When I first encountered butternuts on the ground of the arboretum here in Zürich, Switzerland, I was puzzled. The tree these nuts fell from must have died or been felled years ago, so I only had the seeds for identification. This North American species, Juglans cinerea, is rarely seen in European cultivation outside specialized tree collections, and I didn’t recognize the ridged, oblong nuts. When I took a few home, they were not easy to identify within books on common park trees. After additional research, however, the butternut aroused my fascination and left me with questions about the whole walnut family (Juglandaceae). I had long been familiar with this group of plants, but the more I read about them, the more I realized that, in fact, I knew so little.

Like the butternut, many other members of the walnut family were absent in books that I had at home: hickories (Carya), wingnuts (Pterocarya), and platycarya (Platycarya). As I encountered each new species, new questions arose. After several years of intensive study, my pursuit evolved into a book project, Die Walnuss, which was published (in a German edition) in late 2019. My work with this unique plant family went far beyond scientific analysis; it also involved an artistic exploration of the unique variety of forms of this plant family. I wanted to make the knowledge hidden in scientific papers accessible through a language of drawings and photographs. These different approaches—science and art—offered new ways of observing and understanding the world of walnuts.

I live in a region with no native species of this widespread plant family. Here, you can occasionally find the North American eastern black walnut (Juglans nigra) planted as an ornamental tree in parks. The English walnut (Juglans regia) was most likely introduced by the Romans into the northern parts of Europe and can often be found growing as lone specimens on farms. But the number of these solitary trees has declined in the region since the industrialization of agriculture half a century ago. Walnut farms and orchards are relatively new in the German-speaking part of Europe, and walnuts bought in grocery stores here mostly originate from France (Périgord and Grenoble), the United States (California), or Chile.

Members of Juglandaceae, however, were once among the most common trees of alluvial forests in Central Europe. Fossils allow us to look back on a plant family whose greatest diversity and distribution preceded the ice ages in the Paleogene and Neogene. Many species disappeared only a few hundred thousand years ago. I became fascinated by this history. The fossil record reveals a long, slow story of evolution and shifting ranges, and it provides a counterpoint to the story of the family’s rapid globalization in recent centuries.

Four fossilized hickory fruits
Fossils document the former abundance of the walnut family in Central Europe, where no members of the family naturally occur today. Hickory (Carya) fossils, shown above, were collected from sediments in the Rhine Valley, close to Strasbourg, France, and are around five million years old. Jonas Frei

Not far from Strasbourg, in the Rhine Valley of France, researchers and fossil collectors have discovered fossilized butternuts, described under the name of Juglans bergomensis. These fossils correspond so closely to the North American butternut that it is hard to find visual differences. The nuts must have fallen into the shallow water and sandy substrate of the Rhine five million years ago, but they still have almost the weight and feel of fresh nuts due to carbonization. In fact, this species had a wide distribution: its fossils have been reported in Italy, the Netherlands, and wider parts of eastern Europe and Russia. Similar fossils dating to the Neogene have been found in Japan and in the southern United States. Fossilized hickory nuts are also present in the Rhine sediments, including those of a widespread fossil taxon called Carya globosa, which is similar in appearance to the water hickory (Carya aquatica). Although all the European hickory species went extinct millions of years ago, the nuts look as fresh as if they were only a few years old.

Walnut family species with large, animal-dispersed fruits are only part of the story. Wingnuts (Pterocarya)—a genus that is now known for six extant species—were once dominant trees here in Central Europe along rivers and in mountain slope forests. These are ancestors of the species we now call the Caucasian wingnut (P. fraxinifolia), which today runs wild in parks and gardens in Central Europe, its root sprouts forming dense stands. Some horticulturists have argued that we should cease planting this species in our gardens, given these invasive tendencies, but based on the fossil record, we could also view the wingnut as a returnee from another era. After all, wingnut leaf fossils in the Stuttgart region were found in sediments of the Holstein interglacial and date back only 325,000 years. The few remaining populations of this once widely distributed species are increasingly threatened in their last refuges in the Caucasus. Wheel wingnuts (Cyclocarya) and platycarya—both unusual wind-dispersed genera now found only in East Asia—are also represented in the fossil records in Europe.

