Resurvey of saguaro monitoring plots in Saguaro National Park following the 2011 freeze event

Conver, Joshua; Swann, Don; O’Brien, Kara; Seeton, Valerie

After many decades of decline, the saguaro cactus (Carnegiea gigantea) population in Saguaro National Park (SNP) began to rebound in the 1960s, and recruitment surged in both the eastern Rincon Mountain District (RMD) and western Tucson Mountain District (TMD) through a wet period that ended in the early 1990s. Since then, surveys on long-term plots in the northwestern “Cactus Forest” area of RMD have detected very few young saguaros, suggesting that recruitment has again declined. Changes in recruitment and survival may be related to environmental factors, including higher temperatures, drought, and freeze events, the latest occurring in 2011. We monitored saguaros in both park districts in 2010 and 2014 to better understand recent changes in saguaro demographics across all habitats. We used two sets of data; 1) 45 randomly-located plots established in 1990 and last surveyed in 2010, and 2) 11 plots representative of typical saguaro habitat established in 1941 and 1975 and last surveyed in 2014. We categorized these plots by district and habitat related to the geographic setting (bajada, foothill, and slope) and used generalized growth models based on data from each district to estimate year of germination. Our results indicate that saguaro recruitment has slowed dramatically throughout Saguaro National Park since the early- to mid- 1990s. Establishment has been very low in bajada and foothill areas, and there has been only limited recruitment on slopes. In addition, mortality of older saguaros was very high throughout the park during 2010-2014, suggesting that the 2011 freeze had a widespread effect.