Resistance to invasion is a function of the capacity of a community to limit the population growth of an invading species. Factors that determine plant community resistance to exotic annual brome-grasses (Bromus) are diverse and context-specific. Resistance is influenced by environmental characteristics, attributes of the community, traits of Bromus species, and direct and indirect interactions of Bromus with the plant community. Seasonality of precipitation influences plant community resistance to Bromus through effects on soil water storage, timing of water and nutrient availability, and dominant plant life forms. Differences among plant communities in how well soil resource use by the plant community matches resource supply rates can influence the magnitude of resource fluctuations due to either climate or disturbance and thus opportunities for invasion. Spatial and temporal patterns of resource availability and acquisition of growth resources by Bromus versus native species strongly influence resistance to invasion. Traits of Bromus that confer a ‘priority advantage’ for resource use in many communities include early-season germination and high growth and reproductive rates. Resistance to Bromus can be overwhelmed by high propagule supply, low innate seed dormancy, and large, if short-lived, seed banks. Changes in climate that result in either warmer and drier conditions or shifts in precipitation seasonality may result in both contraction and expansion of Bromusspecies.
Results/Conclusions
Understanding of effects of climate and soils on Bromus and plant communities can be used to determine appropriate management strategies. Bromus are generally more effective than natives at using soil resources on short timescales, and restoration species need the capacity to both establish and compete. Species with similar phenology and resource use patterns such as perennial grasses are typically the best competitors. In less highly invaded areas both passive and active management can increase perennial grasses. In heavily invaded areas decreasing seed production and seed banks is key to restoration success. A high degree of uncertainty in climate predictions necessitates embracing adaptive management. Monitoring natives and bromes at range margins may give advanced warning of range shifts, and management to promote native species diversity and abundance may reduce invasion rates. Assisted migration - purposeful movement of individuals or propagules of a species to facilitate or mimic natural range expansion or long distance gene flow – may be required given rates of change.