The reason the walnut family went extinct in Europe while some species meanwhile survived in North America and East Asia is related to the geographical shape of the continents. Here in Europe, the Alps and the Mediterranean Sea form a barrier for the north-south migration of plant species. In cold periods, trees could survive only in the southernmost corners of Europe; therefore, while in America plant species could migrate according to climate conditions, many European species died out with every cooling and warming. The fossil record indicates that wingnuts survived this back and forth the longest of all Juglandaceae, but in the end, they vanished irretrievably, just like the European magnolias (Magnolia), kiwis (Actinidia), and sweetgum (Liquidambar). Other genera of woody plants, including maples (Acer) and ashes (Fraxinus), are now represented in Europe with only a few species but had much greater diversity before the Pleistocene ice ages that started about two and a half million years ago. The diversity of these genera in Europe was similar to their modern-day representation in North America and Asia.

The fossils reveal more than former distributions and long-extinct species—the record also documents how the walnut family evolved from an entirely wind-dispersed family to one with the charismatic nut-bearing species that we know today. Some of the oldest fossils of Juglandaceae fruits originate from the United States. Fruits of a wheel wingnut named Cyclocarya brownii have been found in different sites from the Paleocene, occurring shortly after the K-T boundary, the geologic marker that separated the Cretaceous and Paleogene a good sixty-five million years ago. This event of mass extinction was both the end of the era of dinosaurs and ammonites and the beginning of a new chapter for the walnut family.

Handdrawn illustrations of walnut-family fruits
The author’s illustrations show both the diversity and beauty of the walnut family: (a) English walnut, Juglans regia; (b) little walnut, J. microcarpa; (c) Japanese wingnut, Pterocarya rhoifolia; (d) Japanese heartnut, J. ailantifolia var. cordiformis; (e) black walnut, J. nigra; (f) butternut, J. cinerea; (g) Arizona walnut, J. major; (h) Platycarya strobilacea; (i) Ma walnut, J. hopeiensis; (j) Manchurian walnut, J. mandshurica; (k) nutmeg hickory, Carya myristiciformis; (l) buart hybrid, J. × bixbyi; (m) Chinese butternut, J. cathayensis; (n) bitternut, C. cordiformis; and (o) Chinese wingnut, Pterocarya stenoptera. Jonas Frei

Cyclocarya looks very typical for early members of the family, especially since its fruits are spread by the wind and not by birds or mammals. Back in Paleocene, some fifty million years ago, mammals only started to specialize in the new ecological niches that became available after the extinction of the dinosaurs. Many other winged walnut species emerged. Some went extinct, but the descendants of others are now populating the tropics of the New and Old World: Oreomunnea in Central and South America, and Engelhardia in Southeast Asia and northern India. It was only with the diversification of mammals, especially squirrels, that some walnut species developed fruits that could be spread by animals.

Squirrels and other rodents drove the evolution of Juglandaceae in two different genera: walnuts (Juglans) and hickories (Carya), which evolved within separate lineages. Birds, especially the crow family, likely played a part in the distribution from the beginning as well. Because animals never find all the nuts they stash in their winter storage places, they contributed to the spread of these groups, and evidently, they were quite efficient. Walnuts and hickories spread through North America, Asia, and Europe, populating much of the Northern Hemisphere. In the case of the walnuts, this process must have taken place during the span of about ten million years. The oldest known fossil record of the genus, a species named Juglans clarnensis, was discovered in North America and dates back forty-four million years, while the oldest European specimen of J. bergomensis is around thirty-three million years old.

