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

Abstract

Some invasive plants perform better in their area of introduction than in their native region, and this is often attributed either to phenotypic responses and/or to adaptive evolution following exposure to new environmental conditions. Genista monspessulana (French broom) is native to Europe, but highly invasive and abundant along the Pacific Coast of the USA. In this study, the population density and age structure, plant growth and reproductive traits, and seed bank characteristics of 13 native (Mediterranean Basin) and 15 introduced (California, USA) field populations of G. monspessulana were compared. Mean population density, plant height and stem diameter were greater in introduced populations, with the latter two traits explained by a greater mean plant age. Age structure also 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). Thus, seed rain and seed bank size was considerably higher in introduced populations. Results from this study indicate that G. monspessulana performs better in its introduced region. We hypothesize that release from natural enemies and competitors together with more favorable environmental conditions in the introduced region may explain the invasion success of G. monspessulana. As a result, an integrated management approach using introduced seed predators to suppress seed production and selected management practices to reduce seed banks may be needed for effective long-term control in California.