Flora, Fauna and Fire

The living things that call the Sonoran Desert and its Sky Islands home are all interconnected. Changes have the potential to impact the entire ecosystem due to the interconnectedness of its organisms. When one creature dies, another creature has lost its food. An excess of one creature’s population can compete with other creatures. When temperatures or water sources change, the food web can be thrown into chaos. And we humans are connected to our surroundings more than we realize. 

The American Southwest has a history of problematic fire suppression that has led to a build-up of excessive vegetative fuels, creating larger fires that have the potential to be more destructive. In Tucson, Arizona, the Santa Catalina Mountains exemplify this issue, whereas the Rincon Mountains display a more ideal situation where fires are allowed to burn and clear out fuels, reducing the risk of more severe fires that can harm ecosystems and reduce habitat availability. 

But fire management in the Sky Islands of Southern Arizona is not all black and white. There are some unique plant and animal species that complicate management goals.

One such species is the Mexican Spotted Owl (Strix occidentalis). According to the  National Park Service , it is listed as a threated species under the Endangered Species Act of 1973 (16 U.S.C. 1531-1544). Hailing from the American Southwest, this owl has experience living in these types of forests and dealing with fire. However, the owl requires a nesting habitat within some sections of dense forests. This conflicts with fire management goals such as forest thinning, minimizing fuels and allowing fire. 

“So, there’s some concern that you know, we let fire back into the system, it’s gonna disrupt their habitat,” says Molly Hunter, an associate professor in the University of Arizona’s School of Natural Resources and the Environment. 

Hunter says fire managers likely had to collaborate with wildlife biologists to determine where fire should be allowed and excluded to protect these habitats. 

Mexican Spotted Owls are usually found on mountains at elevations of 4,000 feet and above. Their habitat can look several different ways, though they do have their preferences. 

“[The owl] can occur in rocky canyons with caves and potholes in cliffs where it can nest, but often it’s in forest,” says Joe Ganey, a retired research wildlife biologist with the Forest Service. “And typically those are older forests.” 

For its forest habitat, this owl prefers mixed conifer – with species including Douglas fir, white fir, Apache pine and Chihuahua pine. In Southern Arizona and Mexico, there are some oaks thrown into the mix, such as silverleaf oak.  

Ganey says for nesting, they prefer large trees that are sometimes at least 200 years old. Like other owls, the Mexican Spotted Owl does not build its own nest. Rather, it finds a tree cavity or another bird’s nest. Or a unique third option for nesting: mistletoe infections on trees, often Douglas fir. 

“The mistletoe infection creates a tangle of branches that then accumulates needles over the years and forms a platform,” Ganey says. 

He says this conceals the owls. 

The Mexican Spotted Owl has a complicated relationship with fire. Ganey says it can be a controversial subject. A 2020 study by Jones et al in the journal Ecosphere states that “the way spotted owls respond to fire is highly variable and context specific.” The study says it might also vary between subspecies – Mexican, California and northern. The authors also argue that “severe fire has not yet resulted in substantial declines in spotted owl populations at regional or subspecies scales,” instead blaming declines on other ecological factors. 

However, Ganey says current fires are “a different beast” and in his opinion, these larger and more severe burns are “largely detrimental.” Research shows that these types of fires can alter the owls’ habitat. Jones et al state that larger and more severe burns can “convert forests used for nesting and roosting habitat to either foraging habitat or vegetation types that are unsuitable for these critical events of spotted owl life history.” 

Mexican Spotted Owl habitat cannot be made just anywhere, and as a result it can be sparse. 

“And given that it’s a rare element on the landscape, losing any of it to fire seems detrimental to me,” Ganey says, “especially losing large chunks at one time, or over short time periods.” 

The Catalinas tend to have larger and more destructive fires than the Rincons, but Ganey says owls still managed to find habitat in the Catalinas after some large fires. 

In the most extreme cases, research does not point to hopeful outcomes for the owls. Jones et al states, “In territories experiencing 100% tree mortality, it is biologically intuitive that a shade adapted species like the spotted owl will be unlikely to persist even over the short term.” 

Aside from fire, another threat facing the Mexican Spotted Owls of the Sky Islands is climate change. In addition to altering the forest type, climate change has the potential to dry up water sources and eliminate cool microsites. 

A cool microsite is defined as “someplace topographically different” with shade and chilly night air. It is cooler than surrounding areas and located at the bottom of a canyon or in a riparian pocket. If cool microsites were to be eliminated, the Mexican Spotted Owl would be forced to travel further up the mountain until it had nowhere left to go. 