Later, humans helped with the worldwide spread of two major species: the English walnut and the pecan (Carya illinoinensis). Whereas squirrels and crows spread walnuts and hickories on three continents over several million years, humans extended the range of cultivation into all other suitable climatic regions within a few decades. The English walnut (a species of Eurasian origin) and pecan (from the southeastern United States) are now cultivated well outside their native range, including in parts of South America, northern and southern Africa, Australia, and New Zealand. So, the tasty kernels of the walnut became the main reason for this widespread distribution—a process started by squirrels many millions of years before the fossil records prove the evolution of humans.

Today, in Central Europe, almost forty species and hybrids of Juglandaceae are cultivated. During my research, I traveled to many parks and arboreta, looking for insight into the diversity of this family. I was driven not only by my scientific interest in Juglandaceae but also by my enthusiasm for the aesthetics of their habits, leaves, and fruits. The readers of my book should be able to make their own journey of discovery through the walnut family, on the tracks I have uncovered with my research.

Often, after days of traveling, I would find out that a tree I wanted to visit had been cut down or that a rare species was simply confused with an ordinary, oft-planted one. I created a collection of seeds of all the cultivated species and a leaf herbarium. The collection soon included hundreds of fruits and nuts from different locations in Europe, which made it possible to distinguish between the species and hybrids. Later, the collection became the basis for the illustrations of all species in the individual portraits of the book.

These trips through Europe searching for the different species of the walnut family also brought to light the stories of other humans—botanists and horticulturists—who moved the walnut family all over the world. While I could find many species within a day or two of searching, many researchers spent years traveling through the natural habitats in North America and Asia a few centuries ago. In the time of Carl Linnaeus, only three walnut species were known to European researchers. Besides the English walnut, Linnaeus included the North American butternut and the eastern black walnut in his Species Plantarum, published in 1753. The hickories—especially the Asian species—were documented much later.

The genus name Carya was proposed by the English botanist and plant collector Thomas Nuttall, who used the name, in 1818, in his work The Genera of North American Plants. He had borrowed this name from ancient Greek, where karya was a word for walnut. The valid botanical name for a genus or species should always be the one from the first official description, and in this case, Nuttall’s proposal wrongly became the namesake of the genus. Ten years earlier, the hickories were described under the name Hicoria by the American polymath Constantine Rafinesque. These circumstances led various scientists to urge for reinstating the earlier name, but the change was never implemented. It would have been a respectful act, not only to honor the scientific rules but also because the Greek word karya refers to the English walnut whereas Hicoria is derived from the Algonquin word for a well-known hickory dish: pocohiquara. That name reveals an obvious fact: these trees have a cultural importance that far predates their scientific documentation.

Kernels of hickories and walnuts
The large kernels of walnuts and hickories have inspired animals to disperse the species widely: (a) English walnut (cultivar), Juglans regia; (b) water hickory, Carya aquatica; (c) shellbark, C. laciniosa; (d) Chinese hickory, C. cathayensis; (e) bitternut, C. cordiformis; (f, g, h) English walnut (cultivars); (i) butternut, J. cinerea; (j) black walnut, J. nigra; (k) Japanese walnut, J. ailantifolia; (l) Japanese heartnut, J. ailantifolia var. cordiformis; and (m) pecan, C. illinoinensis. Jonas Frei

Philipp Franz von Siebold was one of the first Europeans to collect plants in Japan. One of his great collections was Platycarya strobilacea, which was described in 1843. Some botanists initially thought it was a conifer due to its cone-like fruiting structures. In 1844, the famous English plant collector Robert Fortune also found Platycarya in China. Assuming that it was a new, not-yet-described species, he sent herbarium material and seeds to the Royal Horticultural Society in London. John Lindley, the secretary of the society, named the plant after its finder, Fortunaea chinensis, and called the species the most important new find of Fortune. Later, it became known that Siebold had described the species one year earlier, so today the name Fortunaea is only used as a synonym.

