This article is the second of a two-part set that I am writing about the threat of wildfires in the deserts of North America, which are unprecedented and have not been seen before the mid to late 1990s. It is a problem which is accelerating as invasive weeds continue to gain territory, spread in areal extent, and increase in density to the point where desert wildfires are now possible in ecosystems that have never seen them before.

The Sonoran Desert is increasingly at risk for widespread desert wildfires fueled by non-native and highly invasive weeds and grasses. The primary culprits in the Sonoran Desert are grasses such as Mediterranean grass (Schismus arabicus and S. barbatus), red brome grass (Bromus madritensis, aka B. rubens), and buffelgrass (Pennisetum ciliare and potentially fountain grass, P. setaceum). There are also weedy mustard family plants that are very problematic, the most prominent of which would be Sahara mustard (Brassica tournefortii) and rocket mustards (Sisymbrium irio and related species.)

I spent significant time explaining how and why these particular opportunistic exotic invaders are so troublesome in Part One of this series, which dealt with essentially the same issue of fire threat in the Mojave Desert’s Joshua tree woodlands (Yucca brevifolia.) I will link to that below so that readers can catch up with that, rather than me spending too much time repeating it here. Part Two, this edition, covers the saguaros (Carnegiea gigantea) and foothills palo verde trees (Parkinsonia microphylla) of the Sonoran Desert. There are both many similarities with, and key differences between, how saguaros and Joshua trees respond to fires, and it is worth understanding both aspects.

Nevadagascar: The Link Between Invasive Weeds and Wildfires in the Joshua Tree Forests of the Mojave Desert

Basically, the quick recap of the situation is that deserts are normally too dry to burn, meaning the natural vegetation there is too sparse and widely-spaced to successfully carry fires across large distances. The invasive weeds have changed that equation to a point where wildfires are suddenly now a possibility, especially after wet winters, making flames a threat to a plant community ill-adapted to survive even a light and low-intensity burning. I wrote an article on this issue which was printed in January 2006 in the Cactus and Succulent Journal, the official publication of the Cactus and Succulent Society of America. The link to that article in PDF format is below.

Sonoran Crisis article – final proof CSJ Jan 1, 2006

Now, onward to discussing the specific threat of wildfires in the succulent-rich wild habitats of the Sonoran Desert, and the ways in which saguaros differ from Joshua trees in their response to fire….

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A heretofore unheard-of desert wildfire burned through Arizona’s Vekol Valley in June 2005, burning saguaros and numerous other plant species.

In early July 2005 I visited the Vekol Valley, a large watershed that drains much of southern Maricopa County and northwestern Pinal County, southwest of Phoenix, Arizona. Dry lightning storms that passed through the region in late May and early June a month or so prior ignited a series of wildfires across the desert southwest, including the Mojave Desert covered in Part 1 and the Cave Creek Fire Complex north and east of Phoenix, which was at the time the largest fire ever recorded in the state’s history. The Vekol Fire, while smaller in extent, was also part of the outbreak of blazes.

The fuel that carried the Vekol Fire: Invasive Mediterranean grass, Schismus arabicus and S. barbatus.

The Vekol Valley is low Sonoran Desert that sees an average of maybe between 6 and 7 inches (150-175 mm) of annual rainfall. The natural vegetation community is fairly sparse, and comprised mainly of widely-spaced creosote bushes, bursage, and occasional small desert trees such as foothills palo verde and ironwood. Saguaros, chollas, barrel cactus, and ocotillos are among the succulent plants growing here in generally low numbers. Under no normal circumstance would this stretch of Sonoran Desert habitat ever see fires of any type due to the very thin distribution of the plant cover.

A large red barrel cactus (Ferocactus acanthodes) escaped the flames of the Vekol Fire despite the coverage of Mediterranean grass all around.

Obviously the new fire-carrying factor that has arrived upon the scene is two closely related species of Mediterranean grass (Schismus arabicus and the very similar S. barbatus). I used to think that these annual grasses from the Middle East and North Africa could not pose a fire threat, given their short stature and patchy distribution across the landscape.

Another colony of red barrels that was fortunate enough to escape the flames, perhaps by virtue of having grown upon a rocky knoll that has less invasive grass upon it.

