Secondary Invasion of Bromus tectorum Into Salt and Warm Desert Habitats in the Intermountain West is Due to Recent Introductions

Following its introduction to North America in the late nineteenth century, cheatgrass (Bromus tectorum), an invasive winter annual grass, has become dominant on millions of hectares of sagebrush steppe habitat throughout southern Canada and the Intermountain Western United States. Within the last 30-40 years, cheatgrass has expanded its range southward into the Mojave and Chihuahuan Deserts. Preliminary genotyping data suggest that individuals from the sagebrush steppe habitat in the invaded range are most closely related to individuals found in mesic habitats in northern Europe, whereas individuals from the recently invaded warm desert habitats are most closely related to individuals found in climatically similar habitats in south-central Asia. To provide additional support for our hypothesis that the recent invasion of North American warm deserts is due to the introduction of novel genotypes from the native range, we genotyped 40 cheatgrass populations from recently invaded warm and salt desert habitats using 71 SNP markers. Our data clearly demonstrate that populations throughout the Mojave Desert are dominated by a few inbreeding lineages that are closely related to lineages from the warm deserts of south-central Asia. Populations in similar habitats on the northern edge of the Chihuahuan Desert are more diverse, and include previously characterized haplotypes related to populations in northern Europe as well as novel warm desert haplotypes. The present distribution pattern of cheatgrass haplotypes in North American warm deserts supports the model of initial introduction into the Mojave Desert followed by ongoing secondary spread into the Chihuahuan Desert.