Climate change and the saguaro cactus: Saguaro National Park

Swann, Don; Conver, Joshua; Winkler, Daniel; Harper, Carolyn

The iconic saguaro cactus in the desert Southwest is frequently evoked as a symbol of climate change, and Saguaro National Park (NP) has been rated as one of America’s national parks most imperiled by climate change. Establishment of new saguaros at the park has been relatively low since the early 1990s, a period when temperatures in the Sonoran Desert began rising dramatically and the area entered a long-term drought. We explored the relationship between climate and saguaros through a large citizen science effort, the Centennial Saguaro Survey (CSS), and by resampling long-term saguaro monitoring plots, some of which date back to the 1940s. An important secondary goal was to communicate climate change and the status of the saguaro in the Sonoran Desert through social media, video, and outreach to a diversity of audiences ranging from underserved high school students to the scientific community and the general public. During 2015 and 2016, more than 475 citizen scientists donated 3,000 hours working with student interns to collect data on 38 historic study plots established across a range of elevations in Saguaro NP. Citizen scientists measured approximately 10,000 individual saguaros during this study. We also initiated a long-term study on climate and saguaro growth rates at Saguaro NP and resampled historic saguaro plots at two other national parks, Organ Pipe Cactus National Monument (NM) and Tonto NM. In addition, we worked with academic partners to analyze data from the more than 75 years of saguaro monitoring to provide insight into how saguaros have responded to climate and other factors in the past. Additionally, we supported studies on climate and saguaro nurse trees and climate and saguaro flowering phenology. The results confirm that although the population of saguaros in Saguaro NP is quite healthy, establishment of young saguaros has nearly ceased since the early 1990s in nearly all habitats. Of the nearly 10,000 saguaros surveyed, only 70 were less than 0.1 meters in height, or less than 11–15 years old. We found young saguaros almost exclusively in rocky foothill habitats, consistent with the relatively evenaged stand structure observed in these areas. Establishment of saguaros in Saguaro NP in the past appears to be strongly influenced not only by human activities such as woodcutting and cattle grazing, but also by climatic factors. The effects of the current extended drought (>20 years) appears to be exacerbated by higher temperatures, which dry out soils and increase mortality of saguaros that are too small to store sufficient water. In rocky foothills, local microhabitat features such as slope, aspect, soil type, and features that affect the accumulation of water may slightly mitigate these factors. Results from Organ Pipe Cactus and Tonto national monuments suggest variation across the region, as Tonto NM has experienced regular establishment for the past few decades, while establishment at Organ Pipe NM has been more episodic. Evidence also suggests that saguaros survive for longer periods at Organ Pipe NM because of the relative absence of extended, extreme freezing events, whereas, a major freeze in 2011 killed saguaros at Tonto NM and Saguaro NP. Our results will be broadly applicable to other desert areas for predicting how the saguaro and other long-lived desert species may respond to anticipated climate change. Contrary to some models, saguaro survival in a changing climate may not be influenced as strongly by elevation as by microhabitat features not necessarily associated with elevation, such as distribution and type of bedrock, which in turn influences local hydrology and vegetation. We suggest that future conservation and management strategies be based on evaluation of the relative value of different habitats for saguaro recruitment and survival.

Climate change and the saguaro cactus: Saguaro National Park

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