Obviously, my original assessment of fire risk in habitat like this was quite wrong, albeit not entirely so either. Here’s why: Most years the grasses are indeed too small and sparse to carry fires over long distances, thanks to low rainfall. In drought years there will be no grass whatsoever. But every once in awhile a wet winter arrives, such as the record-setting period between September 2004 and April 2005, when most of the southwest saw two to three times the typical rainfall in most localities. That extra moisture enabled the grasses to grow at greater than normal densities over larger than normal expanses, thereby setting the stage for wildfires. Fires might indeed not be a threat very often due to generally low fuel loads, but clearly once every decade or two, when a wet cycle arrives and changes that balance, the odds of fires now are drastically higher.

A patch of rocket mustard crowds beneath an ironwood tree. If this catches fire, the tree is doomed.

Mediterranean grasses are not the only invasive weeds that help carry fire throughout the lower Sonoran Desert. Cruciferous plants such as rocket mustard (Sisymbrium irio) and Sahara mustard (Brassica tournefortii) are now dense enough in many places to also carry fires. They are subject to the same boom/bust cycles of population based upon the quantity of autumn and winter rainfall and follow the same pattern, being incapable of supporting fires most years but occasionally breaking through into a density where they can. The mustards and grasses and other herbaceous weeds all work together to escalate the fire risk in certain wet years at this point.

The burned landscape of the Vekol Valley, with the Maricopa Mountains in the distance.

In the photo above, fire has passed over the landscape. Although it is hard to tell since wind has scattered the ashes in subsequent weeks, the dark marbling on the pale sandy soil is the burned bases of Mediterranean grass. It is easy to mistake this scene as being healthy, because it’s certainly not what we expect a fire-blackened landscape such as a charred forest or chaparral scrub to look like. But the creosote bushes are dead and the saguaro has probably been fatally wounded.

Scorching damage to the skin of the saguaro above.

I’ll discuss the damage that fire does to the trunk of a saguaro cactus in greater detail shortly, but for the moment suffice it to say that it does not take much of a flame for a saguaro to suffer fairly severe burn damage.

The fire scaled up the cactus’ trunk on this side, starting perhaps with spines that were near the very base.

Nest material that somehow avoided being burned in the arms of a saguaro.

The large raptor nest (likely from one of the several hawk species that commonly live in the desert) could have caught fire, but didn’t. The fire did actually creep up each rib as the interwoven spines on the areoles ignited, as evidenced by the dead tan burnt patches on the rib edges and under the arms. Had the nest caught flame, the entire plant would be been so severely burned that it would have died.

Severe fire damage to a saguaro cactus trunk. This plant will most likely die and recovery is extremely unlikely.

As mentioned earlier, we need to explore in more detail why fire, any fire, is so damaging to plants that are basically big columns of water. It’s not like they burn, right? Well, sort of right. While the bodies of saguaros (and cacti in general) are indeed wet, filled with water contained in pulpy storage cells, their skins are covered with spines that are in fact quite flammable. Cactus spines ignite readily and burn quickly, and their density on most of the species in the Sonoran Desert means that the plants can suffer serious third degree burns as a result. Even cacti that don’t have that many spines suffer from this vulnerability.

The formerly green skin of this saguaro has been turned to dead and tan scar tissue, incapable of photosynthesis. Note the horizontal cracks already appearing on the trunk, each of which is a potential entry point for pathogens to the plant’s interior.

It is true that when it comes to fire fuel, cactus spines are fairly meager and flames don’t roar a hundred feet into the air, as they can do when resinous or oily trees like pines or eucalyptus are being immolated. In fact, many pines and eucalyptus are quite fire tolerant, with thick bark that resists ignition by small fires. Meanwhile, even the smallest fire creeping along the ground in a few inches of weeds or dry grass can ignite a saguaro’s spines, sending fingers of flame towards the plant’s apex and scalding the green trunk tissue all along the way.

Think of how awful it would be if your hair were to catch on fire. Your hair isn’t a lot of fuel, but burning it off of your scalp would result in very severe and disfiguring wounds and blisters that would be extremely painful. It could even be potentially life threatening should infection to your damaged skin set in afterwards!

This saguaro has already started dying, dropping the first part of one of its branches only a few weeks after the Vekol fire came and went.

Here is where the true vulnerability of cacti to fires becomes very apparent. Even the slightest singeing by fire, often caused by the burning spines themselves, causes the skin to die. Saguaros are leafless plants, as are almost all cacti (a few tropical species do possess some leaves, for the technical purists reading), and they depend entirely upon their green skin to conduct the critical function of photosynthesis. Once the skin blisters and turns tan and scabby, photosynthesis is greatly inhibited, or in a severe burning, is lost entirely. The plant can no longer make itself the food it needs via sunlight and it will eventually starve to death as stored reserves of sugar and starch are depleted. Some saguaros that are only partially burned on just one side may be capable of surviving on the remaining green tissue and growing new parts that will eventually compensate for the lost parts, but they will be weakened and have an increased chance of death for years afterwards.

