Twenty-Fourth Annual Meeting and Symposium of the Desert Tortoise Council, March 5-8, 1999
Abstracts

One of the most significant effects of plant invasions worldwide is the alteration of natural fire regimes. In most cases invasions lead to increased frequency of fire, especially invasions of exotic grasses. The effects of increased fire frequency can be dramatic in regions where fires are historically rare. Estimates of historical intervals between fires in the Mojave Desert range from 30 to > 100 years. The invasion of exotic grasses has shortened this interval to an average of 5 years in some areas, resulting in significant changes in plant communities and in threats to the desert tortoise (Gopherus agassizii).

Annual exotic grasses have been shown to cause type-conversion of native desertscrub to exotic annual grasslands in the Great Basin and Mojave deserts. Dead stems of the exotics red brome (Bromus madritensis subsp. rubens), cheatgrass (Bromus tectorum) and the Mediterranean grass (Schismus arabicus and S. barbatus) remain rooted and upright into the summer fire season and over successive years, whereas those of most native forbs crumble soon after they senesce. These exotic annual grasses often cover the desert landscape resulting in continuous and persistent fine fuels that facilitate the spread of fire in an otherwise fire-resistant landscape. Frequency and cover of exotic annual grasses can increase within 3 to 5 years after fires, thus increasing the continuity and amount of fine fuels that promote additional fires. This grass/fire cycle reduces fire return intervals and significantly threatens desert tortoise populations and ecosystem integrity.

Perennial exotic grasses can also promote fires. One such species is buffelgrass (Pennisetum ciliare) which was introduced as a forage species to Sonora, Mexico. Thousands of hectare of Sonoran Desert have been converted to buffelgrass stands for livestock. In stands where buffelgrass is cultivated for livestock, fire is sometimes used as a management tool to the keep stands vigorous. Since this plant has naturalized, additional large expanses of desert and thornscrub habitat have been converted to buffelgrass that burn occasionally. This perennial grass has moved northward from Mexico and into the Sonoran Desert of Arizona mostly along roadways. In Arizona, several urban parks have been invaded by buffelgrass and there has been habitat degradation due to fires at these locations. Buffelgrass was only recently discovered in Arizona wildlands and although fires that result from buffelgrass stands appear to be imminent, to date they have not been recorded in great numbers. The potential exists for buffelgrass to invade other desert regions such as the Mojave and Colorado deserts, and studies are planned to evaluate their potential geographic range.

Fires affect desert tortoises directly by killing them with lethal heat or low oxygen levels, and indirectly by altering their habitats. Fires in general appear to be detrimental to the desert tortoise and its habitats. Management of fire in the Mojave and Sonoran deserts should focus on preventing invasions of new exotic grasses, minimizing the dominance of exotic grasses already present, reducing the number of human-caused fires, and suppression of fires when they occur.

References

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