Preliminary results of a re-survey of historic plots established in 1941 in Organ Pipe Cactus National Monument

Conver, Joshua; Swann, Don

The giant saguaro (Carnegiea gigantea) cactus is an iconic species of the Sonoran Desert that has interested researchers for more than 100 years. Permanent plots have been established throughout southern Arizona and northern Sonora, Mexico, in order to investigate the long-term growth and population dynamics of the species relative to its environment (Gill and Lightle 1942; Hastings and Alcorn 1961; Steenbergh and Lowe 1977; Duriscoe and Graban 1991; Orum et al. 1999; Pierson et al. 2013). In this report, we compare current growth rates and age-height structures with findings from a resurvey of five plots established in 1941 in Organ Pipe National Monument (ORPI). These plots were founded with the purpose of keeping detailed observations about saguaro and organ pipe cacti (Lightle 1942). The results of our work will provide insight into saguaro population dynamics. In the 1940s, plant pathologists Lake S. Gill and Paul C. Lightle studied the cause of a black fluid, hypothesized to be bacterial necrosis, oozing from saguaros in Saguaro National Monument (now Saguaro National Park; SNP) and Organ Pipe National Monument. While the hypothesis was incorrect, the data from the Gill and Lightle plots showed an unbalanced, unsustainable population structure at SNP (Gill and Lightle 1942) whereas plots at ORPI showed “an abundance of young healthy plants” (Steenbergh and Lowe 1983). The ORPI plots were revisited annually from 1967 to 1977 by Warren Steenbergh and Charles Lowe in order to construct site-specific growth rate curves as part of the third volume of the Ecology of the Saguaro series (1983). ORPI Resource Management staff visited four of the five plots between 2005 and 2007 with the purpose of modernizing the dataset by collecting GPS coordinates for saguaros and plot markings. In February 2016, staff from SNP repeated the census of all five plots as part of the Centennial Saguaro Census with assistance from ORPI staff.