Post-Fire Recovery in the Arizona Upland of the Sonoran Desert
Fire-prone invasive plants fueled fires that converted this formerly diverse Sonoran Desert landscape of small trees and tall Saguaro cactus into an impoverished shrubland.
By Garry Rogers.
The Sonoran Desert’s diverse vegetation of small round trees, tall cacti, and understory shrubs is remarkably beautiful. I was fortunate to spend my early career studying the desert. One of my projects involved wildfire.
Following fires in 1974, my classmate Jeff Steele and I used repeated observations of permanent plots and transects to measure fire-related adaptive responses of perennial plant species and communities. We expected to find that desert plants were recovering by sprouting from unburned roots and stems and from seeds buried in the soil. We expected this because of the “fire is natural” rebellion that was opposing traditional “Smokey the Bear” fire suppression efforts. We wanted to be rebels too. What we found was that positive adaptations that would allow recovery after burning were common, but they were weak. Most plants just burned to death and stayed dead. Return of the original plant community was taking place very slowly. We projected that several decades would be required for full recovery.
After we published the initial results, both sites burned again. We repeated our observations of the plots and transects several times. In 2008, I reported that 22 years after the second fires, recovery had not occurred (Turner et al. 2010). Only a few fast-growing members of the original plant community had returned, and large numbers of fire-prone invasive alien plants occupied both sites. A brief inspection in 2015 indicated that conditions had not improved. It appears unlikely that the original diverse vegetation dominated by tall Saguaro Cacti and round green Paloverde trees will ever return. Fighting fires in the desert was the right strategy.
Perhaps no fire in the Sonoran Desert has been natural since the introduction and spread of exotic annuals. Both frequency and intensity have increased.
Climate Change and Desert Fire
The lengthening drought in the region occupied by the Sonoran Desert is accelerating the replacement of the original plant communities by fire-prone weeds. Weed landscapes are spreading and fires are becoming more frequent. Watching the disappearance of the original complex desert vegetation is one of my saddest experiences.
Citation: Rogers, Garry, and Jeff Steele. 1980. Sonoran desert fire ecology. Pages 15-19 in M. A. Stokes and J. H. Dieterich, technical coordinators. Proceedings of the fire history workshop. U. S. Forest Service, General Technical Report RM-81. Link to PDF copy of paper.
Reference: Tuner, Raymond M., Robert H. Webb, Todd C. Esque, Garry Rogers. 2010. Repeat photography and low elevation fire responses in the southwestern United States. Pages 223-244 in R. H. Webb, D. E. Boyer, and R. M. Turner, eds. Repeat photography methods and applications in the natural sciences. Island Press, Washington, DC. 530 p.
Reuters: Asia-Pacific nations are failing to halt the loss of natural forests and grasslands, the U.N. Food and Agriculture Organisation (FAO) said on Tuesday, robbing people of their livelihoods and worsening environmental problems like desertification and climate change. Continue reading →
Outdoor recreation does far more than simply transport invasive plants. It disturbs soils and vegetation and takes the lives of animals. Leopold commented on the most violent type of recreation:
“The disquieting thing is the trophy hunter who never grows up. … To enjoy he must invade, possess, appropriate. Hence the wilderness that he cannot personally see has no value to him. Hence the universal assumption that an unused hinterland is rendering no service to society” (Leopold 1949: 176). Continue reading →
Using natural landscapes for any purpose requires caution to prevent plant invasions. An essential task for nature conservation is explaining this to the public.
Cattle in the Sonoran Desert. Heavily trampled soil without soil microorganisms that can absorb and store moisture, convert solar energy to nutrients, increase plant root efficiency, and protect the soil surface from erosion and invasive plants. Photo by George Wuerthner.
Most investigations of plant invasions assign responsibility to Humans. In our ignorance, we introduce potentially invasive plants from foreign ecosystems, and then we disturb native ecosystems and help the introduced plants get established and spread. We have learned that diseases, predators, competitors, and supportive soil microorganisms control plant growth. Move plants to new locations where their natural controls aren’t present and they sometimes explode across the landscape. Continue reading →
Wild Horses on a Former Great Basin Shrubland Destroyed by Livestock Grazing, Invasive Plants, and Fire.
In the invasive plant literature, disturbance refers to an event that removes plants and alters the soil surface. “Disturbance is believed to be the major factor favoring plant introductions” (Radosevich et al. 2007: 58). Without disturbance, invasive plants would find no openings to become established and begin to spread and replace native species.
It is important to understand the nature and origin of disturbance that leads to plant invasions, because, as with global warming, it is often profitable to deny human responsibility for invasions so that a disturbance activity can continue. Continue reading →
The Role of Soil Microorganisms in Desert Ecosystems
There would be no life on the land if there was no soil.
“When you thrust a shovel into the soil or tear off a piece of coral, you are, godlike, cutting through an entire world. You have crossed a hidden frontier known to very few. Immediately close at hand, around and beneath our feet, lies the least explored part of the planet’s surface. It is also the most vital place on Earth for human existence” (Wilson, 2010).
Biological Soil Crusts
Biological Soil Crust (Brown stipplescale) growing in a rocky area in the Great Basin Desert.
In sunny desert environments, various species of algae, cyanobacteria, microfungi, lichens, and bryophytes form thin crusts over the surface of the ground. The crusts protect the soil from erosion, enrich its composition, and enhance plant growth. The crusts are among the most important components of desert ecosystems.
Biological soil crusts (BSCs) are quite fragile. If they are damaged, soils lose moisture and nutrients and become susceptible to erosion and invasion by alien plants. BSCs are susceptible to considerable damage by livestock (e.g., Brotherson et al. 1983). Recovery of BSCs at some sites can occur within 20 years (Anderson et al. 1982), but most studies have concluded that longer periods are required (e.g., Jeffries and Klopatec 1987), and that full recovery can require centuries (Belnap 1993). Continue reading →