Bromus tectorum and Bromus japonicus are both exotic winter annual grasses common throughout the western United States. Both species are well adapted to invading native plant communities alter ecosystem processes, whether it is through shortening the fire return interval or interfering with successional processes. Invasion of these species results in a plant community shift from a native perennial community to an annual grassland. The goal of this research was to quantify vegetation, ground surface, soil, and AM fungal characteristics of a Wyoming sagebrush-grassland that has been invaded by exotic annual bromes.
Field sites were established in north central Wyoming, USA. Each site contained plots within a native sagebrush-grassland vegetation type and plots within an area extensively invaded by Bromus tectorum and Bromus japonicus. Vegetation, surface, soil, and AM fungal characteristics were quantified for both vegetation types.
Results indicated most native vegetation functional groups were reduced when exotic annual bromes exceeded 20% cover. Bare ground and biotic crust cover was also reduced in invaded plots. Levels of persistent litter increased as exotic annual brome abundance increased. Species richness and evenness were 87% different between native and invaded plots, as indicated by a similarity index.
Levels of soil available phosphorus, organic nitrogen, and inorganic nitrogen were significantly higher in invaded soils, especially in surface soils (0-5 centimeters). Soil phosphorus and nitrogen levels decreased as soil depth increased. Levels of soil organic carbon did not appear to be different between native and invaded soils.
Arbuscular mycorrhizal fungal activity, measured by spore counts and a mycorrhizal inoculation potential experiment, were not statistically different between native and invaded soils. However, AM fungal activity decreased as soil depth increased. Growth of both Bromus tectorum and Poa secunda was significantly greater in invaded surface soils, likely due to the greater abundance of soil phosphorous and nitrogen.
This research indicates invasion of exotic annual bromes into a Wyoming sagebrush-grassland result in changes in vegetation and ground surface characteristics. Conservation and restoration efforts of these systems should consider the altered soil properties and functions caused by annual brome invasion. Efforts focusing on suppressing and containing annual brome invasion may be more beneficial in the long term than those focused on mitigating soil responses.