The establishment and impact of non-native Mexican fan palms (Washingtonia robusta) on native riparian habitats in San Diego County, California

The Mexican fan palm (Washingtonia robusta) is a non-native plant species which commonly grows in riparian communities throughout Southern California, but there is no documentation of the extent of its range or quantification of its specific impacts on biodiversity. Invasive plants are one of the leading causes of endangered species and riparian plant communities are critical for the recovery of these species. Mexican fan palms are common ornamental plants in this region so there is a continuous source of seed, but it is unknown how much of the spread of this species is due to increased plantings as opposed to natural recruitment of established trees. Additionally, runoff from storm drains may have altered hydrological conditions to the point that small tributaries that were previously too dry are now able to support palms. This study documents the extent of the Mexican fan palm's range in portions of the San Luis Rey River and Carlsbad watersheds and measures its impact on soil temperature and light availability. At both study sites, Mexican fan palms were more frequent within 100 m of the seed and freshwater source and able to survive 800 m away. Streams of all orders have the potential for invasion by Mexican fan palms if there is a direct source of freshwater and seed. Streams of lower order are at greater risk than higher order streams. Mexican fan palms impact the microclimate in riparian areas because they are more effective shaders and produce cooler soil temperatures and have less light available under their canopy than one native species, Quercus agrifolia. Keywords: Washingtonia robusta, invasive species, Southern California wetlands