Differential Survival of Chaparral Shrub Species in Response to Historic Drought in California

We examined the response of eleven species of chaparral shrubs in the Santa Monica Mountains to historic drought in 2014. Our hypothesis was that plant species with the deepest roots would have highest survivorship because of greater access to soil moisture. We determine the percent mortality of adult species at the Green Valley Preserve off of Stunt road in the Santa Monica Mountains. The chaparral stand was 21 years old, having last burned in 1993 (Old Topanga Canyon Fire). Assessment was facilitated using a Point-Quarter sampling technique. Each random point (63 total) was divided into four quadrants based on compass direction. We recorded the closest shrub species to the sampling point, distance to the point, plant height, basal diameter, and crown diameter. We recorded only plants that were alive or recently dead (dead within the last 6 months due to their retention of dead leaves). These parameters allowed us to characterize our chaparral stand and each species in terms of 1) percent mortality, 2) relative density, 3) density, 4) dominance, 5) relative dominance, 6) frequency, 7) relative frequency, and 8) importance value. Deep rooted species (Rhus ovata, Malosma laurina, Quercus berberidifiolia, Adenostoma sparsifolium, Heteromeles arbutifolia) had mortalities less than 20% whereas shallow rooted species (Prunus ilicifolia, Adenostoma fasciculatum, Ceanothus spinosus, C. cuneatus, Arctostaphylos glauca) had mortalities ranging between 40 to 93%. These data support our initial hypothesis and suggest that severe drought in southern California is already causing shifts in chaparral community structure that may worsen with accelerated climate change