These scientific explorations—and those of other botanists—made it possible to describe, collect, and, of course, cultivate many of the species as ornamentals and orchard trees. But this era of Siebold and Fortune was not simply a time of great scientific discovery; it was also a time of European colonization, in which the gathering of knowledge on expeditions was often combined with ideological, cultural, and religious imperialism. This movement of plants around the world coincided with violations of ethical standards by European maritime powers and a merciless approach to other cultures. The relatively slow but efficient distribution of Juglandaceae by squirrels and mice seems innocent in comparison.

When the walnut family is viewed in the broad sweep of its evolutionary history, the speed of its recent spread is clearly unprecedented. As beautiful as it is to see the worldwide diversity of Juglandaceae close together in many parks today, the globalization of the family has also produced novel threats.

As humans moved the walnut family around the world, fungi and pathogens often migrated with the species. In the United States, a fungal disease known as the butternut canker (Sirococcus clavigignenti-juglandacearum) has brought the butternut to the brink of disappearance. The fungus, which was once native to Asian walnut species, causes little damage to its original hosts, but it is often fatal to the North American butternut. The thousand cankers disease, meanwhile, is the result of the unfortunate encounter of a fungus (Geosmithia morbida) and a beetle (Pityophthorus juglandis) that formed in the western United States due to the proximity of the eastern black walnuts, cultivated in parks, and natural populations of the Arizona walnut (Juglans major). And the walnut fruit fly (Rhagoletis completa), which once lived inconspicuously on the black walnut species of North America, today spreads quickly in walnut orchards of Europe.

Meanwhile, the close planting of related Juglandaceae species leads to the formation of hybrid offspring. This has led to major changes in natural environments, especially in the case of the butternut populations in North America. Many of the butternut trees that can resist the butternut canker also carry the genetic material of Japanese walnuts (Juglans ailantifolia). Resistant hybrids have greater fitness, as they survive and have more offspring, which could be a blessing for the American butternut stocks that survive the strong fungal infestation. On the other hand, conservation of the “real” butternut becomes more complicated. This scenario reveals the cascade of unintended but profound environmental consequences of human actions, which cannot be easily resolved.

Of course, the walnut family experienced various climatic changes over the past fifty million years and therefore changed its distribution again and again. It is assumed that many of the species we know today are the result of hybridization between different populations that collided after a long separation due to climatic fluctuations and subsequent spread by squirrels and ravens. Genetic studies suggest that the English walnut originated from the hybridization of the black walnuts (section Rhysocaryon) and Asian butternuts (section Cardiocaryon). Also, the American butternut is said to carry some black walnut genes in addition to the genetic material of similar Asian species from the Cardiocaryon section. Given this history, one could say that many walnuts, as a lineage, will adapt to human-made influences, although it is unlikely all of the walnut species we know today will survive the pressure.

Photograph of book sitting beside bowl of walnut family fruits
The author’s book Die Walnuss—currently available in a German-language edition—features drawings, photographs, and descriptions of the walnut family. Jonas Frei

Recently, in a second-hand bookstore, I found a small booklet titled Die Quaianlagen von Zürich, from 1889. The author, botanist Carl Joseph Schröter, planned the tree collection at the arboretum where I first encountered the butternuts that started my interest in this exceptional plant family. He states that a butternut tree was planted in 1887 at exactly the spot where I found the nuts pressed into the soil. Now I know that these nuts, almost like modern-day fossils, are the remains of a now-rare species. The tree was planted long before butternut canker was imported to the United States, and before hybridization with imported species changed its natural populations rapidly.

If we did not have our own hands in all the processes that threaten species like the butternut, we could analyze the consequences from a scientific perspective and see with great fascination how some species emerge from this immense pressure and how others disappear, just like during the whole history of this family. But we also have a responsibility towards biodiversity, towards those species that exist now and that enriched the global ecosystem long before the arrival of humans. Today, as the pace of ecological change and movement continues to accelerate, we have to recognize that the story of the walnut family is now entwined with our own.

Jonas D. Frei is a landscape architect, documentary filmmaker, illustrator, and author from Zürich, Switzerland.

Citation: Frei, J. 2021. A Brief History of Juglandaceae. Arnoldia, 78(3): 10–17.