Submitted to: Biological Invasions
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/28/2010
Publication Date: 1/8/2011
Citation: Reddy, A.M., Carruthers, R.I., Mills, N.J. 2011. Introduced populations of Genista monspessulana (French broom) are more dense and produce a greater seed rain in California, USA, than native populations in the Mediterranean Basin of Europe. Biological Invasions. 13:369-380. Interpretive Summary: The introduction of exotic organisms into new habitats is a major global issue due to the detrimental environmental impacts they often cause. One exotic plant originally from Europe, French broom, has established and become widespread in the western U.S. French broom is common in grasslands and disturbed habitats such as road cuts, and can dominate plant communities by forming dense stands and displacing native plants. Exotic plants such as French broom are characterized by more vigorous growth and greater abundance in places of introduction than in their native region. This increased vigor may be due to better environmental conditions encountered in their new habitats or escape from insect herbivores that exist in the region of origin. We investigated whether French broom is more vigorous in California by comparing the population density and growth characteristics of native (Europe) and introduced (California, USA) field populations. On average, plants in California were taller, produced more seeds per pod, and formed denser stands than plants in Europe. Seed rain and seed bank size was also considerably higher in California. Results from this study indicate that French broom is more vigorous and has higher population densities in California. We conclude that escape from insect herbivores together with more favorable environmental conditions and release from plant competitors in California may be responsible for the invasion success of French broom. As a result, an integrated management approach using introduced insect seed-feeders to suppress seed production and prescribed fire to reduce seed banks may be needed to reduce the abundance of French broom in California.
Technical Abstract: Some invasive plants perform better in their area of introduction than in their native region. The increased vigor of invasive plants can result from phenotypic plasticity in response to more favorable environmental conditions or release from natural enemies, or from adaptive evolution of increased competitive ability through less investment in defense in response to reduced herbivory. Genista monspessulana (French broom) is native to Europe, but highly invasive and abundant in the western USA. In this study, the population density and performance of 13 native (Mediterranean Basin) and 15 introduced (California, USA) field populations of G. monspessulana were measured and compared. Mean population density, plant height and stem diameter were larger in introduced populations. Similarly, age structure showed a greater percentage of seedling plants in introduced populations. Fecundity was higher in introduced populations when measured in terms of mature seeds per pod, but lower when comparing seed production per plant (number of pods and mature seeds). Regardless, seed rain and seed bank size was considerably higher in introduced populations. Results from this study indicate that G. monspessulana is more vigorous and has higher population densities in its introduced region. We suggest that release from natural enemies together with more favorable environmental conditions and release from competitors in the introduced region may be responsible for the invasion success of G. monspessulana. As a result, an integrated management approach using introduced seed predators to suppress seed production and prescribed fire to reduce seed banks may be needed for effective long-term control in California.