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| Nonnative Grass Invasions and Fire in the Sonoran Desert |
Click on each photo for a larger image. |
The Arizona Upland Subdivision of the Sonoran Desert is home to the giant saguaro cactus, symbol of this desert. In this mostly arid land, above-average seasonal precipitation heralds copious desert wildflowers in both spring and summer, but recently this occasional boon has added a burden of risk to life in the desert. Rains that promote spectacular wildflower displays also increase the production of nonnative grasses that act as fine-textured fuels and can carry destructive fires. In some respects the risk of fire in the Arizona Upland Subdivision of the Sonoran Desert is double that of the Great Basin and Mojave desert habitats, because approximately half of the annual precipitation falls in winter and half in summer. |
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Winter precipitation in this part of the desert promotes the growth of red brome
(Bromus madritensis [formerly Bromus rubens]), and adequate summer
monsoons promote perennial buffelgrass (Pennisetum ciliare). Red brome is one
of several nonnative annual plants invading the Southwestern deserts; these plants
originated from Mediterranean regions of the world. The annuals grow through the winter,
bloom in the spring, and then lie dormant as seeds during the rest of the year. But
their stems are persistent and can add to fuel loads for 2-3 years after the seeds
have dropped.
Buffelgrass originated from Africa and relies predominately on warm temperatures and precipitation during the summer monsoonal storms. Both types of grasses carry fires during the parched arid foresummer of June and July. The fires can be caused by lightning storms, accidents, or careless motorists tossing cigarettes out of their cars and igniting the tinder dry fuels created by these grasses. The results can be devastating and cause lasting changes to desert communities. One of the clearest examples of this process is in the desert along Highway 87, north of Phoenix, Arizona. |
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That fires are of concern may come as a surprise since recent educational programs have promoted the benefits of allowing natural fire cycles to manifest themselves in some wildland areas. However, fires do not appear to be a natural part of the saguaro-palo verde plant communities that characterize this desert. In fact, if fire had been a common occurrence in these diverse plant communities, there would be far less expansive stands of saguaros. Both saguaros and the smooth, green-barked palo verde trees suffer great losses when exposed to fire because their thin epidermal layers do not provide protection from excessive temperatures during fires. |
| In collaboration with the National Park Service and Bureau of Land Management, USGS researchers have determined that there are increased risks to the survival of saguaros and tortoises by exposure to the fires caused by nonnative grasses. Until recently, buffelgrass was thought to be predominantly a roadside weed. But backcountry surveys show that this grass has spread in the remote areas of the Sonoran Desert. Both long-lived denizens of the desert, the saguaro and the desert tortoise can be harmed by fires that result from these grasses. |  |
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During National Park Service and USGS-sponsored research, researchers determined that 11 percent of a sample of tortoises died as a direct result of a desert fire. Saguaros also suffered a high degree of mortality. Over the course of five years, more than 20 percent of a sample population of saguaros died. Losses such as these are considered to be catastrophic among long-lived species. In fact, the fires that follow invasions by nonnative grasses have the ability to change the structure of the deserts. Even less intense fires are causing long-lasting changes in the composition and diversity of plant communities. | |
| Researchers are only beginning to understand the changes in Southwestern deserts that result from these plant invasions and fires. The problems of nonnative plant invasions, increased fire frequency, and restoration are interrelated and require an integrated research program to gain valuable information for managers. | ||
Buffelgrass Links
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Todd C. Esque Research Ecologist USGS Western Ecological Research Center Las Vegas Field Station 160 N Stephanie Henderson, NV 89014 Phone: (702) 564-4506 Fax: (702) 564-4600 Email: todd_esque@usgs.gov |
Cecil Schwalbe Research Ecologist USGS Southwest Biological Science Center Sonoran Desert Field Station Room 125, Biological Sciences East University of Arizona Tucson, AZ 85721 Phone: (520) 621-5508 Fax: (520) 670-5001 Email: cecil_schwalbe@usgs.gov | |
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