A saguaro with a trunk this badly burnt is virtually certain to die within a few months or less.

Most saguaros that have lost their green skin will not actually starve to death, however. The majority of severely charred saguaros will actually start rotting from rampant infections caused by pathogenic invasion into the numerous cracks, splits, and blisters that cover their blackened trunks within a few weeks. This demise via disease (particularly bacterial necrosis, Erwinia cacticida) will occur long before photosynthetic starvation can set in via resource depletion. Plants that didn’t burn quite as vigorously or have a sizable amount of green skin left might last for a year or two before weakened immune systems or environmental conditions such as drought, heat, and frost do them in.

Again, think of how difficult it is for human burn victims to fight off infections after suffering intense burns, and how long the road to recovery can be. Saguaros are a lot more like humans when it comes to burnt skin than they are like regular trees. A fire like the small, short-grass-fed one that crawled through the Vekol Valley in 2005 would make a thick-barked ponderosa pine or coastal live oak tree laugh. But it proved fatal to a majority of saguaros in this part of the Sonoran Desert within months.

“The Standing Dead”

A superficial glance at the photo above might lead one to assume that the cactus has survived the Vekol fire just fine, since it is standing and the uppermost portion is still largely green. But now that readers understand the unique strain that fire places upon these entirely un-resistant plants, one will grasp the probability that this saguaro just ended up dying within anywhere from a few months to a few years later. I have no idea of whether this individual survived and is still alive today in 2019, over 14 years afterwards. But I would bet money that it did not make it for long before succumbing to its trunk injuries.

A burnt saguaro and its equally burnt “nurse tree”, a foothills palo verde.

To date I have focused most upon the saguaro cactus and how fire affects them as individuals. But this article is actually about fires in the entire desert ecosystem, and it is necessary to also discuss the impacts that flames have upon other species. Perhaps the most important ecological dynamic to examine is the way fire impacts the various desert leguminous trees, including palo verde (Parkinsonia spp.), ironwood (Olneya tesota), and mesquite (Prosopis spp.)

Another nurse-patient pair, again a saguaro and a palo verde tree. Both are very likely to die now that they have been burned.

Briefly, a “nurse plant” is any larger and more mature plant species in the desert that helps to shelter seedling and juvenile cacti such as saguaros during their most treacherous decade or two between germination and becoming large enough to weather the elements unprotected on their own. Nurse plants can be of any species, and shelter juveniles of virtually any other species, so it is a dynamic and not necessarily a tight relationship between palo verdes and saguaros only. (Although that is indeed a particularly common and well-known pairing.) Sometimes an adult saguaro can act as a nurse to its own baby saguaro seedlings, and sometimes a saguaro can shelter a young palo verde tree too in a sort of reciprocity. But very often it is the palo verde tree that first offers protection and comfort to young saguaros. (For the sake of completeness, rocky hillsides can also successfully shelter young saguaros and other cacti, a phenomenon we can easily call “nurse rocks”.)

This foothills palo verde tree (Parkinsonia microphylla) has suffered the same fate as some of the saguaros shown above, which is to say that the green photosynthetic trunk of the tree has been partially killed by heat rising from the flames below them on the undersides of the stems. The green portions are on the upper side, shielded from the heat. It is not certain whether a palo verde that suffered this pattern of injury will be able to survive, or succumb just like the saguaros often do.

It is extremely hard for cactus and succulent seedlings of any type to germinate in open, flat desert without the aid of nurse plants. There are many ways for tender seedlings to die, but the most prominent are extreme summer heat and drought in full sun, frosty winter nights, and exposure to hungry rodents that will readily eat a small grape-sized cactus seedling several years old in a single bite. Nurse plants provide some measure of protection from each of those threats. This particular ecological dynamic’s importance to the perpetuation of desert species can scarcely be overstated.

This palo verde definitely did not make it. I am betting that there was a thick growth of dried mustards and other weeds underneath the canopy, which when ignited caused enough fire intensity to cause the bark to flake off mere weeks after the fire’s passage. This is one potential nurse tree that will never shelter any saguaro seedlings, which is a loss to the desert at large.

