The Cactus Forest at Saguaro National Monument in 1935/NPS

Take saguaros out of Saguaro National Park, and you’d lose more than the park’s namesake cactus, one that gives skyward structure to the vista here.

You might be left straining to catch the drumming of the Gila woodpecker, fail to catch an elf owl peering out from its cavity nest, and not know where to look for Lesser long-nosed bats. All these creatures rely on the saguaro for food and/or shelter.

Realizing those possible losses to this Sonoran Desert landscape begs the question, "How are the park’s saguaros doing?"

To try to answer that question, I headed to Tucson, Arizona, and the national park. The last time I visited Saguaro it was spring, the desert was a figurative riot of color. There were ocotillo, their orange-flowered tips glowing like fireplace pokers, and violet-bloomed Hedgehogs, patches of pink Fairy Dusters, bluish-petaled Scorpionweed, and a hillside studded with sagaruos and painted in yellow brittlebrush.

In early November, most of the color was gone, but the saguaros stood stately as usual.

At the parking lot of Saguaro's Rincon Mountain District I met Don Swann, a wildlife biologist and saguaro expert who has written or contributed to numerous papers pertaining to saguaros and other aspects of the Sonoran Desert. My timing was great, as Swann was heading into his once-a-decade saguaro census in the park.

“I think there are many answers to why we conduct the census,” he said as we walked through the entrance gate and into the park. Overhead, the sky was a cloudless blue, and already at 9 a.m. the temperature had reached 80 degrees.

“But the most basic is we feel it is our responsibility to keep track in a scientific way of the saguaro population in the park. 'Saguaro' is the name of the park, it’s the reason we’re here. It’s obviously a real important plant for the Southwestern deserts and southern Arizona,” the ranger said.

The Cactus Forest at Saguaro National Monument in 1960/NPS

The decadal surveys, which date to 1990, allow park managers to see how the saguaro population is changing across the park’s two districts.

“We’ve been able to really use that information in the last few years to see that there are differences in different parts of the park, in different habitats,” Swann explained as we walked along. “National parks are reference areas for other areas that may be damaged by things that we humans do, and so, this allows us to see how well the saguaro is doing in an area that’s being managed for natural resources.”

Saguaro cacti are fascinating plants, iconic entities that not only benefit many creatures that call the desert home, but are reliable and picturesque backdrops for more than a few cinematographic Westerns. They are tall and silent, but hardly unnoticeable.

“Saguaro is a fascinating plant for us as humans for a lot of reasons. It’s very charismatic. It’s a charismatic mega-plant, really,” Swann told me. “Some people would call it a keystone species because so many animals depend on it. That runs the gamut from the small insects that pollenate the flowers all the way to javelinas and other mammals that depend on the fruit to eat.

“The saguaro is providing food in the form of nectar in the flowers. They have these beautiful white flowers that bloom at night,” he continued. “The fruits themselves are very rich in calories and other things that are really good for animals. They provide shelter. We have elf owls, for example, that nest in holes that are constructed in saguaros by woodpeckers. The holes are constructed and the nest is built inside, then the holes can be reused by other birds in future years.”

Don Swann, Saguaro National Park/NPS

Great horned owls and Harris’s hawks will build their nests on the outstretched arms of saguaros, while the Gila woodpecker and Gilded flicker will carve out cavities that they use for one nesting season before abandoning them to other species.

“Even when they die,” adds Swann, “as the flesh is rotting, its full of these amazing insects that live in the rotting saguaro. And then the wood, what we call the saguaro ribs, lay on the desert floor for many years and provide habitat for lizards and birds even. And so it’s really contributing to the wildlife population throughout its life cycle.”

As we roamed the park landscape, it was easy to spot saguaros that, surprisingly, were quite old. These cacti don’t grow quickly. A quarter-inch tall version might already be two years old, a desert sprite overlooked amid other vegetation. That’s why it can be difficult to gauge just how many saguaros are in the park, despite the census every ten years. There are 45 plots in the park that are part of the census, and each covers about 400 hectares, or almost 1,000 acres.

For the census, volunteers from school groups, hiking clubs, businesses, University of Arizona fraternities and sororities, join park biologists and head for the 45 plots.

"We bring them out, we kind of teach them how to measure the saguaros, how to collect the data so that it’s accurate, how to search for the saguaros," explained the biologist. "We break into small groups and we do that. We check each other’s work, so we double search each area. And then we try to finish that plot with the group. But if there’s 1,800 saguaros and it’s a long hike to the plot, because they’re randomly located, it might take us four or five different groups to finish a plot. So it’s usually done on a Saturday. We also try to get as many weekday groups as possible, and we just keep chipping away at it with the goal of finishing all 45 plots by April.”

Walking along, Swann leads me over to what turns out to be a somewhat young saguaro.

“So we systematically work our way through the plot. And then when we find a saguaro, we stop, and we measure it," the biologist said as he unfolded his measuring stick. “So something like this, this stick is about 180 centimeters, so about 6 feet tall. So what I’ll do is, in this case I can measure this just by hand, so I’ll put my finger right here, at 180 and have you tell me, including the spines when I’m at the top."

While the biologist is wielding his measuring stick, he explains that since 1941 Park Service biologists have been measuring saguaros and so have been able to come up with an average annual growth rate and then attach an age to individual cactuses, depending on how tall they are.

