USGS Home Contact USGS Search USGS

Maps, Imagery, and Publications Hazards Newsroom Education Jobs Partnerships Library About USGS Podcasts/RSS

Non-native Grasses & Fires Create Double Jeopardy

Red brome - Photo by C. Schwalbe

The Arizona Upland Subdivision of the Sonoran Desert is home to the giant saguaro cactus, a 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 exuberant boon of flowers has added a burden of risk to life in the desert.

Sonoran desertscrub after the 1993 fire near Sugarloaf Mountain near Phoenix, Arizona. Photo by C. Schwalbe

Rains that promote spectacular wildflower displays also increase the production of non-native 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 of the pattern of summer and winter rains that occurs near Tucson.

Winter precipitation in this part of the desert promotes the growth of the non-native red brome, and adequate summer monsoons promote the perennial non-native buffelgrass. Red brome is one of several non-native annual plants invading the Southwestern deserts. These plants originated from the 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 to 3 years after the seeds have dropped.

Buffelgrass originated in Africa and relies predominately on warm temperatures and precipitation during the summer monsoonal storms. Both types of grasses cause fires during the parched arid summer weather 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.

Burned saguaros and yuccas as a result of the 1995 Rio Fire in the Phoenix, Arizona area. Photo by T. Esque

That fires are of concern may come as a surprise, because 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 determined that there are increased risks to the survival of saguaros and tortoises by exposure to the fires caused by non-native grasses. Until recently, buffelgrass was thought to be predominantly a roadside weed. But backcountry surveys show that this grass has spread in 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.

NPS and USGS-sponsored research has 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 on this scale are considered catastrophic among long-lived species. In fact, the fires that follow invasions by non-native grasses have the ability to change the structure of the deserts. Even less intense fires cause 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 non-native plant invasions, increased fire frequency, and restoration are interrelated, requiring an integrated research program to gain valuable information for managers. New research should focus on fire behavior, fuels management, seed bank ecology, invasive plant control, and the effects of habitat change on the diverse native plant and animal communities in the Arizona Upland Subdivision of the Sonoran Desert.