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A new study finds that severely hot and dry weather dramatically increased saguaro mortality at two ends of the Sonoran Desert in 2020 and 2021 and that generally, the health of saguaros and other desert vegetation has declined significantly. The continued warming and drying threatens to irretrievably reduce the scale of and, in some cases, possibly eliminate the large saguaro forests, the researchers say.

“It is important that we understand the challenges the saguaros and us are facing,â€� said the study’s lead author, Ben Wilder. “I have doubts that the cactus forests I grew up among around ÃÛèÖÖ±²¥ will still be called forests when my son is my age.â€�

“However, the saguaros of ÃÛèÖÖ±²¥ and Sonora and the giant cacti of Mexico will still be here, and the beauty of the desert will reveal itself in new ways,â€� said Wilder, director of the nonprofit Next Generation Sonoran Desert Researchers and a longtime Sonoran Desert researcher based in ÃÛèÖÖ±²¥.

A walk through the Desert Botanical Gardens in the heart of Phoenix gives a clear, graphic picture of the trend. As always, it takes you past plenty of healthy saguaros, with multiple arms, thick skins and vibrant, bright green color.

But you’re also far more likely to see saguaros under stress, dead or dying than if you had walked the same routes through the gardens five years ago. Some saguaros are leaning over. Some are drooping. Some have shed arms or have collapsed entirely.

Extreme heat and drought have since 2020 killed off about 200 of the 1,000 saguaros that once lived at the 160-acre botanical gardens, officials there say. The annual mortality rate for the iconic cacti was about 1% to 2% prior to the onset of the early 2020s drought; it soared to 7% in 2023 — Phoenix’s second-hottest year on record — before dropping to a little more than 3% last year.

“It’s not sustainable,� Kevin Hultine, a Botanical Gardens official who is one of the study’s lead authors, said of the declining populations as he drove a reporter past numerous examples of slumping and collapsing cacti. “It’s not sustainable without bringing in new plants, and we’ve brought in 40 in the last two years.�

What happened to saguaros at the Desert Botanical Gardens since 2020 matches what happened to five giant cacti species, including the cardon and the organ pipe, in the state of Baja California Sur over the same period, the new study found. There, scientists have concluded that average annual mortality rates for those and other cacti have hit 9.9%.

These and other findings Using both satellite imagery and on-the-ground visits, the researchers examined individual plants, landscapes occupying a particular desert region, and entire cactus populations.

The study found that “long-lived succulent plant species are suffering extreme stress that is causing increased mortality that has the potential to restructure communities throughout the Sonoran Desert.�

Put more simply, it concluded that the stresses and mortalities found among the large cacti in areas such as the botanical gardens and Baja California Sur could well reshape the way our desert looks over the next few decades or longer if the current climate trends continue.

For the very short term, researchers are concerned that if the Southwest’s current extreme dry spell extends into the summer monsoon season, that could spell more stresses and possible deaths for saguaros even in more temperate ÃÛèÖÖ±²¥, which is higher in elevation, slightly cooler and more moist than the deserts of Phoenix and Baja California. Fortunately, the current forecast is for a good monsoon season in summer 2025.

ÃÛèÖÖ±²¥ area not as vulnerable 

Scientists who worked on this study and other cactus experts who were interviewed aren’t forecasting flat-out extinction of the saguaro. It serves as both a nurse plant for other, smaller and less hardy species as well as a symbol of the Sonoran Desert’s resilience and splendor.

But they note that while their study and other research has found that juvenile saguaro populations are remarkably healthy and stable, the older saguaros have suffered the most during the region’s current hot, dry period.

And as has been repeatedly reported across the desert, the recruitment of baby saguaros has dropped dramatically and in some cases is very limited if not nonexistent, putting a squeeze on the overall population at both ends.

Hultine at the botanical gardens was quick to note that while ÃÛèÖÖ±²¥ has experienced the same kind of extreme heat as Phoenix since 2020 — with slightly lower average temperatures — it had a bad monsoon in that period only during the 2020 “nonsoonâ€� season and has had normally good monsoon seasons most years since.

There have been no reports of major saguaro mortalities or population declines from the ÃÛèÖÖ±²¥ area in this decade. ÃÛèÖÖ±²¥ is seen by many experts as less vulnerable to extreme heat and drought events than Phoenix and other Sonoran Desert cities because it is at a higher elevation, is cooler and typically gets a better monsoon.

But Hultine also said, “Phoenix is the future of the rest of the Sonoran Desert if the heat and drought continues. My fear is that if we see these declines in the Phoenix valley, it’s almost certain that they will occur elsewhere.�

Most of the saguaros at the botanical gardens are transplants and that probably did contribute to their demise, he said.

