LAUSR

Abstract We studied changes in a population of saguaro (Carnegiea gigantea) cacti on a 259 ha plot in the northern Sonoran Desert (Saguaro National Park) that was first surveyed in 1941. With the help of citizen scientists, we mapped and measured the height of all detected individual saguaros. The total number of detected saguaros was 13,304 in 1941 and 9023 in 2012, a 31.4% decrease. The population in 1941 consisted of mostly larger, older individuals, whereas in 2012 it was dominated by smaller, younger individuals. Based on estimated years of saguaro establishment, a period of high establishment in the mid-late 1800s was followed by many decades of poor establishment. However, establishment surged from the 1970s to early 1990s but has been very low since. Comparisons with other long-term saguaro studies suggest that the dramatic demographic changes in this area have resulted from climatic factors, most notably recent long-term drought and higher temperatures, in combination with land-use changes, especially loss of nurse trees due to wood-cutting early in the twentieth century.