Extensive Dieback in Malosma laurina in Coastal Exposures of the Santa Monica Mountains Associated with Unprecedented Drought in California

Throughout coastal exposures of the Santa Monica Mountains (SMM) there is a recent emergence of widespread dieback in Laurel Sumac (Malosma laurina). M. laurina is a keystone species of chaparral that dominates the SMM. A significant trait of M. laurina is an extensive root system exceeding 40 feet in depth. We hypothesized that M. laurina was weakened by unprecedented drought in California (2013-2015) and predisposed to fungal pathogens. An initial study conducted in the summer of 2015 revealed all stem samples taken from dieback adult M. laurina plants were infected with Botryosphaeria dothidea, while zero stem samples taken from healthy adult plants contained the fungus. A survey of dieback in M. laurina was undertaken at 10 microsites along coastal exposures of the SMM between elevations of 60 meters to 300 meters. Plants were scored on a scale of 1 to 5 based on the percentage of dieback a plant was experiencing, and for each of the microsites surveyed, the % adult mortality and canopy dieback did not differ significantly among the microsites. Mean score values for the 10 microsites were 1.31 for dieback plants compared to 3.83 for control plants; these predominantly healthy sites and plants possessed greater access to water. Sample sizes (n) at each microsite varied between 15 and 60 plants. From these results, it is clear that the M. laurina dieback is extensive, but may partially recover if the events surrounding El Niño, specifically rainfall, materialize this coming winter