Raymond A. Evans

Seed Production and Germination Dynamics in California Annual Grasslands

The California annual grasslands constitute a productive and diverse ecosystem. Productivity and dominance may be highly influenced by the amount and periodicity of moisture events, but the vast majority of the plant species that make up this vegetation type share a common characteristic; they must originate from seeds annually. In order to persist, the plants must produce seeds before the summer drought and disperse a significant portion of these seeds to favorable locations in the seedbed where they await moisture events of sufficient magnitude in the fall to trigger germination.

Demography of Bromus Tectorum in Artemisia Communities

Bromus tectorum has successfully invaded sagebrush (Artemisia) grasslands in western North America within the past century. The population dynamics of this species under drought conditions and during relatively wet seasons are described. Persistence of populations is ensured by means of dormant seed reserves in the soil. Bromus tectorum has a flexible life history though it typically behaves as a winter annual. The frequency and intensity of natural wildfires regulates its population size, but it can recover rapidly after fire and quickly dominate the vegetation by its high vegetative vigor and reproductive output. The genetic consequences of the large fluctuations in population size which follow fire are considered.