distributed and important deciduous tree species in North America. Repeated outbreaks of insect defoliation have raised the need for more precise monitoring than what aerial surveys are able to provide. A defoliation outbreak of large aspen tortrix in northern Alberta, Canada, was monitored over a 4-year time series of anniversary Landsat Thematic Mapper images from 2001 to 2004. Image models of leaf area index (LAI) based on the infrared simple ratio were related to percent defoliation, resulting in an annual percent defoliation image map from which changes from year to year could be determined. These image maps of aspen defoliation compared favorably to annual aerial surveys conducted independently, and visually compared to aerial photos taken from a survey aircraft that were spatially registered for comparison in a Google TM Earth environment. The image maps produced a similar trend in annual change of aspen defoliation compared to aerial surveys and did so with a much smaller total area of defoliation. This approach results in more precise monitoring of defoliation patterns, and could serve as a locational guide to more detailed field assessments of damage impact caused by defoliation.