The species may be divided into two varieties, vars. californica and hindsii, both endemic to California. Walnut forest is a much fragmented and declining habitat. There are just two or three stands remaining of var. hindsii, although it is widely naturalised in parts of California and was formerly cultivated as a rootstock for J. regia, with which it readily hybridizes.

Juglans californica S. Watson:
United States (North America)

United States

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Year-round

Global Range: Juglans californica is endemic to California and occurs 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).

Juglans californica is the most distinctive western walnut, but some care must be taken in identifying it. The distinctive leaflet shape of J . californica is occasionally replicated by early-season leaves of other species. Furthermore, J . californica is distinctive in lacking simple and fasciculate hairs on the leaves, but like most other walnuts, multiradiate hairs are normally present on the young vegetative growth (stems, petioles, and midribs) in the spring. The hairs are usually deciduous early in the growing season. They have short (0.1-0.2 mm), crisped rays and are never clustered or especially associated with vein axils. The fasciculate hairs found in all of our other species (except sometimes J . microcarpa ) are persistent, have longer (0.3-0.4 mm), straight rays, and are concentrated in clusters abaxially in the axils of the main lateral veins.

Shrubs or small trees , to 6-9 m. Bark light or medium gray, divided into rough plates. Twigs with distal edge of leaf scar notched, often shallowly so, glabrescent or bordered by poorly defined velvety patch; pith brown. Terminal buds ovoid to ellipsoid, somewhat flattened, 5-6 mm. Leaves 15-24 cm; petiole 2-5 cm. Leaflets (9-)11-15(-17), usually narrowly oblong-elliptic to lance-elliptic, occasionally lanceolate, symmetric or weakly falcate, 4.3-9.5 × 1.6-2.6 cm, margins finely serrate, apex rounded to acute; surfaces abaxially without tufts of hair in vein axils, abaxially and adaxially glabrous with scales but no hairs, main veins glandular, often sparsely so, leaflets without nonglandular hairs (except for multiradiate hairs early in season); terminal leaflet well developed. Staminate catkins 5-14 cm; stamens 15-35 per flower; pollen sacs 0.6-1 mm. Fruits 1-3, globose, 2.1-3.5 cm, smooth, at first glandular, with scattered scales, soon glabrescent; nuts depressed-globose, 1.8-2.2(-2.5) cm, shallowly grooved, surface between grooves smooth.

Syntype for Juglans californica S. Watson
Collection: Smithsonian Institution, National Museum of Natural History, Department of Botany
Verification Degree: Original publication and alleged type specimen examined
Preparation: Pressed specimen
Collector(s): W. H. Brewer
Year Collected: 1860
Locality: Sierra Santa Monica., California, United States

Systems

• Terrestrial

Comments: Juglans californica occurs in chaparral, cismontane woodland, and coastal scrub on alluvial soils between 500 and 900 meters in elevation (CNPS 2001). Specifically, the best developed stands of J. californica occur on steep hills with northern exposures, and almost always on soils from Miocene-Pliocene shales (Quinn 1990).

Flowering spring (Mar-May). Hillsides and canyons; 30-900 m; Calif.

Barcode of Life Data Systems (BOLDS) Stats
Public Records: 1
Species: 1
Species With Barcodes: 1

Year Assessed

• Needs updating

Assessor/s

Reviewer/s

United States

Rounded National Status Rank: N3 - Vulnerable

Rounded Global Status Rank: G3 - Vulnerable

Reasons: Juglans californica is endemic to California and occurs in Los Angeles, Orange, Riverside, Santa Barbara, San Bernardino, San Diego and Ventura counties (CNPS 2001). While the California Native Plant Society considers Juglans californica rare, it is currently found in sufficient numbers and is distributed over a wide enough range that currently the potential for extinction is low (2001). Southern California black walnut is, however, threatened by urbanization, grazing, potentially a lack of natural reproduction, increased fire frequency in some areas and exotic understory species (pers. comm. Quinn 2002, CNPS 2001 and Quinn 1990).

Major Threats

Comments: Juglans californica is considered fairly threatened by the California Native Plant Society. It is threatened by urbanization, grazing, and potentially a lack of natural reproduction (CNPS 2001). Walnut forest is also highly fragmented and a declining natural community in California. Walnut forest is rare in Orange, Riverside, San Bernardino and San Diego counties (CNPS 2001). Quinn (1990) comments that the reversal of ecological changes to J. californica communities caused by overgrazing, increased fire frequency and exotic understory species is necessary in order to manage this species. Posing the largest threat is urbanization. Specifically, in 1990 one of the most extensive stands in Puente Hills, Los Angeles co. 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 bulldoze J. californica trees for implementation of a housing subdivision, and an expressway. In the San Jose Hills one of the most developed stands of J. californica was demolished due to expansion of a landfill (Quinn 1990). 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).