Clarifying the conservation status of northern California black walnut (Juglans hindsii) using microsatellite markers

Authors: POTTER, DANIEL - University Of California; BARTOSH, HEATH - Nomad Ecology, Llc; DANGL, GERALD - University Of California; YANG, JUDY - University Of California; BITTMAN, ROXANNE - Department Of Fish And Wildlife; Preece, John

Submitted to: Madrono
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/27/2018
Publication Date: 7/1/2018
Citation: Potter, D., Bartosh, H., Dangl, G., Yang, J., Bittman, R., Preece, J.E. 2018. Clarifying the conservation status of northern California black walnut (Juglans hindsii) using microsatellite markers. Madrono. 65(3):131-140. https://doi.org/10.3120/0024-9637-65.3.131.
DOI: https://doi.org/10.3120/0024-9637-65.3.131

Interpretive Summary: The conservation status of the Northern California black walnut, Juglans hindsii, has been a source of considerable confusion and controversy. Although not currently legally protected by either federal or state regulations, the species has been listed as rare and threatened by the California Native Plant Society and of special concern by the California Department of Fish and Wildlife, and some California counties require mitigation for removal of individuals of this species, especially older trees. The primary reason for these concerns is that, despite the current widespread distribution in northern California and southern Oregon of trees that match J. hindsii morphologically, there are only three or four sites where the species is known to have occurred prior to extensive settlement by Europeans of California in the mid-19th century. This has led to the suspicion that trees found in other places may not be genetically pure J. hindsii, but may instead be descendants of lineages that experienced past gene flow from one or more other species. Nonetheless, many of those trees have been confirmed as pure J. hindsii based on DNA sequences, although evidence of occasional past hybridizations with closely related other North American Juglans species has also been detected. In addition, despite its more distant relationship, the cultivated walnut (J. regia) readily hybridizes (as the male parent) with J. hindsii, producing morphologically identifiable ‘Paradox’ hybrids, which occur spontaneously and are widely planted as rootstocks and street trees. Finally, although J. hindsii is generally accepted as a distinct species from Southern California black walnut, J. californica, based on morphology, herbarium specimens identified as J. hindsii collected in southern California over the last five years have raised questions about the respective geographic distributions of the two taxa. We analyzed genotypes at 10 microsatellite loci for 160 apparently wild trees of J. hindsii from one county in southern Oregon and 10 counties in northern and southern California, including representatives of putative original native populations, as well as several Paradox hybrids, 10-20 standards for each of the five North American black walnut species, and six standards for J. regia. Bayesian cluster analyses with the program STRUCTURE revealed that two-thirds of the putatively wild J. hindsii represent genetically pure members of that species, while the remainder show evidence of past hybridizations with one or more of the other North American black walnut species. The results suggest that J. hindsii should not be considered a rare species.

Technical Abstract: The conservation status of the Northern California black walnut, Juglans hindsii, has been a source of considerable confusion and controversy. Although not currently legally protected by either federal or state regulations, the species has been listed as rare and threatened by the California Native Plant Society and of special concern by the California Department of Fish and Wildlife, and some California counties require mitigation for removal of individuals of this species, especially older trees. The primary reason for these concerns is that, despite the current widespread distribution in northern California and southern Oregon of trees that match J. hindsii morphologically, there are only three or four sites where the species is known to have occurred prior to extensive settlement by Europeans of California in the mid-19th century. This has led to the suspicion that trees found in other places may not be genetically pure J. hindsii, but may instead be descendants of lineages that experienced past gene flow from one or more other species. Nonetheless, many of those trees have been confirmed as pure J. hindsii based on DNA sequences, although evidence of occasional past hybridizations with closely related other North American Juglans species has also been detected. In addition, despite its more distant relationship, the cultivated walnut (J. regia) readily hybridizes (as the male parent) with J. hindsii, producing morphologically identifiable ‘Paradox’ hybrids, which occur spontaneously and are widely planted as rootstocks and street trees. Finally, although J. hindsii is generally accepted as a distinct species from Southern California black walnut, J. californica, based on morphology, herbarium specimens identified as J. hindsii collected in southern California over the last five years have raised questions about the respective geographic distributions of the two taxa. We analyzed genotypes at 10 microsatellite loci for 160 apparently wild trees of J. hindsii from one county in southern Oregon and 10 counties in northern and southern California, including representatives of putative original native populations, as well as several Paradox hybrids, 10-20 standards for each of the five North American black walnut species, and six standards for J. regia. Bayesian cluster analyses with the program STRUCTURE revealed that two-thirds of the putatively wild J. hindsii represent genetically pure members of that species, while the remainder show evidence of past hybridizations with one or more of the other North American black walnut species. The results suggest that J. hindsii should not be considered a rare species.