Factors affecting Euphorbia terracina recruitment and establishment in coastal sage scrub

Euphorbia terracina is an aggressive and invasive non-native plant that has recently become abundant in some parts of southern California. Its distribution has dramatically increased over the last eight years, with large populations scattered throughout Ventura and Los Angeles Counties, including the Santa Monica Mountains National Recreation Area. Management efforts have focused on controlling expanding populations and restoring invaded parkland. Nevertheless, the species continues to establish in intact native plant communities, including coastal sage scrub. Successful invasion and spread of a non-native species relies on the physical attributes of microhabitats within the recipient plant community that are conducive to seedling recruitment and establishment of the pest species. In Mediterranean plant communities, including coastal sage scrub, the factors that have been shown to impact the establishment of other non-native species include the amounts of seed predation, overhead canopy, and understory litter. How these factors affect seedling germination, growth, and survivorship of a non-native species can determine whether or not it will successfully pass through the recruitment and establishment phases of invasion. In this study I sought to identify features of the native plant community (coastal sage scrub) that could be manipulated in order to reduce the establishment and reproductive success of E. terracina, and possibly even favor establishment of native plant species. I experimentally manipulated seed predation, leaf litter, and shrub canopy and evaluated the impacts on E. terracina as well as Salvia leucophylla, the most common co-occuring native shrub in invaded coastal sage scrub. I found that although predation significantly reduced emergence in both species to less than 1%, E. terrracina and S. leucophylla responded somewhat differently to the experimental treatments. Predation combined with the presence of leaf litter significantly increased seedling emergence and survivorship in E. terracina whereas S. leucophylla was most affected by the combined effects of predation and lack of canopy. These findings indicate that additions of leaf litter mulch would not be an effective means of suppressing E. terracina. In addition, they suggest that canopy gaps should be prioritized for weed treatment because these microhabitats are favorable for S. leucophylla regeneration and probably other native species.