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'I almost needed a machete' — burned areas brim with new growth after monsoon

Ben Wilder, director of Tumamoc Hill, Maya Stahl, ecologist, and Joe Black, a botanist, walk through thick desert brush to get to a plot set up in Catalina State Park. The researchers are studying the growth of the desert after the Bighorn Fire in 2020.

 Mamta Popat, Arizona Daily Star

After last year’s Bighorn Fire, researchers set up scientific plots in Catalina State Park to see how the scorched desert might start to recover with a little monsoon moisture.

The answer came in a green explosion so thick it nearly blocked their path to the study area and made their plots tricky to find.

“It felt like I almost needed a machete, walking through the tropics, with so many great morning glory vines and the amaranth — the pigweed — growing up,” said Ben Wilder, director of the University of Arizona’s Desert Laboratory on Tumamoc Hill.

Wilder, research ecologist Maya Stahl and botanist Joe Black hiked back to their vegetation plots on Sept. 23 to see how they looked after the third wettest monsoon season on record. Precipitation gauges in the area have recorded well over 12 inches of rain since Wilder’s last visit in mid-June.

“The annual flora seems to be responding vigorously from the seedbank that wasn’t damaged by the fire,” Wilder said, as he walked with notebook in hand across one of the 50-by-50-foot plots about a mile from the park’s main road. “My first pass looking at it, this is majority native (growth). And it’s diverse, too.”

The team set up a total of 20 plots last fall, just a few months after firefighters gained full control of the Bighorn Fire. Ten of the plots are staked out in patches of desert and grassland burned by the fire, while the other 10 are in similar habitat types that escaped the flames.

The plan is to monitor the sites in the coming years and decades to better understand how wildfires impact Sonoran Desert ecosystems.

Ben Wilder, director of Tumamoc Hill, takes notes in one of the plots set up in Catalina State Park. On his first observation Wilder says he sees much more native regrowth in the area.

Too many plants

Wilder said such fires used to be rare, but they are becoming more frequent due to the spread of invasive grasses and hotter, drier conditions brought on by human-caused climate change.

He said it remains “an open question” whether Tucson’s iconic backdrop of saguaros and palo verdes can survive this new fire regime or if it will be gradually replaced by some other landscape type.

Roughly one third of Catalina State Park was scorched by the fire. Wilder said it will probably take five years for native trees and cactuses to reestablish themselves and another 25 years for the desert to resemble what it looked like before the fire, assuming it ever does.

So far, the recovery looks promising.

Though Lehmann’s lovegrass and other invasive weeds have sprouted in some of their charred desert plots, “there’s a healthy mix of native plants and a good diversity of shrubs and grasses,” Wilder said. “I’m encouraged by how many natives there are.”

As recently as mid-June, these hills remained mostly bare, after being swept with flames and seared by drought. Now the land is almost completely covered in plant life.

A desert tortoise lumbers across Catalina State Park. The tortoise was seen as researchers hiked to a plot in the park to study the regrowth of the desert after the Bighorn fire in 2020.

The growth is so dense in places that the researchers couldn’t find some of the short, metal posts they drove into the ground to mark the corners of their study areas.

“We searched for a while on that first plot and never found them,” Stahl said.

All the growth has forced them to rethink their study protocols, which originally called for a much more intensive survey of every plant and every species in each of the plots.

That seemed “totally reasonable when there was nothing there,” Stahl said with a laugh.

Trying to count every plant now would take too much time, so they’ve settled on a new way to sample the plots for coverage and diversity.

“It’s truly astounding how much the desert has responded to this rain,” Wilder said. “You know, just add water and life will flourish.”

Download PDF 100321-tuc-news-green-desert-g1

More to burn?

So what happens to all these plants when they eventually dry out and die?

Molly Hunter is a research professor specializing in fire science with UA’s School of Natural Resources and the Environment.

Like many Tucsonans, she has enjoyed seeing our mountains swathed in green and bursting with life. “But I have also been looking up there and thinking, ‘Whoa, that’s a lot of biomass,’” she said.

Researchers studying the effects of the Bighorn Fire have found their scientific plots outside of Tucson overrun with new growth. Video by Henry Brean/Arizona Daily Star

Historical records and tree-ring research show that “big fire years” often occur “when you have a confluence of a wet period followed by a dry period,” Hunter said.

In other words, what happens between now and next summer could make all the difference. “If we have one of those winters when we just don’t get any rain, it could be dangerous,” she said.

But Wilder isn’t overly concerned about the fire risk, mostly because of the timing of the desert’s recent growth spurt. Though the wet monsoon did produce “a ton of biomass,” he said, much of that plant material should die back and break down over the winter, before the start of our traditional fire season.

From left to right, Joe Black, a botanist, Maya Stahl, ecologist, and Ben Wilder, director of Tumamoc Hill, talk about the growth of the desert after the monsoon rains in Catalina State Park.

Unless, of course, climate change decides to throw a wrench into that, too. As Hunter put it, “The whole notion of a fire season is just going out the window. You can get starts year round now.”

There is some good news, at least for the Catalina Mountains.

Hunter said she has noticed the same thing while hiking in the foothills that Wilder is cataloging in Catalina State Park: Perennial, native plants seem to be doing better right now than invasive weeds like buffelgrass, which burns hot and respawns quickly after a fire.

Ultimately, she said, the chances of another large, destructive blaze in the Catalinas are diminished right now, because the Bighorn Fire consumed so much fuel across such a large area.

The mountains could see some smaller wildfires in the late spring or early summer, after all this green turns brown, but Hunter said it’s important to remember that fire is part of the natural system and not inherently bad for the landscape.

“Just having a fire doesn’t mean the end of the world to me,” she said. “Fire is not binary. That’s a hard message to get across.”

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Contact reporter Henry Brean at hbrean@tucson.com or 573-4283. On Twitter: @RefriedBrean