Juglans californica

Juglans californica is endemic to California, USA, and occurs in Los Angeles, Orange, Riverside, Santa Barbara, San Bernardino, San Diego and Ventura counties. While there are over 300 occurrences, the species habitat is increasingly fragmented and declining in quality due to threats of urbanization, grazing, fire, and non-native species.

Juglans californica is endemic to southern California in the western United States. It is found in Los Angeles, Orange, Riverside, Santa Barbara, San Bernardino, San Diego and Ventura counties (CNPS 2001). More specifically, J. californica occurs most extensively in Ventura, Los Angeles and Orange counties in the Santa Clarita River drainage near Sulphur Mountain, and smaller stands occur in the Simi Hills and Santa Susana Mountains, the northern slope of the Santa Monica Mountains, the San Jose Hills, Puente Hills and Chino Hills (Quinn 1990). In addition, J. californica does occur outside this range, but is usually found with other species of trees, and most often with oaks (Quinn 1990).

Based on herbarium records and photo-based observations, there are over 300 occurrences rangewide (GBIF 2023, iNaturalist 2023).

Juglans californica is considered fairly threatened by the California Native Plant Society. It is threatened by urbanization, competition from non-native plants species, grazing (especially historically), potentially a lack of natural reproduction, increased fire frequency, and hybridization with orchard trees Walnut forest (R. Bittman, pers. comm., 2018; Quinn, pers. comm., 2002; CNPS 2001, and Quinn 1990). It is also highly fragmented and a declining natural community in California.

Posing the largest threat is urbanization (Quinn 1990). Specifically, in 1990 one of the most extensive stands in Puente Hills (Los Angeles County) was under threat of removal of thousands of trees due to construction of a golf course (Quinn 1990). Additional threats that existed in 1990 included proposals to remove trees for a housing subdivision and an expressway. As of 2002, plans to build the golf course were halted by Cal Poly Pomona campus protests, the expressway was not built, however, some interest does still exist, and the subdivision proposal did not come to fruition (pers. comm. Quinn 2002). Further, there are extensive stands of walnuts at the eastern portion of the Puente Hills that are currently not threatened by development, but withstand extensive cattle grazing. While development efforts in the past have been thwarted, the threat of urbanization or land development continues to exist, as currently new proposals to develop land containing J. californica are being explored (pers. comm. Quinn 2002).