The Saguaros of "Section 17" in Saguaro National Park: Re-survey of a One-Mile-Square Section First Surveyed in 1941

Conver, Joshua; Weber, Irene; Foley, Theresa; Swann, Don; MacEwen, Becky; Fajardo, Emma; Gamez, Yesenia

We re-surveyed all saguaro cacti (Carnegiea gigantea) in a 1-square mile section (section 17, R16E, T14S) in the Rincon Mountain District, Saguaro National Park, that was first surveyed in 1941. This survey was a “citizen science” project initiated during the 2011 BioBlitz at the park. Student interns trained and supervised more than 300 middle and high school students and adult volunteers during October 2011 through July 2012. We systematically searched 64 4-hectare subplots, measured each saguaro detected, and recorded its coordinates, nurse tree, and the number of bird holes and arms. In 1941-1942, researchers at the park measured 13,304 saguaros, with 80% being > 3.66 m (12 ft) tall. In 2011-2012, we measured 9,034 saguaros, with 80% being < 3.66 m tall. In general, the saguaro population in 1941 was dominated by large individuals estimated to be more than 50 years of age, whereas today the population is dominated by younger individuals that have germinated since 1960. Our results complement and are consistent with a long-term study of six smaller plots surveyed annually within Section 17 for every year since 1941. They indicate that the saguaro population in the Cactus Forest, which was in decline for many decades before and after the establishment of Saguaro National Monument in 1933, has re-bounded due to a surge in establishment of young saguaros during the period from the 1960s through the early 1990s. However, establishment of new saguaros has all but ceased since the mid-1990s, presumably in response to drought conditions.

The Saguaros of "Section 17" in Saguaro National Park: Re-survey of a One-Mile-Square Section First Surveyed in 1941

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