“We have a really good kind of average growth rate for saguaros in this district of the park," Swann said. "It’s slightly higher than the average growth rate of saguaros in the west district of the park, which doesn’t get as much rain. And during rainy years they grow faster, and during dry years they grow slower. But I can say, on average, this saguaro would be about 38 years old.”

Once upon a time, back in the 1930s when the park was established as a national monument, the area known then, and now, as the Cactus Forest was thick with saguaros reaching into the sky. But over the decades the forest thinned out, dramatically when you look at photos spanning the decades.

The Cactus Forest, 1985, Saguaro National Monument/NPS

But, I learn as Swann leads me around in search of saguaros of different ages, that diminished panorama is not evidence of a staggering downfall of this desert icon.

“That’s a great question,” he replies when I ask if we'll see those dense saguaro forests one day return, “and the answer is no, I don’t think we will. But not for the reasons most people think.

“I think those saguaros that we have out there today will grow up and they’ll get arms and they’ll be impressive, but part of what makes that photograph so impressive is that there aren’t trees, and so that you can really see the saguaros,” he said. “So in order to get back to that same scene we’d probably have to cut down the trees again, which we’re not going to do.”

As we walk, the effort to appreciate how old an individual saguaro is is an ongoing challenge. Coming upon a group of three, Swann asks how tall I think one is. After I guess it’s about 20 feet tall, the biologist tells me that corresponds to nearly 70 years in age.

Not 10 yards away Swann spies a much, much smaller one, roughly a foot tall.

“So this one is certainly tall enough to be able to store water and be pretty resilient to a drought situation. But, it’s older than most people would think,” he said, bending down with his measuring stick. “Let’s see, it’s about about 48 centimeters (18 inches), which makes it about 21 years old.”

Looking around at various saguaros, I ask the biologist at what age saguaros sprout their "arms," or branches.

“They don’t always appear. You don’t always get arms on saguaros. I always like to ask people, so I’ll ask you: Why do you think saguaros have arms?," he said.

“For bird perches," I offer, sending us both into laughter.

Saguaro in bloom, Saguaro National Park/NPS

"The answer is that they produce the flowers at the tip of the growing stems, and they can produce flowers at the end of every arm as well. So a saguaro that’s doing really well, is getting enough water for survival, is then beginning to put its energy into reproduction," explained Swann. "So once they reach a certain age or a certain size, and it’s usually not until they’re older than I am, they will start to put out arms if they’re in a favorable place, and then they can put out multiple arms.

“If you look around here, you can see some small arms that are just starting, some big arms that have been there for a long time. So they can put out those arms and they can basically produce more flowers, more fruit, and more seeds. And so they’re getting their seeds out on the landscape in larger numbers because of those arms.”

Those uninitiated to the Sonoran Desert and saguaro biology likely wouldn’t notice, or not think it unusual, that these smaller saguaros are growing up next to trees or bushes. These trees -– mesquites, palo-verde, acacia -– are called “nurse” trees for the benefits they bring young saguaros until they’re old enough to make it on their own. They provide shelter from the hot sun in summer and even from the cold of winter.

Swann tells me that research is under way to try to determine if one particular species is a better nurse tree than another.

“Scientists have long suspected that one of the critical factors for young saguaros is protection from the cold during these freeze events that we occasionally get,” he said. “There’s also potentially a moisture benefit. The nurse trees provide, just from the shade itself, greater moisture underneath the tree, but there’s a fascinating line of research being pursued by one of our collaborators who’s looking at nurse trees, like palo-verdes and mesquites, bring up water through a process called hydraulic lift from ground water that’s deeper in the ground.

“There’s some evidence that their roots at the surface leak a little bit, and that little bit of lost water potentially could be taken up by saguaros and help their survival,” said the park biologist. “So it may be that the nurse trees are benefiting the saguaros through this kind of water transfer.”

Sometimes, rocks provide the “nursing” by shielding young saguaros from the sun and providing a measure of radiated warmth in winter.

Gazing around at the Cactus Forest landscape, and recalling those photos from the 1930s and ‘40s, my mind wanders to the bottom line of the census.

“People hear the word ‘census,’ and they might imagine that you’re coming up with a finite number of how many saguaros you have in the park,” I said.

“The word is inaccurate, it is not a census,” replies Swann with a grin. “We are not counting every saguaro in the park. However, we use the number from the plots to estimate how many we have in the park. And we estimate, based on the 2010 census, that we have about 1.9 million saguaros in the park.”

“And how does that relate to the first census back in 1990?” I ask.

“It’s increased dramatically. I don’t have the exact numbers, it’s not quite doubled, but it’s increased significantly since 1990,” is the answer.

The reason the landscape doesn’t seem to hold that many is that young saguaro can be hard to spot to the untrained eye.

“In 1990 they didn’t find a lot of the very young saguaros that were not visible yet,” the biologist explained. “They were there, but we didn’t find them. But yeah, there’s been a tremendous increase in the number of saguaros in the park in the last four decades.”

As we head back to the visitor center, Swann reminds me of the mission of the National Park Service.

“In national parks, our job is to protect resource, like saguaros, for future generations,” he said. “It’s not just about us, it’s about will people be able to come here 30 or 100 years from now and see a saguaro, and what can we do, how can we gain knowledge that will allow us to make good decisions to protect the desert and protect the saguaro.” 

Cactus Forest, 2010, Saguaro National Park/NPS

Traveler footnote: Catch Traveler's entire interview with Don Swann in  our podcast, National Parks Traveler Episode #40.