In Phoenix, the summer of 2020 was so hot that even the cacti couldn’t handle it – literally.
In the weeks following August, the hottest summer hottest month on record in a place already notorious for its scorching heat, the Desert Botanical Garden was beset by calls from concerned residents .
The saguaros were falling.
Some had completely toppled over, the thick concertina trunks slamming on the 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 signature curved arms.
“We still expect to lose saguaros,” said Tania Hernandez, researcher at the Desert Botanical Garden. “But people felt that 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 strongly tied to our identity,” Hernandez said. “People really like this plant. They care.
Now, Hernandez hopes to harness that enthusiasm in a community science project with an ambitious goal: find all the saguaros in the Phoenix metro. Dubbed the Saguaro Census, the tally was born out of that terrible time when urban plants fell left, right, and center.
“Everyone felt that it was somehow related to climate change,” Hernandez said. “That’s what our intuition tells us.
“But the truth is, 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 lose each year.
And that’s where you come in. But first, a little background.
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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 humidity. The existing plants – mesquites and palo verdes – act as “nursing trees” for the budding saguaro giants.
“It’s a life cycle for saguaros,” Hernandez said. “They are protected from the sun and the temperatures thanks to the nourishing tree. They keep growing, growing, growing to a point where they outgrow the tree and the tree dies and the saguaro thrives.
For cacti young and old, she says, the Sonoran Desert is “the perfect setting for saguaros to grow, survive, and 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 than to take root in the wild.
Add to that the “nonson” of 2020, the driest summer rainy season on record, and the massive saguaro drop starts to make sense.
While the calls about the cactus dropping have slowed, Hernandez said, there are still regular reports of sick saguaros.
Some are too thin. Others don’t bloom as they should. Many suffer from bacterial necrosis, its telltale ugly gashes marking trunks that were once a healthy, rubbery green.
Not all damage to cacti is caused by disease. Hungry javelin squads occasionally nibble on saguaros. There are instances of vandalism, although the census so far suggests they are rare.
Hernandez thinks sick plants can be in a state of great distress because of their environment.
“Their immune system, because plants have an immune system, may be weaker, may be weak, because the plant is facing this great stress, pollution, high temperatures,” she said, ” and the plants might not be able to heal for if they were healthy.
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Finding saguaro ‘twins’ could be the key
For better or worse, genetic testing companies like 23andMe have built huge databases of people willing to hand over their DNA.
Now Hernandez wants to do the same for saguaros.
The census will not stop at documenting the number, location and condition of the saguaros scattered throughout the Phoenix metro. Part of the project will involve obtaining DNA from urban saguaros in an effort to trace their origins.
Most of the saguaros in the city of Phoenix were transplanted from long-forgotten wild areas, Hernandez said. “These plants are so old that in most cases people have no idea where the plant came from.”
Hernandez and his team have already taken DNA samples from saguaros ranging from northern Phoenix to southern Sonora and plan to sequence them.
This summer, they’ll sample city saguaros in hopes of finding a mate. Hernandez is looking for people with saguaros on their property who are happy to offer the cacti for sampling.
“We want to identify the closest relative of urban plants so we can compare. How do they cope with their high stress in the city, compared to a sister plant in the wild? she says.
“It’s a similar approach to comparing twins who grow up in different conditions.”
The saguaros growing in urban areas of Phoenix are a huge natural experiment, Hernandez said, that would take a lot of time and money to replicate.
“If we were to artificially study the effect of climate change on saguaro plants, we would need to bring wild plants, put them in a very special, high-tech greenhouse, and grow them there for years. Raise the temperature with a lot of energy,” she said. “However, we naturally have them here in the city.”
This accidental experience could have two major benefits.
First, take a slow-growing urban cactus. Its slow progression could be due to high temperatures driven by climate change, for example, or urban pollution. Or it could just be genetic. In isolation, there is no way of knowing. But finding a sister plant could unlock the answer.
Second, it could save the Phoenix Metro saguaros. “Ultimately, we’d like to propose that we can adapt our urban saguaro population,” Hernandez said. “We can adapt it to future conditions by bringing saguaros here that are already adapted to nature’s driest and hottest conditions.”
But we have to start now.
“Because we have to remember that these plants take many, many years to grow. So if we don’t start now, our children or grandchildren won’t see saguaros in the city. »
How you can help
Participating 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 the living things around them to other nature lovers and biologists around the world.
Second, create an account and search for the Metro Phoenix Saguaro Census project.
Third, document any saguaros you see in the city of Phoenix.
“You take a picture of the saguaro, even if the picture is not good, even if you’re in your car and passing by, that’s fine, because we want to generate a database of saguaros in the city,” said Hernández.
“If you haven’t looked closely, like 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 how many we have and where they are.”
The best thing about saguaros
Hernandez completed her doctorate in cactus evolution in Mexico, home to the greatest diversity of cacti in the world.
For her, the best thing about saguaros is their ability to inspire.
“Culturally, it is a very important species. It’s not endangered in the wild, it’s quite successful, abundant, it’s not threatened,” she said.
“Some cactus species are more endangered than the panda or the white rhinoceros. But people don’t know that.
“What I love about saguaro is that this plant, this cactus, 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 on how you can get involved, visit dbg.org/saguarocensus2022.
Those interested in nominating saguaros on their property for further study can contact Dr. Tania Hernandez at [email protected]