Giant. Majestic. Old. And, brown? Deep in the Sonoran Desert near Gila Bend, Arizona, the giant saguaro cactuses thrive. Or they once did.
Driving along the 8-Interstate, visitors pass miles of saguaro forests, where these towering cactuses dominate the rocky and arid landscape. In past decades, the saguaro is a green stripe in the surrounding brown region.
In spring of 2017, however, these cactuses were brown – or in the process of turning brown. It creeps up from the bottom, where pack rats and beetles chew up the base of the cactus, giving it a splotchy look. This brown, however, is not caused by the native animals of the desert.
“There has been research from Saguaro National Park on epidermal browning.” said Joshua Conver, a Ph.D. student at the University of Cincinnati. “Nobody really knows what causes it.”
Much is unknown about the saguaro cactus, including the reasons behind the browning. It could be caused by pack rats, beetles and woodpeckers, but it could also be caused by disease, frost, drought or sunburn.
“A lot of saguaro research as a whole comes with some caveats,” Conver said. “Because the species is so small when it’s young, we’re always chasing the current state of the population. Much of the saguaro research, like saguaro geography and populations numbers, is about on a ten year lag. When you go out to do field research on saguaros, we find about half of the ones that are ten years old because they are so small.”
From the saguaros that are ten years old and over, researchers are able to deduce that sunburn is most likely the cause of the saguaros browning.
“There’s theories that the spines create micro-shadows on the surface of the saguaro and it reduces temperature,” Conver said. “They’re used to diffuse incoming light energy and spread the energy.”
On-going drought in the region along with a warmer winter and more sunshine causes the spines of the cactus to fall off. It also makes the cactus more vulnerable to sunburn, thus the browning.
“The drier the site, the higher probability for browning,” Conver continues. “So if the site it wetter, the roots of the saguaro can draw in excess water and the saguaro won’t get that burn to it – it’s able to compensate.”
Conver explained that the range along the 8-Interstate, towards Yuma, is the drier and hotter region of the Sonoran Desert, so “it would make sense” that there is a higher rate of browning in that area.
A probable reason behind the entire saguaro forest browning at the same time is because the saguaro cactuses grow in pulses, according to Benjamin Wilder, interim director of Tumamoc Hill Research Center.
“You have even-aged cohorts of saguaros,” Wilder said, “so when you look at a landscape you often see that the saguaros are more or less the same height. It has to be a very specific set of conditions for the seeds to germinate and then for the babies to actually establish, and those are pretty rare. They honestly occur only every couple of decades.”
In order for these conditions to be perfect, there has to be moisture, cooler temperatures with less chance for sunburn or frost, and there has to be enough nurse trees in the area. Nurse trees are often Palo Verde’s or ironwoods, or any shady tree in the region.
“They create a microclimate underneath, ‘facilitation’ is the ecological term,” Wilder said. “They create this area where it’s a little cooler. You get some more precipitation from accumulated runoff from the tree and more shade and on really cold nights it’s the opposite – you don’t get the harsh freezes. The saguaros establish under those.”
But with prolonged drought and a hotter, drier climate, it is difficult for juvenile saguaros to survive.
“What I have noticed is that temperatures are rising,” Conver said, “so that’s adding moisture stress and it’s limiting the ability of the juveniles to survive from year one to ten in their life span. If there’s not water and they don’t have the capacity to store water to survive, then they can desiccate like any other seedling.”
The cause of the prolonged drought, warmer winters and hotter summers? Climate change.
“We have increasing heat waves across the whole Southwest,” said Daniel Ferguson, director of the Climate Assessment of the Southwest (CLIMAS) research group at the University of Arizona. “You see more clusters of hot days close together, few cold snaps, that kind of stuff.”
Changing seasonality effects not only saguaro cactuses – they affect every native plant in the region.
“We see plants getting started early and some of the native species not being as well suited to an earlier warm season or precipitation timing,” said Ben McMahan, researcher and outreach coordinator with CLIMAS.
The effects of climate change in the Sonoran Desert are not limited to the browning phenomena amongst the saguaro cactuses: it affects every living organism on this planet. The impacts of climate change throughout the world are already visible but are difficult to reverse.
“I don’t think that there is any way to prevent it,” Conver said.
As this region of the Sonoran Desert grows drier and hotter, there will be more moisture stress on natural flora and more saguaro cactuses will brown.
The saguaro cactus has been an icon for the southwestern United States since the first eastern settlers arrived in the area. Its majesty is awe-inspiring to visitors from around the globe and the number of saguaros found in the forests are astounding. So what will happen to these historic saguaros as the world heats?
“I, personally, don’t worry about the saguaro going away from the landscape,” Conver said. “Because this species lives so long, it’s able to wait out the bad periods and then reproduce during the good periods.”
The saguaro cactus outlives a human by at least 50 years. It can wait for humans to figure out how to save the world. It may brown, it may adapt to the new climate, but it is unlikely to disappear forever.
15,000 years ago the Sonoran Desert rearranged itself to what it is now, Wilder explained. Today, citizens living in the desert will see a continuous rearranging of the desert as climates change.
“The desert can be a profusion of life,” Wilder mused.
Thank you for reading! Have you ever traveled to the American southwest and seen a saguaro cactus? Have you ever seen saguaro cactuses with this condition?
Also! What are your thoughts on the use of “cactuses” versus “cacti?” AP Style rules that the plural of cactus is cactuses!
This story was originally written in the spring of 2017.