As readers can see via the photos, fire kills palo verde trees just as surely as it kills saguaros, and for the same reason: Thin, green, photosynthetic bark. Palo verde literally translates to “green stick” in Spanish and refers to the bark of the main trunk and branches, as well as the smaller twigs. By being nearly leafless, palo verdes are able to conserve immense amounts of water, which is a critical life skill in a desert with as little rain and as much heat and sun as the Sonoran gets. But this same thin bark has absolutely zero wildfire tolerance, and the effects of even small weed-fueled fires upon palo verdes is as devastating as it is for the cacti.

Ironwoods (Olneya tesota) also serve as nurse trees in the Sonoran Desert. Despite the saguaro being much taller and heavier than its adoptive parent, it is the slow-growing ironwood that is surely much older. Ironwoods have been dated to live for 1200 to 1500 years, and even moderately small individuals such as this one are likely to be 100-200 years in age. When they are removed by fire, it will be centuries before they can return, if in fact they can do so at all.

As if it is not bad enough to have most of the individual trees of the Sonoran ecosystem killed by fire, it is made worse by the removal of the nurse plant function that might otherwise enable the damaged desert ecosystem to recover more rapidly. Once desert soil is cleared, it is amazingly difficult for most plant species to establish themselves upon it given the predominance of intense summer heat and drought that kills tiny seedlings of most species, and nearly all of the succulent ones. Sure, some desert plants can get going in the wide open (bursage and creosote are two) but it takes decades for them to become large enough to provide a useful amount of protective cover for other seedlings to join them. The succession between a cleared landscape and a mature climax one is very slow in deserts.

Mesquite trees (Prosopis velutina and others of the same genus) are also common Sonoran nurse trees. This one was burned down to white ash and stands no hope for regenerating.

Some nurse trees might be capable of resprouting from the base, or even shallowly buried sections of trunk, after the tops are killed by fire. This is more likely to happen in mesquites than in ironwoods or palo verdes, but all three groups can on occasion do it. The level of burning matters when considering the possibility of returning from the base. A tree whose bark was damaged just enough to kill the crown, but not kill basal buds, stands a chance of regeneration. But a tree burned completely down to white ash, as in the photo above, is so thoroughly cremated that there is no surviving tissue.

Because older desert trees have often survived where they are for many decades, and sometimes even centuries, dead leaf and branch litter underneath them is commonly significant. As shown before, annual weed growth often is too. These two dead fuel sources will provide the correct conditions to entirely cremate many desert trees that might otherwise have been able to survive and resprout. And again, without the new weed invasion to support the spread of such fires in the first place, the amount of dead litter underneath desert nurse trees was never an issue before. Sadly, it is now….

A mixture of pale Mediterranean grass seeds, black ash, and charred creosote bush twigs on the desert floor.

Most of the biggest weedy invaders of desert biomes within the past 50 to 100 years are annual plants, rather than perennials. (Buffelgrass, Pennisetum ciliare, is a perennial but produces large numbers of seeds every year, which enables its rapid spread.) Annual plants are often very fire resistant because a large number of their seeds survive burning simply by being on the ground where heat levels are minimal, especially in deserts where the conflagrations are similarly low intensity due to thin fuel loads. Vast numbers of grass and mustard seeds survive a fire very well, and are poised to germinate with the next suitable rainfall in the fall or winter months.

Moreover, upon germination the weeds find themselves in a newly-cleared landscape with many fewer competitors than they once faced, ash-enriched soils, and niches open to colonization. In these open spaces the weeds often thrive and become even more abundant that they were pre-fire, which simply sets the table for the next fire to happen again. Any secondary fires, or tertiary and beyond, will kill off whatever surviving desert shrubs and nurse trees and cacti manged to escape the first time around. In a few decades with repeated weed-fueled fires, the entire diverse desert ecosystem can be transformed into a barren, weedy landscape populated by a few invasive species and a handful of fire-tolerant native plants. It is a greatly impoverished ecosystem which contains far fewer species than the original Sonoran Desert did. This is a gigantic loss on so many levels.

Fires kill everything, great and small.

I presume that by now it is becoming clear just how deadly and damaging wildfire is to the desert, and to species that did not evolve any fire resistance because they never had to. Weeds that we introduced, often by accident via agriculture and global trade, are posing this type of a systemic threat to not only the Sonoran and Mojave Deserts, but to deserts worldwide and indeed to other ecosystems too. Invasive animals and insects, and invasive diseases such as fungi and viruses can have similarly devastating effects upon natural ecosystems, even if fire is not the primary mode of altering the ecosystem dynamics.

