Juglans hindsii

There are two native species of black walnut to California, one in the north (Juglans hindsii) and one in the south (Juglans californica). J. hindsii, historically, was considered imperiled even though it was known from many occurrences and was widely planted in California as a street tree (Potter et al. 2018). The reason for the previous imperilment rank was due to the belief that only a few populations had not hybridized with other Juglans species post 19th century settlement, and therefore, pure J. hindsii was only known from a handful of populations. Potter et al. (2018) discovered through genetic assays that many other known populations of what was believed to be hybridized J. hindsii, is actually in fact pure J. hindsii.

Juglans hindsii was considered an imperiled species prior to 2018 because only a few populations were known to be genetically pure. Potter et al. (2018) ended a long standing controversy over the distribution and conservation status of this species by conducting genetic assays from trees across California. They determined that a high percentage (>76%) of the trees sampled were pure J. hindsii, and not hybridized. The study by Potter et al. (2018) provided the evidence for the conservation status rank for this species to move from 'critically imperiled' to 'secure'.

The range extent of this species has been complicated and confusing because of a lingering question over the indigenous range of the species in California. The species has always been considered native to California, however, given that hybridization commonly occurs in this genus, only those few populations known to be pure genetic lineages (only 3 or 4, Contra Costa, Sacramento, and Napa) were counted in previous conservation assessments since they were known to exist pre-settlement. It is now understood, that the range extent is throughout northern and central California and southern Oregon as evidenced by many naturalized populations in unmanaged habitats (Potter et al. 2018).