Bromus tectorum and its relatives: Using a database to examine patterns of invasiveness and invasion in brome grasses

Background/Question/Methods

The genus Bromus contains approximately 150 grass species that grow in temperate regions of the world and in mountainous areas of the tropics.  A number of Bromus species are invasive, while many others are not.  Bromus tectorum is of major concern in the western United States as a weed of winter cereal crops and as an invader of natural communities. Other Bromus species introduced to the United States are weeds of crops, disturbed areas, and natural environments. Alterations in temperature and rainfall patterns associated with climate change may affect distributions of introduced Bromusspecies in the United States.

We developed a database of information about Bromus grasses and used it to look for correlations between distribution, weediness, and species traits including taxonomic section, lifecycle, seed awn length, seed weight, polyploidy, human use, and availability of cultivars. The genus Bromus is divided into six sections:  Bromopsis, Ceratochloa, Neobromus, Bromus, Genea, and Nevskiella. Based on existing literature, we hypothesized that species in annual sections Bromus and Geneaare more likely to be invasive than perennial species, that low seed weight and long seed awns are correlated with invasiveness, and that human use and cultivar availability also contributions to invasion.

Results/Conclusions

For this study, we defined invasiveness as high distribution outside of the native range combined with listing as a weed. Taxonomic section, short lifecycle, long seed awns, polyploidy, human use, and availability of named cultivars were all significantly correlated with both wide distribution outside of the native range and with weediness. Seed weight was not. A large number of the species in annual sections Bromus and Genea are invasive, including both diploid and polyploid species.  Only a few species in perennial section Bromopsis are invasive, and invasiveness in this section is correlated with polyploidy, human use and availability of cultivars. The species in New World sections Ceratochloa and Neobromus have life cycles that range from annual to perennial. Invasiveness in these sections seems to be related both to human use and to other factors associated with weediness in species with short lifecycles. 

A number of introduced species in annual sections Bromus and Genea currently grow in warmer areas of the North America.  Research to evaluate effects of climate change on distribution of these species would be valuable.  Further research on use and cultivation of grasses in ways that reduce risk of damage to natural communities would also be useful.