A fallen Ferocactus acanthodes lies in the 109 F July summer sun. It will surely rot and die within weeks given the severe damage it sustained.

Burned soaptree yuccas (Yucca elata) lie fallen in the ashen landscape.

I’ll add some photos of other desert species in the Vekol Valley that also suffered losses in the 2005 fire. While saguaros and leguminous trees are the flagship species of the Sonoran Desert, there is a whole range of other plants that also participate in the ecosystem that should not be ignored. I’ll discuss some ecological dynamics with respect to other species as well.

Above, soaptree yucca plants (Yucca elata) lie dead after the fire. Unlike some other yuccas, soaptrees have a substantial subterranean rootstock that can usually manage at least one regeneration after the loss of top growth via fire or other means. This plant will probably resprout, but since yucca growth is generally slow it will be a decade or two before it is capable of blooming again. And what if another weed-driven fire passes by in that 10-20 years and kills the plant back to the ground again? Can it resprout a third time? What about the small yucca moth (Tegeticula spp.) that pollinates yuccas? Can these tiny insects survive 10 to 20 years without any flowering yuccas to feed their brood and keep generations alive? What if yuccas do survive without further burning for 20-plus years, only to discover that there are no yucca moths around anymore to pollinate them once they are blooming size again? Yucca moths are the sole insect capable of pollinating these plants and setting future seed crops, so their presence in the ecosystem is mandatory for long-term yucca survival. It is highly likely that fire disrupted this delicate balance.

Ocotillos (Fouquieria splendens) are no more fire tolerant than any other plant thus far discussed, and for the same general reasons. The same forces that act to kill saguaros and palo verdes such as thin bark and the need for nurse plants to aid in seedling survival operate upon ocotillos just as much as they do upon the other plants covered.

A barely-recognizable Engelmann hedgehog cactus (Echinocereus engelmannii) lies dead at the base of a fatally wounded ocotillo. Neither plant will survive, and there will be no more seeds from either one to help potentially repopulate the missing parent plants.

A burned chainfruit cholla (Cylindropuntia fulgida) surveys the deeply damaged desert valley it grew up in. The entire background has been burned and odds are high that every ocotillo and most of the creosote bushes (Larrea tridentata) have been killed.

I know that cholla cacti (Cylindropuntia spp.) are not on most people’s list of favorite desert plants, but it is not wise to ignore their importance to the desert ecosystem. The Sonoran Desert is home to the greatest diversity of these plants on earth, both in terms of number of species (about 15 to 20) and number of individuals (tens of thousands in some particularly extensive stands). Chollas provide critical food, shelter, and habitat for numerous desert animals and they are an integral part of the functioning of the ecosystem, since many creatures have evolved to use them. These animals range from rodents that use the joints for food, to insects that pollinate the flowers, to cactus wrens and doves and thrashers that nest among the spiny branches, and to deer, desert bighorn sheep, and Sonoran pronghorn that use the juicy fruits of chainfruit chollas as a summer food and water source. Yes they are spiny and unpleasant to encounter too closely, but they are too important to write off.

Chollas have dense spines that ignite very easily and burn fiercely, if briefly, like a firework. They suffer from the same fate of charred skin, being unable to photosynthesize, and infection that the saguaros do once burned.

Rocky areas are somewhat protected from fire when compared to areas with more soil, which can support more weeds, and therefore potentially burn. To be clear the fires did still happen up on the slopes, but more plants escaped from burning there.

Not every saguaro on the flats burned either. This twin pair of adolescents seem to have escaped. But with weeds an ever-evolving threat whose density can change for the worse every few years, will this continue to be the case in perpetuity?

A post-fire stand of saguaros. Despite what must have been a fairly small and low-intensity burn, the fact that it happened at all is devastating to the plant community.

Most of the plants in the scene above are burned and I suspect that many if not most were killed. Studies in the late 1990s and early 2000s on previous Sonoran burn zones have shown that post-fire saguaro populations dropped by about 70% on average within the first two years, as plants succumbed to rot and infection and inability to resist weather extremes. This scene was taken in July 2005, and I am writing this in November 2019. Although I have not been back to the Vekol Valley in those 14 plus years to do a follow up photo session, if I were to somehow be able to re-photograph this scene exactly I am fairly sure that nearly all of the saguaros would have fallen and most of the larger palo verde and ironwood trees be skeletons. I don’t think the Vekol Valley has burned again in the past 14 years, but the lingering effects of the first and only fire ever seen here will still be very evident by comparison of then and now.

