Importance of soil and plant community disturbance for establishment of Bromus tectorum in the Intermountain West, USA

The annual grass Bromus tectorum has invaded millions of hectares in western North America and has transformed former perennial grass and shrub-dominated communities into annual grasslands. Fire plays a key role in the maintenance of B. tectorum on the landscape but the type of disturbance responsible for initial invasion is less well understood. We conducted an experiment in a perennial shrub/grass/forb community in eastern Idaho, USA to examine the roles of plant community and soil disturbance on B. tectorum emergence and establishment prior to state-changing fires. Our experiment consisted of a plant community disturbance treatment where we (1) removed the shrub component, (2) removed the grass/forb component, or (3) removed all shrubs, grasses, and forbs. We followed this treatment with seeding of B. tectorum onto the soil surface that was (1) intact, or (2) disturbed. Each experimental plot had an associated control with no plant community disturbance but was seeded in the same manner. The experiment was replicated 20 times in two sites (high and low aboveground biomass). We measured emergence by counting seedlings in late spring and establishment by counting, removing, and weighing B. tectorum individuals in mid-summer. We also examined the influence of plant community disturbance on the soil environment by measuring extractable NH4+ and NO3- four times each summer. Soil disturbance greatly influenced the number of B. tectorum individuals that emerged each spring. Plant community disturbance, specifically disturbance of the grass/forb component, increased N availability in the late growing season and biomass of B. tectorum the following summer. We conclude that soil disturbance and plant community disturbance interact to promote the initial invasion of B. tectorum in Intermountain West valley ecosystems.