Mapping saguaro cacti using digital aerial imagery in Saguaro National Park

Abstract

Population research on the saguaro cactus (Carnegiea gigantea) has historically been limited to repeat monitoring efforts in small plots with areas of 1 to 4 hectares. Large extent saguaro censuses of areas >2.59 km2 have not been undertaken. This research developed an automated shadow detection method for mapping mature saguaros in digital aerial imagery using saguaro shadows as proxies for saguaro locations. The shadow method detected 446,092 saguaro shadows in aerial imagery of 231 km2 of the Saguaro National Park (SNP) near Tucson, Arizona. The results were validated against saguaro location data in 11 small plots provided by SNP staff. The shadow method correctly identified 58% of mature saguaros in these 11 plots. Twenty-one percent of the saguaros in the plots did not produce a shadow signature in the imagery. Fieldwork was conducted in two plot locations to investigate reasons why these mature saguaros failed to produce shadow signatures in the aerial imagery, which showed that vegetation interference, rocky slopes, and shadow alignments were the primary factors that precluded shadow signatures from appearing in the imagery. This research provides a methodology for automated large-area saguaro mapping, which will be useful in long-term population monitoring and saguaro ecology research.