One chainfruit cholla escaped the flames. Can it replace all of its burned brethren in the background over time?

The flats of the Vekol Valley suffered the most burning since the sandy soils supported the most weeds there. The hills of the Maricopa Mountains weren’t as badly affected since rocks inhibited both extensive weed growth and fire spread, fortunately.

A giant collapses.

Signs of rotting were quite evident in many of the burned saguaro cacti only about 5 to 6 weeks after the Vekol Fire had passed. This is not much time in the scheme of things. It also illustrates the damage fire causes even when the plants were not technically “burned” in the same way we tend to think of forest trees burning, which means total foliage removal.

I observed at least a dozen such fallen giants in only a couple of hours, and I was there only a few weeks after the fire. Had I shown up about 3 or 4 months later, I would probably have witnessed hundreds, or even thousands of them lying about, decaying.

As shown in many photos here, the saguaros remain standing for some weeks to months after the fire, often still partly green at the tops, and most people will misinterpret this as them “having survived”. Few people are around to witness the systemic rot and collapse of thousands of the plants in the months to year after the fire has passed. By then wind and rain has dispersed the black ashes, returning the landscape to the expected tan color. Once fallen, the skeletons are quickly degraded by termites and other decomposer organisms, and in 20 years, this scene will show few or no traces of them having been there at all. Unless you knew what used to exist in these valleys up until the year 2005, you wouldn’t know it from the 2019 perspective.

Ghostly ashes of a burned skeleton trace the gravesite of a former saguaro.

This particular skeleton above had to have fallen before the fire and been dry and ready to burn, so the fire didn’t cause this individual’s death. But once again it illustrates a larger point, which is that a second fire will remove nearly all traces of saguaros ever having been here at all. All of the fresh and still wet skeletal remains of rotting saguaros post-fire will dry out within a few months at most. If another fire passes by again in the future, those dry skeletal remains, primed to burn, will disappear. And that will be it. No trace whatsoever that large numbers of saguaros ever lived upon the land here. Just boring flats filled with exotic invasive grasses. The ecological type conversion from Sonoran Desert to weedy savanna will be complete.

There is a sort of shocking beauty to this cleanly-scalped barrel cactus in front of the dead palo verde tree that helped nurse its survival in its original years. It also makes me sad.

Final harvest: Dry saguaro fruits litter the burnt soil at the base of a dying saguaro cactus.

The Vekol Fire (and other Sonoran fires like the Cave Creek Complex north and east of Phoenix) occurred at the end of May and the beginning of June, 2005. This is the blooming and fruiting season for saguaros. The plants typically start blooming in early to mid-May, set fruits that enlarge throughout the rest of the month, and start ripening to red and dispersing seeds in June and early July. Most of the saguaros that burned in the Vekol Fire were in their immature fruit stage of development. By the time I arrived in early July they had dispersed the majority of those fruits, which had largely been emptied of seeds by birds and insects, with the red rinds dropping to the ground.

A latecomer saguaro manages to push out a last few fruits. Nearby neighbors have already finished. Note the extensive tan scabbing on the trunk where the spines burned off of the ribs. This plant likely died shortly after this picture was taken.

This shows yet another important dynamic of saguaros in the ecosystem. These massive succulent plants contain tons of water reserve in their stems and arms and this is more than enough to support the production and ripening of a crop of flowers and fruits, even when they are dying. And once again, people will often take one glance at a fruiting saguaro with a burned trunk and think, “Wow, it’s got flowers and fruits and it looks happy! There is nothing wrong here after all! Fire is the way of life and brings renewal to the desert.” As shown, however, this is the final act for the saguaros, and a majority of plants collapse into piles of rot shortly after producing their last seed crop. Had the Vekol Fire occurred in October, when the cacti don’t bloom, then this last reproductive step would have been skipped. Fire might indeed bring renewal and fresh growth to ecosystems that have evolved to handle it. But the Sonoran Desert is not among such places.

A few floral remnants cling to the branch tips of these doomed saguaros. They will never reproduce again.

A coyote scat filled with saguaro seeds.

This will be the last time such fruit abundance will be possible in much of the Vekol Valley, since the majority of saguaros were killed by the fire. Coyotes are just one of the many animals that rely upon saguaro fruits for food in season. They are also effective seed dispersal agents, along with other highly mobile animals such as birds and bats. But when there is a mass death of the entire saguaro population, how possible is recovery at all, when so much of the seed crop necessary to keep the population going has been suddenly eliminated? And what about all of the missing and dead nurse trees that shelter the seedlings? Those are gone too!

