Determining the Mediterranean origins of invasive red brome in California: a preliminary assessment to understanding plant-soil feedbacks

Allen, Edith B.1, Allen, Michael F.1, Ellstrand, Norman C.1, Silvera, Katia1, Sanders, Andrew1, Redak, Richard A.1, McDonald, Chris2, Bell, Carl E.2, Aronson, James3, Baréa, José Miguel4, Blank, Robert5, Cristofaro, Massimo6, Cruz, Christina7, Manrique, Esteban4, Gokhan, Serdar8, Kashefi, Javid9, Lopez, Alvaro4, Miranda, Juande4, Nemli, Yildiz8, Ochoa-Hueso, Raúl4, Pérez, Estefania10, Querejeta, Ignacio4, Rector, Brian5, Roldán, Antonio4, Secmen, Ozcan8, Sforza, René9,  and Türkseven, Suleyman8

1Center for Conservation Biology, University of California, Riverside, CA 92521, USA.
2University of California Cooperative Extension, Southern California, USA.
3CNRS-UMR, Montpellier, France.
4Consejo Superior de Investigaciones Científicas (CEBAS-CSIC), Spain.
5USDA-Agricultural Research Service, Reno, NV, USA.
6ENEA C.R. Casaccia, UTAGRI, Rome, Italy.
7Dept. Biologia Vegetal, Faculdade de Ciencias de Lisboa, Portugal.
8Ege University, Izmir, Turkey.
9European Biological Control Laboratory, France and Greece.
10Misioneras Concepcionistas, Algeciras, Spain.

Invasive species from the Mediterranean have transformed California ecosystems by causing vegetation-type conversion of native plant communities, reducing biodiversity, and changing plant-soil feedbacks. We initiated studies of plant-soil feedbacks as a mechanism of invasion for Bromus rubens and B. madritensis subsp. rubens (both known as red brome). These are among the most abundant invasive grasses in undisturbed as well as disturbed southern California native vegetation, but are ruderals in the Mediterranean. We hypothesize that their abundance is controlled by differences in soil nutrients and rhizosphere microorganisms in native and invaded habitats. California has more nutrient-rich soils and different rhizosphere microbes that may allow red brome to proliferate. We are currently undertaking a study of the biogeographic origins of red bromes prior to further rhizosphere studies, to determine sites of origin from the Mediterranean. During spring 2011 we surveyed red bromes across the Mediterranean from the Iberian peninsula to Turkey. The greatest movement of genetic material was likely from Spain with the California Mission settlements of the 1750’s and southern Italy after California statehood in 1949, but settlers arrived from other Mediterranean countries as well. The Flora of Europe and local floras list Bromus rubens in all Mediterranean countries, but we located it primarily in Spain in areas that matched the precipitation of southern California (250-450 mm). Other southern Europe Mediterranean countries and Turkey have precipitation > 450 mm and had abundant Bromus madritensis subsp. madritensis, but not B. madritensis subsp. rubens or B. rubens. We have not yet searched the Mediterranean islands, some of which have precipitation < 400 mm and may be sources of red bromes, but are proceeding with a genetic analysis to determine the relatedness of Spanish red bromes with California populations. We look forward to communication with colleagues concerning the presence of red bromes across the Mediterranean.