PARENT SESSION
Oral Session 18: Ecological Theory I: Modeling.
Presiding: R Zabel
Monday, August 2, 8:00 AM to 11:30 AM, Meeting Room F 152.
Life history evolution of seed dormancy cycles in desert winter annuals.
Mathias, Andrea1, Chesson, Peter1, 1 University of California, Davis, CA, USA
ABSTRACT- The seeds of annual plants exhibit yearly dormancy-nondormancy cycles ensuring dormancy at unfavorable times and germination when reproduction is expected to be successful. The physiological change of seeds is a dynamic continuum induced by environmental cues: the seasonal change of temperature and photoperiod. The pattern of changes in dormancy states is a result of life history adaptation. In desert climates some winter annuals germinate only in autumn and their seeds become dormant during winter, while the seeds of others can germinate in mid winter as well. The cause of the diversity of dormancy cycles in winter annual communities is not understood. We use a modeling approach created to investigate the mechanisms of competition in variable environments, which enables us to study the joint effects of predictable seasonal temperature change, unpredictable rainfall variation and competition on the life history evolution of seed dormancy cycles. The model consists of a differential equation system describing the growth of biomass and the change of soil water content through the growing season. The dormancy sates of seeds respond to the cyclical changes of ambient temperature, and germination occurs when temperature coincides with the temperature required for germination. Germination in mid winter may be disadvantageous: seedlings have to compete with plants from autumn germinated seeds, and plants from late germinating seeds must have faster life cycles to reproduce before summer drought. However, although water has to be abundant for germination, subsequent water availability to growing plants varies due to high variation in yearly patterns of precipitation. The advantage of germinating in mid winter may be that seeds can benefit from a broader rainfall distribution and reproduce even if autumn rains were not sufficient for establishment. We use the model to test these hypotheses and find further explanations for the observable patterns of seed dormancy cycles.
Key words: germination cueing, winter annuals, dormancy cycle, life history