This is why fires, even small ones that seem insignificant to us, are so wretchedly destructive to the desert. Joshua trees and saguaros alike suffer from ANY FIRE AT ALL.

Firefighting efforts in the Vekol Fire stain the hills red after air tanker drops of fire retardant.

The Vekol Fire occurred at approximately the same time as numerous other fires across the region, not just in Arizona but also in California, Nevada, and Utah. Firefighting resources were spread thin as a result of all of this demand, costing hundreds of millions of dollars to boot. Most of the effort went towards protecting human infrastructure and buildings, while hundreds of thousands of acres of remote rural land in all four states were left alone to burn simply because they were assigned the lowest priority. Even ecologically valuable lands that should have received priority firefighting protection were left behind. That said, clearly some resources were dedicated to fighting the Vekol Fire in the form of several red swaths where dyed fire retardant was dropped from aircraft across the hills.

A stretch of desert hillside after retardant was applied.

This teddybear cholla (Cylindropuntia bigelovii) shows some signs of new growth, post-retardant drop. These chollas put forth new joints in the late spring and so the natural pale yellow spines appear to have emerged after the drop was done, while older growth shows the staining.

The area covered by this fire retardant drop was not burned, and not necessarily because of the retardant doing its intended job, either. I didn’t see any burning within several hundred yards of this spot. Perhaps fire crews managed to stamp it out before it got here and the retardant was a precaution? Either way, the vegetation and rocks are stained red as a result.

Fire retardant is usually a mix of about 85% water, 10% ammonium phosphate fertilizer, and 5% other agents such as thickeners and emulsifiers that help it stick to targeted vegetation. The red dye helps airplane pilots track where drops have already been made so as to avoid re-treating the same zone. Retardant is not a cheap way to fight fires, either, costing somewhere between $2.50 and $3.50 per gallon for Phos-Chek, the brand name of the most commonly used retardant. Add to that a flight operating cost for large air tanker planes of about $16,500 per hour as of 2017 prices. Load sizes vary based upon the particular aircraft in use but range from 1,200 gallons for small planes up to 18,500 gallons for the newest and largest 747 plane, which was originally purposed as a commercial passenger plane before being converted into a firefighting plane. It would seem that the retardant drops here in the Vekol Fire in 2005 were done by a small plane (possibly a helicopter?) dropping not much more than the 1,200 gallons at the bottom of the scale.

In an arid environment the red staining can persist for years after the fires are extinguished.

In addition to the high financial cost of using retardants to fight wildfires, there are rising concerns about the ecological impacts that can occur in their wake. Since retardants are ammonium phosphate, basically they are enormously concentrated doses of fertilizer containing huge quantities of both nitrogen and phosphorus, two of the three most critical elements that all plants (and eventually animals) need to grow. Many soils in the western US, where most fire retardants are used upon wildlands (as opposed to in the eastern US) are of naturally low or modest fertility, including desert soils. The plants of the Sonoran Desert are adapted to this low fertility, and are generally slow growing by nature. It is unclear whether they will be negatively impacted by large retardant drops in years after the fires are gone, and in what ways if so. But it’s pretty likely that the effects will be noticeable in one form or another.

The fire retardant didn’t help this Ferocactus acanthodes, sadly.

One newly emerging threat that fertilizers in retardant drops are bringing about is that the very same invasive weeds that brought the presence of fires to begin with will respond quickly and favorably to the spike in fertility. These annual weeds are designed by evolution to grow rapidly and survive easily in a variety of conditions; therefore sudden increases in fertility in and around zones where retardant was applied may well serve to increase their growth and fecundity in terms of seed production. As rainfall and runoff spread the water-soluble nutrients down the slope and into the drainage channels below, could the weed and grass growth in those places become even more abundant? If so, will they make the fire threat even worse in the next wet cycle a decade or two from now? This issue needs further study but the suspicion that such a linkage exists is not entirely implausible.

Fire-wilted pencil chollas (Cylindropuntia arbuscula) are another casualty of the Vekol Fire despite the smear of retardant on the hill in the distance.

Additionally, I have noticed from retardant drops in the Mojave Desert near St. George, Utah that traces of red dye still remain upon the stones and trunks of Joshua trees after a decade or more. I have visited these areas repeatedly over the course of about 12 years, between the original fires in 2005 and my latest visit in 2017, and with some careful searching you can still see the red dye where it penetrated into the crystalline structure of the rocks it coated. How well the Joshua tree woodland has recovered in those same years will be the subject of a different post that I have yet to write. Suffice it to say that the recovery of the Joshua tree forest ecosystem post-fire is abysmal, with nearly no tree regeneration. I would have to go look with an eye towards the still-present cheatgrass and Mediterranean grass fields to see whether the fire retardant areas are noticeably different from non-drop zones, however.

