The Demography of Bromus Tectorum: The Role of Microclimate, Grazing and Disease

(1) Correlations between weather and the emergence of cohorts and correlation between weather, disease and grazing and subsequent cohort survivorship were investigated for the introduced annual Bromus tectorum in three habitat types in eastern Washington, U.S.A., for three consecutive generations. (2) Emergence of twenty out of twenty-four cohorts among the three sites in late summer to early autumn closely followed showers; emergence in spring could be related less commonly with isolated showers. (3) Emergence in winter probably did not occur until soil surfaces were free of snow and air temperatures were at or above 0 OC; spring emergence was less predictable but occurred even as available soil moisture progressively declined. (4) Many members of cohorts emergent in late summer and early autumn at all three sites were often killed by drought in September and October. (5) In winter, plants, regardless of age, were disturbed by frost-heaving at the moist and mesic sites; grazing by voles was common under snow cover. (6) Infestation by Ustilago bullata was most common at the mesic site. (7) The abundance and distribution of Bromus tectorum seem to be determined by the chronology of environmental events.