PS 70-91
Defoliation and habitat effects on Medusahead demography

Thursday, August 13, 2015
Exhibit Hall, Baltimore Convention Center
Elise S. Gornish, Plant Sciences, University of California, Davis, Davis, CA
Jeremy J. James, Sierra Foothills Research and Extension Center, University of California, Davis

The winter annual grass Taeniatherum caput-medusae (L.) Nevski, commonly known as medusahead, has invaded over 17 western states, significantly decreasing livestock forage production, reducing biodiversity, and promoting increased wildfire frequencies. Targeted control efforts are highly variable, and have not demonstrated long-term success. Contributing to the limited success of controlling medusahead is an absence of fundamental demographic knowledge about this noxious weed, which is essential for predicting the behavior of future invasions. By gaining a better understanding of the life cycle of medusahead, we can assess the susceptibility of life stages to different abiotic factors, and create models to assist with combating future invasions.

We examined the demography of medusahead across its life cycle within oak woodland and open grassland habitat types at different seeding rates (0, 10, 100, 10,000, and 50,000 seeds per m2), both with and without targeted defoliation, in a randomized design. Survival, growth, and fecundity data was collected to construct periodic matrix models over 1 year. Perturbation analysis was used to determine the sensitivity of each stage to experimental treatments. 


We found overall population growth rate of medusahead to be lower in oak woodland habitats compared to open grasslands, as a result of reduced germination and seedling establishment. We also found that defoliation served to decrease overall medusahead density in both habitats. However, this reduction in density led to an increase in flower production by individuals that escaped the defoliation treatment (due to a late phenology). We expect that this increased flower production will be a positive feedback on population growth for the following year. This work highlights the importance of integrating demographic considerations into invasive weed management efforts. Specifically, our work suggests that effective eradication methods for medusahead require a series of defoliation treatments in order to target both early and late phenology individuals.