Gardens officials don’t have exact age data for all the dead saguaros, “but the majority of those that failed were 12 feet or more tall, likely making them 70-plus years old. Some were likely over 100 years old.� They were transplanted to the gardens as far back as the 1950s, with some being transplanted in the 1980s and more recently, he said.

But the fact that so many of the dead saguaros were transplants doesn’t refute the point that these deaths could foreshadow still more if the climate keeps warming and drying, he said.

“Because we have data that goes back many years, the condition of plants before the recent heat waves showed the plants here were thriving,� Hultine said.

Lead study author Wilder added, “We’re seeing stress results (in cacti) across the landscape. In conversations with other scientists who are not part of the study, they say they are seeing similar things.�

‘Cactus scorching’

The genesis of the study was the extreme heat-drought damage to cacti seen on Baja California Sur, the southernmost state on the Baja peninsula.

“Our friends in Baja California Sur started reporting extreme yellowing of the stems of pitaya dulce (the organ pipe cactus),� recalled Wilder. “We saw these pictures and decided we needed to see what was going on. What we found was a landscape level stress response we believed was directly related to the 2020-2021 extreme event.�

Besides the extraordinarily high mortality rate, the researchers also found widespread epidermal decay among that region’s tall, columnar cacti. It was shown by yellowing of organ pipe; graying in stenocereus gummosus, commonly known as sour pitaya; and browning in the cardon, the tallest known cactus in the world, the researchers found. Decomposing and dying saguaros in other parts of the desert also showed browning skin.

The researchers came to call this phenomenon “cactus scorching�, a rapid discoloration of cactus skin during periods of extreme heat and drought that leads to permanent photosynthetic dysfunction. They found the scorching was scientifically associated with a three-fold increase in giant cactus mortality following the 2020-21 period of extreme heat and drought.

“That discoloration is not recoverable. Just like you or me, a cactus has its skin for life, it does not have leaves that it can drop and regrow,� Wilder said. “If the cactus scorching covers too much of the plant’s body, it can no longer produce the resources it needs to live. In many cases, the plant can produce new stems and weather the storm, though we are seeing many cases when they cannot.�

They were also surprised by the discovery that the large columnar cacti such as saguaro, organ pipe and cardon along with large trees such as the boojum were the desert plants most deeply damaged by the extreme hot and dry weather, said another researcher on the study, Exequiel Ezcurra, an ecology professor in the Department of Botany and Plant Sciences at University of California-Riverside.

“A lot of people wonder with climate change, things are going to be hotter and drier, what will be most affected? Until we did this study, there didn’t seem to be any clarity as to which life forms will suffer the most when warmer conditions prevail,� he said.

“The reason seems to me that any living things have their own weak point, an Achilles tendon,� Ezcurra said.

With cacti, a weak point is that they don’t have leaves and must do their essential task of photosynthesis through their skin, he said. Trees carry out photosynthesis by shedding leaves and producing new ones. But with cacti, any damage to the skin will stay there forever, he said.

“It will end up killing the plant because they cannot regenerate that,� Ezcurra said of the cactus’ epidermis, or skin. “It was very, very warm and very, very dry in 2020 and 2021.

“The plants lost their ability to do photosynthesis, started to decay and in a couple of years they died.�

While cacti are “wonderfully adapted to the desert,� the shallow rooted species reach hard limits when temperatures get as high as we are starting to see and there is no water, said researcher Hultine, a plant physiologist. “What we are seeing is that the photosynthetic system of the plants shuts down. . .when high temperatures exceed thermal protection the plant is normally able to provide. The water that is normally available in these succulent plans has been baked out of them and is not there.�

‘Unprecedented’ spread of extreme heat

The extreme heat and drought of the 2020-21 period not only set or tied records for high temperatures and low precipitation, it spread to unprecedented distances, Ezcurra said. It ranged from 23 degrees latitude, which includes Baja California, parts of Mexico and Cuba, to 38 degrees latitude, which crosses California’s Napa Valley.

“It was a huge expanse of almost 1,500 miles of drylands,� Ezcurra said. “We’ve had locally strong droughts, but a generalized drought of that magnitude, we realized it had never happened. It was a huge anomaly on a regional level.�

Plus, the researchers analyzed temperature trends since 1980 at all weather stations in the Sonoran Desert from Las Vegas south to Cabo San Lucas in Baja, and found that mean summer temperatures have risen at the rate of about 1 degree per decade into 2025.

Ezcurra and Wilder agree that human-caused climate change lies at the root of the extreme weather pattern that led to the deaths and severe damage to saguaros and other cacti.

“The correlation between dramatic increases in temperature and the human induced climate change is extremely strong,� Wilder said. “The large amount of independent studies globally all point to them being connected.

“The increased drought events happening are also very strongly connected to climate change,� he said.