It is a truth universally acknowledged that closely related plants occur in widely varying locations around the globe, yet going from a truth universally acknowledged to a truth personally observed is a profoundly gratifying process. In September 2018, along Lake Yuno, within Japan’s Nikko National Park, we collected fruits of the forked viburnum (Viburnum furcatum). The fleshy drupes were reddish-orange, although a number had already turned blackish and were mushy to the touch. We reached them by standing on tiptoes in the muck, and pole pruners came in handy for those higher up. When not browsed by deer, the shrubs stood up to ten feet tall and sometimes just as wide, the sprawling branches positioning the round, saucer-sized leaves like open hands to capture light in the shady understory of maples and conifers. While this species is native to eastern Asia, we could have fooled ourselves into thinking we were standing in front of the hobblebush (V. lantanoides) of eastern North America, given the striking similarities between the two species’ appearance, habitat, and associated genera.
It was already a great morning collecting, and not far away, deciduous azaleas (Rhododendron wadanum) and hollies (Ilex geniculata) grew near the shoreline of the mountain lake, as did the occasional clethra (Clethra barbinervis). Higher up on the ridges, large beech (Fagus crenata) and hemlock (Tsuga diversifolia) filled the overstory. Because these genera also occur in eastern North American forests, we could have sworn we had already experienced this collecting day before, a hemisphere away. While we have long understood the profound concept of biogeographical disjunctions, it has taken compounded years of plant exploration for us—as colleagues at sister institution that share a long history of cooperation and collection goals—to personally observe these connections in far reaching places.
Our collaborative expedition to Honshu, Japan, also included Steve Schneider from the Arnold Arboretum, Todd Rounsaville from the Polly Hill Arboretum, and Mineaki Aizawa and Tatsuhiko Shibano from Utsunomiya University. During our two-week trip, we visited six locations and made fifty-eight collections of fifty-five different taxa. Like the viburnum, half of the genera we collected co-occur in eastern Asia and North America, creating even more moments of déjà vu.
At Lake Yuno, most of the forked viburnum’s wide, rounded leaves had heavy insect herbivory, giving it the Japanese common name of mashikari (always eaten by insects), yet the habit and leaf shape instantly reminded us not only of the American hobblebush but another eastern Asian species, Viburnum sympodiale. The genus Viburnum (Adoxaceae) has a wide geographic range, with approximately 165 species occurring across the Northern Hemisphere and dipping into South America and Southeast Asia. Botanists have organized these species into sub-generic groups or clades, based on how closely related these are to one another. These three viburnums occupy the Pseudotinus clade, which also includes a fourth species, V. nervosum (also from eastern Asia). The four share a distinctive branching architecture, which Erika Edwards and colleagues have dubbed the “furcatum pattern” (drawing on work Michael Donoghue published in Arnoldia in 1981). The branches grow horizontally with the ground (known as plagiotropism) until the end of the season when one of the terminal buds turns upwards to produce a short, reproductive shoot. The next spring, the main, horizontal growth continues from the other terminal bud, creating a sympodial growth pattern, where one of two forked branches becomes dominant. This branching structure is acknowledged in the specific epithet of V. sympodiale.
Although there are no European representatives, this quartet is a fascinating case study of the history of botanical description and horticultural introduction. The description of these four species follows a familiar arc from the initial exploration and cultivation of North American natives in the late eighteenth century, followed by the introduction and study of Chinese and Japanese species through the early twentieth century, and it coincided with an increasingly refined understanding of biogeography and the eventual explanation of lookalike species that are spaced a world away from one another.
The native range for Viburnum lantanoides stretches between Canada’s Maritime Islands and northern Georgia. This species—the first of the Pseudotinus to be attributed a scientific name—was described as V. alnifolium by Humphrey Marshall, a cousin of botanists John and William Bartram, in an alphabetical catalogue of plants growing in the eastern United States. Marshall’s publication in 1785 described the leaves as “heart-shaped, oval, sharp-pointed, deeply sawed on their edges, strongly veined, and placed opposite upon long slender footstalks,” but the most telltale characteristic was the halo of sterile flowers encircling each inflorescence—a feature also shared with V. furcatum and V. sympodiale.
