Seed germination regulation in Bromus tectorum (Poaceae) and its ecological significance

Bromus tectorum is a winter annual grass that has become extensively naturalized in western North America. Its seeds are usually at least conditionally dormant at dispersal and lose dormancy through dry afterripening. Germination response to temperature for recently harvested seeds and rate of change in germination response during afterripening were examined for collections from 21 western North American populations representing a wide array of habitats. Analysis of variance showed highly significant among population differences in germination response variables. Principal components analysis of 20 germination variables revealed groups of populations that could be characterized by distinct response syndromes. Degree of dormancy at summer temperatures in recently harvested seeds as well as rate of dormancy loss during dry storage could be related to the risk of premature summer germination in different habitats. Mojave Desert populations showed the most clearly differentiated response. Populations from Intermountain desert find foothill habitats showed intermediate responses and did not form distinct groups. Montane populations showed the widest variation. Fully afterripened seeds from all populations were nondormant and could germinate quickly across a wide temperature range. These results demonstrate the existence of adaptively significant variation in germination response. Such variation probably represents the beginning of genetic differentiation as a result of selection among and within founder populations. Lack of a consistent relationship with habitat reflects the stochastic nature of colonization and the fact that diverse germination strategics may permit persistence, especially in less extreme habitats.