The Mexican Spotted Owl serves its ecosystems by consuming rodents. According to Ganey, in many places “it probably is the top nocturnal avian predator.” If this species were to be completely wiped out, Ganey believes another species might step up to fill its talons. But he cannot say with certainty that this will happen, because the Great Horned Owl – a potential replacement - might not use their same habitat. 

“So, I don’t know if they would be readily replaced or if that niche would be vacant, hard to say,” Ganey says. 

He says there needs to be more research on how Mexican Spotted Owls use landscapes and respond to climate change. Although mechanically thinning forests can prevent destructive fires, Ganey says there needs to be more research on the various forest treatments and what effect they have on these owls. Jones et al express concern that “landscape-level fuel reduction projects have the potential to remove key habitat elements required by old-forest-associated species” such as spotted owls. 

Ganey emphasizes the importance of improving forests’ resilience to protect them from “large canopy fires” that present a threat to habitats. This protection is beneficial to the owls. The ideal landscape for this purpose is one that resembles a pre-fire suppression landscape, which will improve the forests’ climate change resilience as well if they are thinner. 

But in order to allow habitat for the Mexican Spotted Owl, a balanced landscape is essential. 

“We still need those pockets of closed canopy forest for them,” Ganey says. “We just need to kind of protect those by thinning the landscape around them.” 

Jones et al echo this sentiment, stating that “management actions that can demonstrably reduce the extent of severe fire within spotted owl habitat in a changing climate may contribute to owl conservation if those actions do not remove critical structural habitat elements positively associated with spotted owl vital rates.” 

Another species with unique fire concerns is the lowland leopard frog. It lives not in the mountains, but below them in desert habitats including springs. When a fire burns in the mountains above, especially one at a higher intensity, these frogs face the threat of post-fire floods bringing debris into their home. Hunter believes this type of erosion can still occur after a lower-intensity fire, but not to such an extent as to cause a complete disruption of the frogs’ habitat. 

In contrast, some species thrive in recently burned ecosystems. Woodpeckers depend on standing dead trees for nest sites as well as the wood boring insects present after a fire that provide a food source for them. Hunter says woodpeckers are one of several species that rely on the ecosystem conditions right after a burn. 

The Lesser Long-nosed Bat (Leptonycteris yerbabuenae) is a nectar-sipping bat species whose roosts can be found in both the Santa Catalina and Rincon Mountains. However, they are only found in this region part of the year as they are migratory. Debbie Buecher, bat scientist and independent consultant at Bucher Biological Consulting, tells the story of their journey. 

They spend their winters in Central Mexico. In the spring, they begin their desert odyssey. As they migrate across the border, they stop at various types of columnar cacti along the way. These include cacti species such as cardon, organ pipe as well as saguaro, which is also found closer to the Catalinas and Rincons. 

“And these bats are following the flowering and they’re visiting the flowers,” Buecher says. “They’re drinking nectar and they’re getting pollen all over their heads and bodies.” 

Buecher says this allows them to cross-pollinate, which maintains genetic diversity among plants. Around May, the bats arrive at Organ Pipe Cactus National Monument, which is located just across the border near the town of Ajo, Arizona. Here, the pregnant female bats will remain until at least four weeks after their pups have been born. In the meantime, they will eat the pulp of saguaro fruits. Buecher says this is where they perform the vital task of seed dispersal when they fly around and poop “pre-fertilized packets of seeds,” ensuring the new saguaro will be far enough away from its “parent plant.” She recalls witnessing this phenomenon for herself when she was camping near a bat roost and woke up to see small, red piles. 

When the saguaros have concluded their flowering phase and the bat pups are capable of flight, the group shifts its course to the East, heading for grasslands that encompass Sky Islands such as the Rincons and Catalinas. In this region, Buecher believes the Lesser Long-nosed Bat makes its habitat in areas including the perimeter of the grassland biome and the higher elevation of the grassland biome itself, possibly extending into higher elevations where the oak woodland biome starts. She says they reside in caves as well as abandoned mines. 

An important aspect of their habitat is feeding on agave plants. Buecher emphasizes the impressive size that some agaves can grow to. 

“And when they flower in about 25 years, they only flower once, and they put up a flower stalk and the bats come and feed on,” she says. “It’s a paniculate agave, which means it’s a big column.” 

Out from this stalk protrude arms which bear flowers. This gives the bats easy access, as the flowers are not crowded within other vegetation. This arm-flower structure is also found in saguaros and other types of columnar cacti. According to Buecher, this is only one of several agave adaptations to attract bats instead of other pollinators. 

Other adaptations include opening their flowers at night while the bats are out and about, producing a “musty odor” and having a lighter color for visibility. 

