Invasives in the Southwestern U.S.

Adapted from: Rogstad, Alix, Thomas DeGomez, and Carolyn Hull Sieg. 2007. Invasive plants in Arizona’s forests and woodlands. University of Arizona College of Agriculture and Life Sciences. Publication No. az1436

Competition for resources (moisture, light, nutrients) is fierce in the often resource-limited environments of the desert Southwest. Species native to the desert Southwest have special adaptations to take advantage of short-lived moisture sources and to endure long drought periods (Dimmit 2000). Other arid places in the world besides the southwestern United States support similar vegetation communities, but the climate of Arizona has unique features important to the interaction between native species and non-native invasive species.

Climate influences vegetation patterns at many different scales across the desert Southwest. Small-scale, short-duration monsoon-season thunderstorms can bring much-needed precipitation to small patches of vegetation, or they can initiate widespread flooding, creating a window of opportunity for invasive plants to thrive where flood-prone native plants are lost. Long-term variations in climate related to ocean circulation patterns (ENSO) can create multi-decade wet or dry periods that can promote large-scale increases in the populations of certain species (wet periods) or large-scale mortality (dry periods) (Allen and Breashears 1998).

Tamarisk or saltcedar (Tamarisk spp.) growing along the Colorado River. (Photo courtesy of Steve Dewey, Utah State University,

Increasing evidence suggests that direct or indirect impacts of extreme climate variations may give some invasive plants a competitive edge over a number of native species (Dukes and Mooney 1999). For example, extreme droughts may result in the loss of some native species but enhance populations of invasive plants that are better adapted to warm, dry conditions and have high reproductive rates.

Riparian zones of the Southwest are highly influenced by annual variations in both precipitation and temperature. In Arizona lowlands, riparian zone degradation has been linked to the invasion of tamarisk or saltcedar (Tamarix spp.), a non-native invasive shrub that is both flood- and drought-tolerant. With long-term continued warming and drought, tamarisk has the potential to invade higher elevation riparian zones and displace native riparian plant species such as cottonwood (Populus spp.) and willow (Salix spp.). Once established, tamarisk is very difficult to remove, even with a return to wetter and cooler conditions.

Adapted by Thomas DeGomez, University of Arizona and Amy Grotta, Oregon State University

References Cited
Allen, C. D. and D. D. Breshears. 1998.: Drought-induced shift of a forest-woodland ecotone: Rapid landscape response to climate variation. Proceedings of the National Academy of Sciences 95: 14839-14842.

Dimmitt, M. A., 2000: Biomes and Communities of the Sonoran Desert Region. In The Natural History of the Sonoran Desert, S. J. Phillips and P. W. Comus, eds. Tucson: Arizona-Sonoran Desert Museum Press.

Dukes, J.S. and H. A. Mooney. 1999. Does global change increase the success of biological invaders? Trends in Ecology and Evolution 14:135-139.

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