Status of Endangered La Graciosa Thistle Cirsium scariosum var. loncholepis (Asteraceae) in Coastal Southern California

Abstract

La Graciosa thistle Cirsium scariosum var. loncholepis (Asteraceae) is a biennial or short-lived perennial plant (usually 10 to 100 cm tall) that is endemic to southwestern San Luis Obispo County and western Santa Barbara County in coastal southern California, and little has been published regarding it. The taxon was listed as threatened under the California Endangered Species Act in 1990 and endangered under the U.S. Endangered Species Act in 2000. At Federal listing in 2000, La Graciosa thistle was known from 17 occurrences, 8 of which were likely extirpated. As of 2019, it is known from 21 occurrences of which five are extant, 15 are likely extirpated and 1 has unknown status. La Graciosa thistle exists as groups of individuals in wetland habitats in an arid and semi-arid landscape, and the five extant occurrences are associated with the Callender Dunes and Guadalupe Dunes in San Luis Obispo County. The plants flower once and then die, with a probable life span of 2 to 6 yr. Seed dispersal is by wind and also likely by water, and the taxon appears to have only a minimally persistent soil seed bank. The primary threat to La Graciosa thistle in 2019 is reduced water/lack of water, with groundwater decline as the likely major cause, along with hydrological alteration, drought and climate change. The groundwater decline appears to result primarily from extraction for urban, agricultural and industrial uses, and it is exacerbated by drought and climate change. In 2019, La Graciosa thistle meets the IUCN criteria for endangered.