Suppression is not the only force standing in the way of a healthy fire regime. The Sonoran Desert is being encroached upon by invasive grass species, such as Lehmann Lovegrass. This creates enough fuel to cause fires to burn hotter. Buecher says that while some people are unconcerned about large agave plants getting burned, she and her colleague beg to differ. 

Buecher explains that these desert plants of grand size start out “the size of our thumbs” - small enough for a fire to kill the entire group of young agave. The effects of this mortality will be felt many years later, when they are supposed to be flowering. 

“If there’s a fire, then there’s going to be a gap of flowering agave for the bats down the line,” she says, “because these have to grow up and get bigger and so in 25 years there’s going to be a gap...because they lost a whole generation.” 

Buecher’s thoughts on these two contrasting fire regimes echo those of other experts. 

“Allowing smaller fires in the Rincons prevents those catastrophic climax fires that you see in the Catalinas,” she says. 

It reminds her of a story of when she and a colleague were hiking through the Catalinas in the first few weeks following the Aspen fire. She recalls seeing parts of the mountains in which the fire had been hot enough to completely burn all of the “duff” - something she defines as a layer of “leaf litter,” pine needles and other plant matter resting on top of the soil. 

“It was ash,” she remembers. “Your foot went down through ash.” 

Buecher says this lengthens plant recovery time. A seed that falls on this layer of ash will not be met with nutrients to aid its growth. 

Although there is not yet any research specific to this issue in these two locations, based on her knowledge of other mountain ranges Buecher believes the Catalinas’ overall hotter fires could mean a higher mortality rate of small bat pups compared to the Rincons’ overall cooler fires. 

The Lesser Long-Nosed Bat faces several other threats aside from fire. 

One of these is habitat loss. Buecher says humans love to make their homes in the grasslands. She mentions Sonoita as an example, saying that when people clear land for livestock it removes agave by altering the habitat. 

Another concern is, of course, climate change. As the climate continues to warm, Buecher says agaves are flowering earlier. When this happens, it does not match up with the bats’ migration. 

“And before the bats migrate from the maternity site, so when they get to the grassland there’s a break in the plant phenology and the arrival of the bats,” she says. “And that will be detrimental, and you’ll start seeing them hitting more hummingbird feeders because they don’t have enough natural food.” 

Buecher says this issue has not yet been documented and that research is necessary. 

Although there is limited knowledge on it, Buecher says light pollution affecting agave fields is a possible concern, explaining that bats are active at night “to avoid predation.” 

The Lesser Long-Nosed Bat is also threatened by recreational spelunkers who disturb their roosts, according to Buecher. She cites the historic example of this species of bat vacating their roost on Fort Huachuca due to frequent visitation by local enlisted people. 

Lesser Long-Nosed Bats have encountered some trouble with problematic practices in the production of tequila, which is made from agave stalks. If stalks are cut from the wild, some sustainability issues for the agave can arise. 

“Well, if you cut all of these bolting agaves that the bats would be using, they’ve lost their food source,” Buecher says. 

Lesser Long-Nosed Bats serve their ecosystem by dispersing seeds in their poop to allow better placement of new plants. They also serve as pollinators – moreover, as cross-pollinators that promote genetic diversity among plant communities. 

“They will fly 30 miles from their day roost to go forage so you know all of that foraging and visiting flowers is critical for genetics, plant genetics.” 

If the Lesser Long-Nosed Bat were to disappear completely, it would change things for plant communities. 

“Well, the columnar cacti that they follow up along the west edge of Mexico, on the Sea of Cortez, would then only be pollinated by hummingbirds and moths,” Buecher says. 

This would reduce genetic diversity, causing the columnar cacti to become inbred. 

The answer to producing tequila without harming Lesser Long-Nosed Bats is to cultivate agave farms, and do so sustainably. Buecher says the industry has already made this transition to some extent. The bats’ role in pollination and genetic diversity benefits the tequila’s flavor. In return, the farmers leave enough stalks to flower all the way for bats to pollinate. 

“So, it’s sort of a partnership,” Buecher says. 

A solution that Buecher isn’t necessarily a fan of is replanting agaves in wild grasslands. She says although done with positive intentions, it creates stress for the plant, causing disruption in its flower and stalk habits. In Buecher’s opinion, it would be preferable to conserve grassland areas that include agaves. This would mean shielding those areas from potential threats such as the proposed Rosemont Mine in the Santa Rita Mountains and large, land-hungry housing developments. 

A highway winding through the Catalinas takes visitors to Mt. Lemmon – a place sought after for hiking, rock climbing, scenic views, cooler weather, fall colors and even snow. Many people stop at the village of Summerhaven. But for those who continue their drive up, more treasures await at higher elevations. 

Closer to the top sit a ski lift and a restaurant across the street. In the summertime, the ski lift continues operating to give visitors a view of the mountain. And in October, the ski lift and restaurant are both surrounded by trees boasting their brilliant autumn colors. 