At first, this portrayal seems unmistakable, but as it turns out, Marshall’s description was not of the hobblebush viburnum. Marshall based his description upon an earlier one (by British botanist Philip Miller in 1768), which was confused, in part, with smooth hydrangea (Hydrangea arborescens) given the shared floral characteristics and large leaves that are arranged opposite one another along the stem. Almost twenty years after Marshall’s publication, André Michaux proposed the alternative name, Viburnum lantanoides, which is now widely accepted. (For further background reading on the confusion between the two names, see Ferguson, 1966, and Mackenzie, 1927.)
In some sense, it’s surprising that such a widespread shrub would have escaped the attention of earlier botanists. After all, one of the shrub’s characteristic qualities is its ability to lay branches and root, creating bulky masses of foot-catching vegetation that give the species its common name: hobblebush. When describing the species’ habitat, phrases like “damp woods,” “moist woods,” “cool moist woods,” “rocky woods,” “deep woods,” “swampy woods,” and other combinations thereof leap off the labels of most specimens at the Harvard University Herbaria, and when it comes to describing the shrubs themselves, another word dominates: “thickets.”
Even now, however, the species is scarce in the nursery trade and often slips under the horticultural radar. We first jointly encountered the species near its southernmost range edge in May of 2002, while hiking along the Blue Ridge Parkway in Transylvania County, North Carolina. On either side of the path, the shrubs bore large and roughly heart-shaped leaves that stuck out at wide angles to catch the light. Just two years before, and not far away, Tony had first seen and collected the species in very similar habitats at Nantahala National Forest in North Carolina and near Clingman’s Dome in the Great Smoky Mountains National Park.
The next Pseudotinus species to enter the botanical literature was Viburnum nervosum—the most basal of the four species. While we often think that the earliest Western botanical explorations to Asia occurred in Japan and coastal China, the European presence in India meant that trans-Himalayan species were often described earlier in the nineteenth century, before their eastern counterparts. In 1825, forty years after Marshall’s putative description of the hobblebush, David Don, the librarian for the Linnaean Society, described V. nervosum, based on collections made by Nathaniel Wallich and Francis Hamilton in Nepal.
Neither of us has encountered Viburnum nervosum in the wild, but this species occurs at exceptionally high altitudes and is common in open, boggy habitats at (or even above) the tree line in Sichuan, Xizang, and Yunnan Provinces of China, as well as in Bhutan, India, Myanmar, Nepal, and North Vietnam. The Flora of China notes that it can grow up to 14,700 feet (4,500 meters) above sea level. Unlike the other three Pseudotinus species, V. nervosum does not produce the distinctive halo of sterile flowers around each inflorescence; instead, the fertile flowers within each inflorescence are larger. Showy fertile flowers are an alternative to showy marginal flowers for attracting pollinators, according to Brian Park and colleagues.
The Japanese species that we observed at Lake Yuno—Viburnum furcatum—was next to be described. Japan was predominantly closed to Europeans and Americans until 1868, the year of the Meiji Revolution, but in 1858, the German botanist Carl Ludwig Blume obtained an herbarium specimen of a Japanese viburnum and labelled it with this name. The species would not be properly described, by Carl Johann Maximowicz, for another two decades. Nonetheless, in 1859, Harvard botanist Asa Gray noted the presence of a Japanese viburnum that he suspected was the same or a closely related species as V. lantanoides, based on herbarium specimens collected by Charles Wright, botanist on the United States North Pacific Exploring Expedition between 1853 and 1856. This and other early specimens, initially referred to as V. plicatum (and occasionally V. tomentosum), supported Gray’s theories of disjunction of species, particularly between the eastern United States and eastern Asia.
Gray concludes his momentous essay, published in 1859, with a provocative summation: “Under the light which these geological considerations throw upon the question, I cannot resist the conclusion, that the extant vegetable kingdom has a long and eventful history, and that the explanation of apparent anomalies in the geographical distribution of species may be found in the various and prolonged climatic or other physical vicissitudes to which they have been subject in earlier times.”
