Saguaros in a warming desert: what drought and heat are changing at Saguaro National Park

A 1935 photograph of the Cactus Forest in the Rincon Mountain District of Saguaro National Park, Arizona. NPS photo.

Saguaros can live 150 to 200 years, which makes them an unlikely subject for studying climate change. But park scientists say climate shapes how new saguaros establish, grow and reproduce, and the picture is changing. This article, published in Intermountain Park Science in 2021, was written by Don E. Swann (biologist, Saguaro National Park, National Park Service), Daniel E. Winkler (research ecologist, Southwest Biological Science Center, U.S. Geological Survey), Joshua L. Conver (PhD candidate, University of Cincinnati) and Theresa Foley (climate science specialist, Sonora Environmental Research Institute, Inc.).

A slow-growing icon

The saguaro (Carnegiea gigantea) is a symbol of the desert Southwest and the namesake of Saguaro National Park, created in 1933, initially as a national monument, to study, interpret and protect it. Young saguaros grow very slowly and reach reproductive age only at 35 to 65 years. They bloom at night with white flowers from April through June, followed by large red fruits that many desert animals eat. The fruit is also a traditional food of the Tohono O’odham people, and its harvest is an important part of their culture. Mature plants produce thousands of seeds a year, but seedlings survive only in favorable periods of several consecutive years of cooler, wetter weather.

Height gives a fair estimate of age. In the park’s west district a saguaro is about 4 inches tall at 15 years and about 7 feet at 50. That lets researchers read when plants established, and how land use and climate affected them.

Two photographs, one view

Two photographs from the same vantage point show how much the Cactus Forest changed. In 1935 the landscape was dominated by large, mature saguaros that had established in the mid-1800s. Protective “nurse trees,” mostly foothills paloverde and mesquite, were largely absent because they had been heavily harvested over the previous five decades. Without them, few young saguaros survived the summer heat and winter cold. By 1998 most of the older saguaros had died, but under the Park Service’s protection the nurse trees had returned.

The same view in 1998, with fewer mature saguaros and more nurse trees. Photo USGS/Raymond Turner.

What the 1998 photo cannot show is young saguaros growing slowly beneath those trees. New establishment was also helped by the end of cattle grazing and by favorable weather, especially in the 1980s.

Nurse trees such as foothills paloverde protect saguaro seedlings from the Sonoran Desert’s extreme heat and cold. NPS photo.

What a warmer, drier climate does

The Sonoran Desert entered a prolonged drought in the mid-1990s, and human contributions to climate change made it more severe. Adult saguaros store large amounts of water and are very drought resistant, but seedlings need moist soil for their first few years. Many studies link new saguaros to periods that were cooler and wetter. A 2018 study by Winkler and colleagues, based on 10-year surveys on randomized plots, found that establishment parkwide is strongly and positively correlated with the Palmer Drought Severity Index, which runs from -6 (dry) to +6 (wet): when it falls below -2, few seedlings survive. Establishment surged in the wet 1980s and has slowed since. Higher temperatures dry the soil, which may keep young plants from growing large enough to store the water they need. Some saguaros have lately established in very rocky areas, which suggests they may use water captured in rock cracks; the authors’ research also indicates that the cracks and shade create stable microclimates.

Saguaros live and reproduce so long that they may withstand years of low recruitment, but the authors still call the rarity of saguaros under 25 years old cause for concern that justifies continued monitoring. Average temperatures in the park have risen by approximately 2.16 °F (1.2 °C) per century, and the park has had more frequent extreme summer heat events.

Freezes, fire and moving uphill

Saguaros are subtropical plants, and below-freezing temperatures kill adults and juveniles alike. The long-term plots set up in the 1940s were designed to study freezes, and earlier researchers concluded that die-offs in the Cactus Forest in the 1940s, long blamed on disease, were probably caused by freezes in the late 1930s. Since 1978 the park has had one major freeze, in 2011. Surveys before and after it found mortality of nearly 17% among large adults (taller than 24 feet) and under 3% among younger plants (shorter than 18 feet), probably because nurse plants protected the young ones. Between 20 and 28% of saguaros taller than 18 feet showed some frost damage, and the authors say these are minimum estimates because damage can take up to nine years to show.

Might warming winters push saguaros uphill? Surveys between 4,000 and 5,000 feet in the Rincon Mountain District found no upslope movement. Instead, wildfires, probably fed by invasive grasses, are burning at lower elevations than before and into saguaro habitat, and saguaros did not evolve with fire.

Growth, flowers and buds

Growth and the age of first reproduction track summer rain. The west district gets about 2 inches less rain than the east and its saguaros grow more slowly, and in the east they gain height faster in wetter decades. A study using plots first measured in the 1970s finds that growth has slowed significantly in recent drought years, and that growth rates depend heavily on habitat, with slower growth in rocky habitats where survival may be higher. Slower growth means later reproduction, which could affect the population over time.

The Fourth National Climate Assessment found that warming is shifting the timing of events such as flowering across much of the United States. For saguaros, one study in the Santa Catalina Mountains found first flowering three days earlier over 20 years, while a study in the Rincon Mountains found flower timing and number vary widely with temperature and winter rain.

Staff and citizen science volunteers photograph saguaro flowers with a long selfie stick and tablet. NPS photo.

In a citizen science project with partners including the USA National Phenology Network, interns and volunteers photograph the tops of about 55 saguaros from a standard direction six days a week from April through July, using a camera on a 29-foot selfie stick. Early-season buds appear first on the east side of the stem, then the northeast, north, northwest and west as the season goes on. The authors note that spring 2019 was very cool compared with 2017 and 2018, and that where buds appear may prove a more precise indicator of environmental change than traditional measures.

What the authors found

• After high establishment from the 1960s to the early 1990s, establishment has slowed dramatically.
• Poor survival of young saguaros is correlated with drought and may be due to higher temperatures drying soil faster than in the past.
• It has been speculated that saguaros may move to cooler, higher elevations as winters warm, but the park has found no evidence of this yet.
• The park and its partners are still studying possible changes in reproduction and growth.

Volunteer citizen scientists in the 2010 saguaro census at Saguaro National Park. NPS photo.

Citizen science is central to the work. The Saguaro Census has been the main monitoring program since 1990, held every ten years on large, randomly located plots. In 2010 more than 300 volunteers counted and measured more than 20,000 saguaros. The 2020 census began in October 2019 and ended just after the start of the COVID-19 pandemic. The authors close by noting that the saguaro’s slow growth, episodic recruitment and long life make it hard to forecast and also make it a fitting plant for talking about how today’s decisions play out over long periods.

Sources

• Don E. Swann (National Park Service, Saguaro National Park), Daniel E. Winkler (U.S. Geological Survey), Joshua L. Conver (University of Cincinnati) and Theresa Foley (Sonora Environmental Research Institute, Inc.), Climate Change and Other Factors Influencing the Saguaro Cactus, Intermountain Park Science, 2021, National Park Service; rewritten in hubnx's own words.