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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. [ABSTRACT FROM AUTHOR]

Cultivation legacies affect native vegetation in old fields of the Great Basin, USA for nearly a century after these fields are abandoned. We hypothesized that cultivation lowered soil nutrients and that this legacy would differentially impact plant performance of four representative Great Basin species. To test these hypotheses, we compared soil nutrients (C, N, P, K, Mg and Ca) between two formerly cultivated and adjacent noncultivated sites in two soil series. We then compared the plant growth and foliar nutrient content of an exotic grass ( Bromus tectorum L.), two native grasses ( Elymus elymoides [Raf.] Swezey and Achnatherum hymenoides [Roem. and Schult.] Barkworth), and a native forb ( Sphaeralcea grossulariifolia [Hook. and Arn.] Rydb) grown in these soils in the greenhouse and in the field. Only one sampling site had reduced soil nutrients associated with cultivation legacies, where most of the negative effects on plant performance were found. E. elymoides appeared to be less affected by cultivation legacies than did A. hymenoides, which had a reduced survivorship and 20 % less above-ground biomass in cultivated soils. No species, including B. tectorum, were favored by cultivation. Our findings suggest that cultivation legacies can affect plant performance of different species in different ways and that altered soil nutrients may interact with other abiotic and biotic cultivation legacies in complex ways. [ABSTRACT FROM AUTHOR]

Our objective was to determine the resource most limiting to plant growth within a semi-arid plant association. We hypothesized that the addition of nitrogen (N) would elicit an increase in biomass more than additions of water, light, phosphorus (P), potassium (K), or sulfur (S). Treatments included the addition of water, N, P, K, or S, or reduction of ambient light on two sites in southwestern Montana within the Festuca idahoensis/Agropyron spicatum plant association. Nitrogen was the only treatment that increased plant community biomass, specifically native grass biomass (about 50% greater than the control). The addition of water decreased above-ground plant community biomass. Our results suggest that N is the primary limiting resource for the dominant functional group within the F. idahoensis/A. spicatum plant association. [Copyright &y& Elsevier]