By DR. CAMERON BARROWS

“There are no historical records of large fires ... in the Southern California deserts before the invasion of non-native grasses. Now, such fires are becoming increasingly frequent, endangering the continued existence of native desert ecosystems.”

This quote was in reference to the Sawtooth Fire which burned a large area of the southwestern Mojave Desert in 2006. The more recent Cima Dome fire (2020) and York fire (2023), both within the Mojave Desert Preserve in the central Mojave Desert, have spurred questions regarding whether these fires, while once rare, and now more frequent, may have some historical, natural ecological context. There are no standard methods for answering these questions but thinking both using a comparative approach and within a “deep time” framework may provide insights.

There are southwestern habitats in which wildfire has an essential ecological role. Think grasslands, chaparral, oak woodlands, and ponderosa pine forests. Native grasslands depend on fires to keep invading shrublands at bay, for removing dense layers of dead fuels, clearing the way for new growth, and for exposing bare soil so that fire-following wildflowers can germinate. Without fire, grasslands would be much less species-rich as fire-following, fire-adapted species would be lost, unable to germinate through the thick grass thatch. Biodiversity would be lost. To sustain the biodiversity of grasslands, wildfires must be relatively frequent, on a decade to multiple decades return interval.

Chaparral is a “poster child” for a fire-adapted landscape. Many chaparral shrubs have root “burls” that allow those shrubs to resprout weeks and even days following a wildfire. Here too are a myriad of annual wildflowers that are fire-followers, needing the stimulus of heat or smoke or open ground to germinate. These fire-followers have their own pollinators, multiplying the biodiversity associated with this fire-adapted habitat. The open ground, wildflowers, and pollinators provide a boon to lizards, birds, and deer, providing both food and ample opportunities for the reptiles to thermoregulate. Still, there are other shrubs here that are not nearly so fond of wildfires and may take many decades before they are able to sprout and become incorporated into the palette of species that call this habitat home. It could take several post-fire decades before a chaparral landscape reaches its peak in biodiversity, and then begins a slow decline until the next wildfire rejuvenates this habitat once again. The responses in oak woodlands and ponderosa pine forest follow a similar pattern. After fires the oaks resprout and fire-following wildflowers abound. Mature ponderosa pines’ thick bark makes them able to survive fires, fires that clear the forest floor of down and dead branches, as well as competitors, and again, stimulates the germination of fire-followers.

There are few if any parallels of the benefits of fire in grasslands, chaparral, and oak woodlands, compared with the impacts of wildfire on desert and near desert (pinyon-juniper woodlands) biodiversity. Typically, desert wildflowers are stimulated by rainfall, not wildfires. Although there is one exception. In portions of the Mojave Desert there are native perennial bunch grasses and some annual grass species that respond to summer rainfall and then provide enough continuous fuels to carry a lightning-ignited wildfire. Otherwise, deserts usually lack the continuous fuels to carry a fire, and desert shrubs are usually killed by fires, not stimulated to sprout, or flower. Iconic Joshua trees are reported as having 70-80% mortality, or more, following wildfires. Shrubs such as blackbrush have a similar or worse fire-related mortality, and those shrubs are critical in providing essential nurse-plant refugia for seedling Joshua trees.

The recent increase in desert wildfires has been attributed mostly to non-native, invasive grasses that were introduced during the cattle grazing era of the 19th and early 20th centuries. The exception being those areas with dense native perennial bunch and annual grasses. Those grasses do very well after fires, with higher seed production and higher establishment on the open ground provided by the fires. It is that exception that has ecologists wondering whether there is some natural role for wildfires in the Mojave Desert.

The afore-mentioned Sawtooth fire burned through the Pioneertown Mountains Preserve owned and managed by the Mojave Desert Land Trust. I hiked there before the fire, through Joshua trees and at the higher elevation, a pinyon-juniper forest. After the fire there was a near complete cover-type shift to desert chaparral, filled with fire adapted species. The pinyons are gone. There are some Joshua trees starting to fill in, but they are the 10-20% whose roots were not killed by the fire and are now slowly resprouting. Here there were few if any native grasses, just non-native grasses and extremely flammable pinyon pines and junipers. Eighteen years after the fire it does not appear that this landscape will ever completely recover to its former self. But maybe we should think of centuries, or millennia, not decades.

The long-term, deep-time, view could conclude that our deserts are in transition. Some 20,000 years ago and before this landscape supported giant ground sloths, camels, horses, mastodons, mammoths, deer, pronghorns, and more. To support so much animal biomass there must have been trees, shrubs, and lots of grasses. It was cooler and wetter, and wildfires were probably common. Ever since then it has become incrementally warmer and drier. There wasn’t enough forage for the big mammals, and they went extinct. The first arrival of people here was likely 10,000-20,000 years ago. These people likely witnessed the last huge herds of giant mammals on this landscape. They were also masters at using fire to manage their environment and to provide the resources they needed to survive. They used those perennial grass seeds for food and the grass stalks for baskets, but they did not want fires that consumed 1000s of acres. They likely used their fire management skills to keep fires small, and from consuming the other plants they depended on, including yuccas and bear grass (Nolina), pinyons, junipers, mesquite, and oaks. Then, some 100-150 years ago a new group of people arrived, and they brought cattle and non-native annual grasses. The cattle ate the grass and so there was little or no fuel to carry a wildfire, and it kept getting warmer and drier. Finally, even the cattle couldn’t survive.

It is interesting to imagine that those Mojave Desert perennial native grasses could be some of the last vestiges of a world of mammoths and camels, and then of a culture of masters of fire. Whether the current climate trajectory might support those native grasses is too early to tell. If it did, today’s land managers would be better off trying to emulate those indigenous fire masters, keeping the fires small and in control.

Nullius in verba – Go outside, tip your hat to a chuckwalla (and a cactus), and think like a mountain