Noyer noir de Californie

The species is most common in Los Angeles, Orange and Ventura counties. Small stands are found in the Simi Hills, Santa Susana Mountains, Santa Monica Mountains, San Jose Hills, Puente Hills and Chino Hills (Esser 1993). 3,500 living individuals were recorded from Sulphur Mount, Ventura County, California making up 49% of the basal area (Keeley 1990). In the same study, 1,130 living individuals were found on Mount Washington, LA County contributing to 90% of the basal area (Keeley 1990). Finally, at Montecito heights, LA County 1,710 living individuals were counted with a basal cover of 95% (Keeley 1990). At each of these three sites, there were also 50 to 100 dead individuals of the species (Keeley 1990).  Keeley (1990), also looked at the size structure of the species and most individuals had a small dbh. Based on estimates from these three sites, population size is suspected to exceed 10,000 mature individuals. Nature Serve (2018) describes the population size as 'sufficient' and widespread. It can also be dominant in woodlands (Esser 1993). A lack of natural reproduction is also reported (NatureServe 2018). Subpopulations are found in protected areas and due to the species dominance in woodland, these can be large and stable subpopulations. 

Riordan et al. (2015), looked at the possible behavior of this species and its' preferred habitat type in response to climate change. A potential 20,493 km² of climatically suitable habitat was identified for the species, 31% of this habitat has already been subject to decline due to anthropogenic development of urban areas and agriculture and before 2080 a further 33% could be lost. Climate change also has the potential to cause 48 - 61% of currently suitable habitat to be lost, which alongside anthropogenic changes could see J. californica experience habitat loss of between 64 and 70% (worst case scenario). However, climate change may not have a negative impact and could see the range of the species expand along the coast by 218% (Riordan et al. 2015). More information is needed on how the species will disperse, and reproduce alongside fine scale habitat and ecological information (soil moisture etc.) is needed before these models can better predict what will happen to the population size of the species in response to climate change.

Considering these details, overall the population is experiencing decline due to loss of habitat and grazing of the natural habitat. Californian Walnut forests are also becoming more fragmented. There has been great historical decline due to human population growth in the state and this growth is predicted to continue if not increase into the future. This will have a major impact on the population of J. californica oustide of protected areas, as available habitat may be significantly reduced. However in the protected sites large stable subpopulations will persist.