The final crop of fruits that this large, multi-armed saguaro will ever produce litters the soil surface underneath the plant.

The damaged valleys of the Maricopa Mountains in a side channel to the larger Vekol Valley, which runs to the west of this scene. Note the retardant drop in the background for a sense of distance.

Let’s for a moment consider a zone such as the scene above. Here, a small drainage wash coming out of the Maricopa Mountains was burned while the nearest hillsides, protected by rockiness and less hospitable to the weedy grasses that plague the flats, were not. In a situation such as this, where the relatively narrow valley floor is not very far from the still-vegetated hillsides, I imagine that ecological recovery could be significantly quicker than other areas nearby. This is because there are seed source plants only a few hundred meters at most from the entire valley floor, on both sides, and some of those seeds can migrate down there via animal dispersal, wind, or water flow to repopulate the saguaros, palo verdes, and other plants that died. Barring any future fires reappearing, it is possible that the valley floor will look somewhat like normal Sonoran Desert again within a few decades, possibly with a new crop of juvenile saguaros even. But only if (and it’s a big “if”!) it does not burn again.

The main Vekol Valley is unlikely to ever host viable populations of saguaros ever again, sadly.

Moving out into the much wider flats of the miles-wide main Vekol Valley, the situation appears to be much bleaker in terms of potential recovery. Here, the nearest stands of blooming-size saguaros and mature seed-bearing palo verde and ironwood trees are located a mile or more away, generally upon the distant hills which escaped burning, which means that seeds from parent plants need to migrate much farther. Even if some seeds do manage to traverse the distance, they will be faced with a hot, flat, sunny landscape nearly free of substantial nurse trees and shrubs (and no “nurse rocks” either) and seedlings will have very little chance of surviving their first few weeks or months after germination. The valley’s vegetation has been permanently altered by one single low-intensity fire event, one that could be considered beneficial in other, more fire-tolerant landscapes and ecological regimes.

The summer monsoon season arrives in early July, 2005, presaged by a dust storm on the southeastern horizon thanks to building thunderstorm complexes over the Tucson area. Outflow winds from the thunderstorms gathered dust and within about 15 minutes they overcame my position.

Clearly the new threat of fire to North American desert ecosystems which have historically never faced such pressure before is potentially quite devastating to the biodiversity of the region. The only long-term viable answer to the presence of overly-abundant fire-carrying weeds and grasses is to find biological controls that help bring these invaders into better balance with the ecosystem. They are now too widespread and integrated into the landscapes of the West to eliminate. They are here to stay, so learning ways to live with them and reduce their impact is the only real path forward.

Biological control methods such as finding appropriate pathogens, parasites, or plant predators is probably the best way to rein in these pestiferous weeds and stop them from taking over as much of the land as they have done. Who would think that simple grasses only a few inches tall would create such a threat, not just to saguaros and Joshua trees, but to millions of acres of forest and sagebrush and other biomes that they have invaded elsewhere as well? It’s all because they make too much fire, too possible, too frequently. It is also possible that genetic modification to make the pest weeds less troublesome might also pay dividends. But the main point is that ignoring the problem has gotten us here, to this point upon the brink of calamitous ecological type conversion and biodiversity loss.

The charismatic vegetation of the Sonoran Desert has a future that looks murky, just as this early monsoon season’s first dry thunderstorm raises dust that partially obscures the sun. We can’t lose hope that it will clear, if we apply ourselves to proper solutions, however.

In order to find solutions to this particular ecological crisis of widespread fires everywhere across the West, we need to fund some research into those biocontrols and genetic modification methods which show promise. For a few tens of millions of dollars, we could probably start finding genuinely workable ways to address the worst invasive weeds and start returning western ecosystems to better health and a much less frequent burn cycle, or no burn cycle whatsoever, as the case may be. With billions of dollars spent annually in terms of fighting weed-fueled wildfires anyway, not to mention billions of dollars more of lost human infrastructure, housing, and even dozens of lives, isn’t it sensible to spend a few million bucks on trying to find solutions?

How much do we care? Can we find the political will to fund some science research into solving the issue, rather than just pouring billions into relentless fires every season? The deserts await our answer….