In Phoenix, the summer of 2020 was so hot even the cacti couldn’t stand it — literally.
In the weeks after August, the hottest month in the hottest summer on record in a place already famous for its blistering heat, the Desert Botanical Garden was beset by calls from worried residents.
The saguaros were falling over.
Some had tipped entirely, the thick concertina trunks slamming onto sizzling sidewalks or, in at least one case, a house. Other saguaros offered a less dramatic — though still concerning — manifestation of their internal stress: dropping one of their iconic curved arms.
“We always expect to lose some saguaros,” said Tania Hernandez, a Desert Botanical Garden research scientist. “But people felt that this was not normal.”
It was not normal. In fact, it was a health crisis. And it struck a chord in Phoenix, where people take their saguaros very seriously. The famous cacti are unique to the Sonoran Desert and grow almost exclusively in Arizona and Sonora, Mexico.
“For Arizonans, this is a plant highly tied to our identity,” Hernandez said. “People really love this plant. They care for it.”
Now Hernandez hopes to harness that enthusiasm in a community science project with an ambitious aim: Find every saguaro in metro Phoenix. Dubbed the Saguaro Census, the count is borne of that terrible time when the urban plants were dropping left, right and center.
“Everybody felt that this was somehow related to climate change,” Hernandez said. “That’s what our intuition tells us.
“But the truth is that we don’t have enough information to understand what’s going on,” she said. “We don’t even know how many saguaros we have in the cities. We don’t know where they are, how healthy they are. We don’t know how many we are losing every year.”
And that’s where you come in. But first, some background.
Iconic Saguaro cacti:How fast they grow, how big they get and can you cut one?
Why are saguaros so stressed?
The Sonoran Desert is a perfect nursery for baby saguaros. The heat is the right temperature, and the annual monsoon rains provide the right amount of moisture. Existing plants — mesquites and palo verde — act as “nurse trees” for budding saguaro giants.
“It’s a life cycle for saguaros,” Hernandez said. “They receive protection from the sun and the temperatures from the nurse tree. They keep growing, growing, growing at a point that they outgrow the tree and the tree dies and the saguaro thrives.”
For both young and old cacti, she says, the Sonoran Desert is “the perfect framework for saguaros to grow and to survive and to live.”
But in the heat island of Phoenix, life is different. There is concrete. Air pollution. Higher temperatures. Saguaros are more likely to have been transplanted, rather than taken root in the wild.
Add in 2020’s “nonsoon,” the driest summer rain season on record, and the mass saguaro fall starts to make sense.
While calls about falling cacti have slowed, Hernandez said, there are still regular reports of sick saguaros.
Some are too thin. Others are not flowering as they should. Many are suffering from bacterial necrosis, its telltale ugly gashes marring trunks that were once a healthy, rubbery green.
Not all damage to cacti is caused by illness. Squadrons of hungry javelinas sometimes munch on the saguaros. There are instances of vandalism, though the census thus far suggests these are rare.
Hernandez believes the sick plants may be in a state of high distress from their surrounding environment.
“Their immune system, because plants do have immune systems, may be lower, may be weak, because the plant is dealing with this great stress, pollution, high temperatures,” she said, “and the plants might not be able to heal as if they were healthy.”
Saguaros:Will the iconic cacti start to disappear from parts of the Southwest?
Finding saguaro “twins” could be the key
For better or worse, genetic testing companies like 23andMe have built enormous databases from people willing to hand over their DNA.
Now Hernandez wants to do the same for saguaros.
The census won’t stop at documenting the number, location and condition of saguaros dotted across metro Phoenix. Part of the project will involve obtaining DNA from urban saguaros in a bid to trace their origins.
Most of the saguaros in urban Phoenix have been transplanted from wild areas long forgotten, Hernandez said. “These plants are so old that in most cases people have no idea where the plant came from.”
Hernandez and her team have already taken DNA samples from saguaros spanning from north of Phoenix to south Sonora, and plan to sequence them.
This summer, they will sample city-dwelling saguaros in the hope of finding a match. Hernandez is looking for people with saguaros on their property happy to volunteer the cacti for sampling.
“We want to identify the closest relative of urban plants to be able to compare. How are they coping with their high stress in the city, in comparison with a sister plant in the wild?” she said.
“It’s a similar approach as when they compare twins who grow up in different conditions.”
Saguaros growing in urban Phoenix are a huge, naturally-occurring experiment, Hernandez said, one that would take significant time and money to replicate.
“If we were to study the effect of climate change on saguaro plants artificially, we would need to bring plants from the wild, put them in a very special, high tech, greenhouse, and grow them there for years. Increase the temperature with a lot of energy,” she said. “However, we have them naturally here in the city.”
This accidental experiment could result in two major benefits.
One, take a slow-growing urban cactus. Its sluggish progress could be from high temperatures driven by climate change, say, or urban pollution. Or it could just be genetic. In isolation, there’s no way to know. But finding a sister plant could unlock the answer.
Two, it could save metro Phoenix’s saguaros. “Eventually, we would like to propose that we could adapt our urban saguaro population,” Hernandez said. “We can adapt it for future conditions by bringing here saguaros that are already adapted to the driest and hottest conditions in nature.”
But we need to start now.
“Because we need to remember that these plants take many, many years to grow. So if we don’t start now, our children or grandchildren are not going to see saguaros in the city.”
How can you help?
Taking part in the saguaro census is easy.
First, download the iNaturalist app. The free app, developed by National Geographic and the California Academy of Sciences, allows the public to observe and upload information about living things around them to fellow nature-lovers and biologists across the world.
Second, make an account and search for the Metro Phoenix Saguaro Census project.
Third, document any saguaros you see in urban Phoenix.
“You take a picture of the saguaro, even if the picture is not good, even if you are in your car and passing by, that’s okay, because we want to generate a database of saguaros in the city,” Hernandez said.
“If you didn’t have a good look, such as to assess the health of the plant or the size of the plant, that’s okay. We can go back and take more data on that same plant,” she said.
“But the important thing is to know how many we have and where they are.”
The best thing about saguaros
Hernandez completed her Ph.D in cactus evolution in Mexico, home to the greatest diversity of cacti in the world.
To her, the best thing about saguaros is their capacity to inspire.
“Culturally, this is a very important species. It is not endangered in nature, it is quite successful, abundant, it is not threatened,” she said.
“Some cacti species are more endangered than the panda bear or the white rhino. But people don’t know about this.
“What I like about saguaro is that this plant, this cacti, is so important to people that we can use it to (raise) awareness about the conservation of other cacti.”
The Saguaro Census
The Saguaro Census is running through May 2022. For more details about how you can get involved visit dbg.org/saguarocensus2022.
People interested in volunteering saguaros on their property for further study can contact Dr Tania Hernandez at [email protected]