If you were to keep driving past the ski lift, further up Ski Run Road you would stumble upon a stand of trees with quite the unique story. Two-thirds of the way up this road, just downhill from the dirt lot and collection of spare ski lift parts, on the upslope of the hill lie some Arizona Corkbark Fir trees, known as Abies lasiocarpa var. Arizonica, according to the American Conifer Society. 

They live at higher elevations with a moist climate, often at the summit of the mountain. For these reasons, it is more commonly found in Northern Arizona. According to Thomas Swetnam, University of Arizona Professor Emeritus of Dendrochronology, this species is only found in one other mountain range among the Sky Island of Southern Arizona aside from the stand on Mt. Lemmon. 

The population of this tree on Mt. Lemmon is small – a few thousand at most. 

“And that’s such a small, little tiny population of these unique trees that it’s totally conceivable that a big fire will take them all out,” Swetnam says. 

They are typically found at elevations of around 9,000 feet and are shade-tolerant. 

“They like growing under the canopy of other trees,” says Chris Baisan, senior research specialist at the University of Arizona’s Laboratory of Tree-Ring Research. 

Corkbarks are often paired with spruce trees. This spruce-fir mix is found on Mt. Graham near the University of Arizona’s telescopes – but not on Mt. Lemmon. Here, the Corkbarks grow without their spruce companions. This is only one of several mysteries surrounding Mt. Lemmon’s stand of Arizona Corkbark Firs. 

There is some uncertainty as to how and when this particular stand of Arizona Corkbark Fir was established on Mt. Lemmon, but Baisan believes the species has been present there since the Pleistocene era – during which it would have thrived more in this region due to cooler temperatures. The trees that we see in this stand on Mt. Lemmon today have been alive since the 1800s. 

Baisan says it is uncertain why this specific population of trees is so small and how it has managed to survive there for thousands of years, especially given that fires have occurred there. The Arizona Corkbark Fir is not made to survive fire. It can be harmed or killed by a low-intensity fire. 

Historic fire suppression and subsequent excessive fuels buildup in the Catalinas have led to large, high-intensity fires, such as the Aspen Fire that destroyed the town of Summerhaven. But somehow, the stand of Corkbarks is still here. Baisan guesses it might have to do with the topography of the slop where they are located. 

“I mean it’s favorable, it’s north-facing, and it’s, you know, fairly cool and moist there,” he says. “And it’s probable that the topography is such that it’s unlikely that a fire could come through and kill all of them.” 

Baisan brings up an almost comical threat – if you could even call it that – to the Arizona Corkbark Fir. While conducting research on Mt. Graham, he noticed that many trees had injuries from bears peeling off the bark to snack on it. He attributes this to them being extra hungry in the spring after hibernation.  

But although bears are present on Mt. Lemmon as well, they simply do not peel the Corkbarks in this particular stand. This adds another layer to the mystery of how Mt. Lemmon’s Corkbark Firs are able to protect themselves from various threats. Baisan believes this is explained by Mt. Lemmon having a different “bear culture” that isn’t aware of this trick. 

A more somber threat to the species is found in climate change. Warmer temperatures mean less snow. Baisan says this won’t kill off the species as a whole; rather, it will migrate elevations and latitudes to reach remaining cool environments. But the population on Mt. Lemmon is already at the highest point. 

“So, if it gets warmer, those things have no place to go,” Baisan says. “They would...it would be replaced by something else.” 

According to Swetnam, climate change can also threaten these trees by inducing drought and insect infestations. 

Swetnam says The Arizona Corkbark Fir likely serves its ecosystem by providing food – such as seeds and cones – for birds and rodents. 

If this tree were to disappear entirely from Mt. Lemmon, Swetnam says we would lose the genetic component of this unique population on this specific mountain. 

“So, it’s like, you know, plants, maybe they belong to the same species, main genera and species,” Swetnam says. “But if they grow in different mountain ranges, they’ll often have their own unique genetic makeup and their own evolutionary history.” 

He says the Corkbark Fir is an example of this and refers to it as an “endemic subspecies of a fir tree.” If this population were eliminated from Mt. Lemmon, it would “reduce the gene pool of that particular species.” Whenever an ecosystem loses a species, it also loses biodiversity and “stability.” 

In addition to these issues, “People would be sad,” says Baisan. 

He emphasizes the aesthetic importance of the Arizona Corkbark Fir, and says that Mt. Lemmon stands out for its biodiversity, owing in part to its elevations that are “just high enough. 

“Mt. Lemmon is a pretty special place that it has these associations that some of the other mountains down here don’t have,” Baisan says.