Southern California Black Walnut [Juglans californica Wats. var. californica]

Southern California Black Walnut [Juglans californica Wats. var. californica]

Listing CNPS List 4 R-E-D Code 1-2-3

State/Federal. Status -- None JUGLANDACEAE Mar.-May

Global Rank G3T2 State Rank S2.2

Distribution: Orange County, Riverside County, San Diego County, Santa Barbara County, San Bernardino County, Ventura County, Los Angeles County

Habitat: This tree grows in open savannah, oftentimes in habitat best labeled walnut woodland. Typically, the understory is limited such as in the Chino Hills. Heavy grazing has occurred throughout most of the historical range where this tree is concentrated. Clays seem to be prevalent in many of the locales where this small tree is concentrated, and this walnut may be more tolerant of clay soils than are most trees and woody shrubs.

Known Sites: In San Diego County walnuts are scattered along the Sweetwater River near Dehesa and Sloane Canyon. These populations are questionably native, although isolated individuals upstream from Sequan are not close to recent historic settlements. Diegueno or Luisueno Indian transport from the north may account for the introduction of the walnut into the area. This walnut is also reported from Cottonwood Canyon near the Santa Rosa Plateau, Bonsall, Jamacha Junction, and near the Tenaja Truck Trail and De Luz. Substantial walnut woodlands occur in Orange County such as at Tonner Canyon near the boy scout camp. Walnuts grow on mesic north-facing slopes of Telegraph Canyon near Yorba Linda and the Diemer Filtration Plant. Slopes with walnut woodland were seen throughout the Chino Hills near the Prado Basin. Concentrations of trees tended to occur here on the steepest slopes where cattle grazing was limited by the terrain. There are numerous other locales for Orange County. Herbarium specimens examined include Carbon Canyon and Brea Canyon Road in Orange County; the San Jose Hills near Pomona, Topanga Canyon, and Claremont Wash in Los Angeles County; also Santa Catalina Island. Smith reports this tree in the Santa Barbara region is sparsely scattered from Carpenteria to Las Cruces Hot Springs, Alisal and Alamo Pintado creeks near Solvang, to Los Olivos and the mouth of Jalama Creek; also along State Highway 150 near Rincon Creek east towards Casitas Pass; Padre John Canyon; Ojai. Raven reports this tree from throughout the Santa Monica Mountains of Los Angeles County, growing with oaks or in chaparral on mesic north-facing slopes.

Status: The Southern California Black Walnut is declining throughout its range. Hybridization with introduced horticultural walnut trees is reported to pose a problem. Overgrazing has been the primary impact of recent decades with an apparent loss of young trees among increasingly senescent stands; however, urban development focusing on substantial regrading of steep slopes is becoming a major threat to foothill populations.

Copyright May 1994 Craig H. Reiser.

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