Duke University Herbarium Project on black walnuts

Phylogeography of the black walnut group (Juglans)

Donald E. Stone, Dept. of Biology, Duke University.
In collaboration with Paul S. Manos, Dept. of Biology, Duke University.

Research supported by Duke University, the American Philosophical Society, and the National Geographic Society.

Black walnuts are a New World section of 16 highly prized species of trees that range from southern Canada to northern Argentina. The ancestral species are almost certainly North American in origin where the oldest fossils are found and diversification of both fossil and extant species is greatest. Migration into the southern hemisphere through Mesoamerica occurred 3-4 million years ago when the connection between North and South America was established.

This study is designed to examine the evolutionary relationships of the five South American species that are aligned along the eastern slopes of the Andes. Morphological and molecular techniques are being used to assess relationships, patterns of formation and aspects of diversity that will be useful in conservation.

This research has both academic and practical significance.
> On the academic side it has the potential to address questions about the origin, colonization, and evolution of the largest subgroup of walnuts, a group that occupies geographic areas that only became available in relatively recent geologic time. The formation of the Panamanian landbridge approximates the time of discovery of the first record of fossil walnuts in Ecuador, and yet the genus per se has been around for 40 million years. This evidence reinforces the idea that black walnuts had a northern origin and then dispersed and diversified southward as land in Central America, South America and the West Indies became available for colonization.
> On the practical side the black is generally acknowledged as the most valuable timber species in temperate North America and walnut also produces important crop nuts and food for wildlife. The California black walnut (Juglans hindsii) is the preferred rootstock for the common, commercially-produced English (Persian) walnut (J. regia). In South America and the West Indies black walnut wood is equally prized for its high quality, and, as a consequence, is very, very rare. Seminal conservation programs have been initiated in Cuba, the Dominican Republic, Puerto Rico and Venezuela, but information about the genetic diversity of the stock is lacking at this time. And thus there is a real role to be played using molecular markers.

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Duke University Herbarium