Gray recognized that the occurrence of distant doppelgängers like these North American and Japanese viburnums could only be explained through change on a geologic timescale. This argument would gain substantial theoretical weight with the publication, in 1860, of On the Origin of Species, written by Gray’s friend and scientific correspondent Charles Darwin. It is noteworthy, therefore, to see this group of Pseudotinus viburnums—or even just the paired Japanese and North American species—along this timeline of scientific breakthroughs. As botanists like Gray began to work with a greater volume of herbarium specimens from Asia, an evolutionary and geologic story began to emerge. Global exploration and paradigm-changing scientific discovery necessarily moved in tandem.
The final viburnum in the quartet—Viburnum sympodiale—was described by Paul Graebner in Ludwig Diels’ epic Die Flora von Central-China, published in 1901. Graebner named this species using herbarium specimens collected by Augustin Henry and Arthur von Rosthorn, and he noted how its forked (or sympodial) branching structure resembled that of V. furcatum. Ernest Henry Wilson observed V. sympodiale during his initial trip to China (between 1899 and 1902), while working for Veitch Nursery, as well as later while working for the Arnold Arboretum, and he noted in his collection notebooks that the species was “very rare!” This characterization stands in stark contrast to his observations of the common Japanese species.
No matter the botanical interest of these species, it was another thing to bring them into more widespread cultivation. In 1889, Charles Sprague Sargent, the founding director of the Arnold Arboretum, wrote a short article about Viburnum lantanoides for Garden and Forest. He praised the species as “one of the most beautiful plants of our flora,” yet he noted that the hobblebush was “impatient of cultivation” and “the most difficult of all our native shrubs to cultivate.” At the Arnold Arboretum, only two early hobblebush accessions achieved long-term success. Both arrived as plants and perished in the mid-twentieth century. Repeated attempts to cultivate the species since then—via seeds as well as layers from the wild—have been in vain until recently. Seeds refused to germinate, and the plants rarely lived beyond a few years after planting out. However, several plants collected in the Adirondacks in 2008 have survived near a swampy spot adjacent the North Woods, known for a planting of corkwood (Leitneria floridana)—the same location where one of the two earlier successes had grown.
The other three species have been equally slow to enter North American gardens. Sargent collected seeds of Viburnum furcatum from Japan in 1892, yet no records of successful germination (or planting) exist. It was not until early 1915 that two collections from Ernest Henry Wilson in northern Honshu made their way to the Arnold Arboretum. One collection originated as plants, the other as seeds, and the resulting accessions (numbers 17988 and 17989, respectively) were also sited near the Leitneria. The year those accessions arrived, Sargent noted that the species was “as handsome a plant as the American species, and will probably prove equally difficult to manage.” However, the seed-derived accession survived until 1997, occasionally dying to the ground and sending up suckers. The other accession (containing three plants) has persisted in the location for a century, putting on dramatic floral displays typically in late April to early May. Another seed-derived accession arrived in 1998.
Perhaps because this species takes more readily to various propagation techniques, Viburnum furcatum seems to be the most widely cultivated of the four species, showing up in some forty-three public gardens in fourteen different countries according to Botanic Garden Conservation International’s PlantSearch database. The American species is growing in twenty-seven gardens in ten countries, while V. nervosum and V. sympodiale are growing in only eleven and nine gardens, respectively. These numbers, while likely accurate in the ranking of each species’ popularity (or amenability to cultivation), are likely underestimates, for the database requires individual gardens to self-report what they are growing. For instance, no gardens in Japan are noted as cultivating V. furcatum, and they likely are.
The Morris Arboretum has two specimens of Viburnum nervosum, which were acquired as cuttings from a specialist nursery in 2010. The Arnold Arboretum has only attempted to cultivate the species twice—two seed collections in the early 2000s—and, in both cases, the seeds failed to germinate. Wilson had made several herbarium collections of V. nervosum (as V. cordifolium) in western Sichuan, where he observed it was “common” and formed “thickets.” Joseph Rock also collected the species in high plateaus of Yunnan and noted it growing as both a shrub and even a tree up to twenty-five feet in height. Yet, the earliest record of the plant’s introduction to North America seems to be a 1927 accession donated to the United States Department of Agriculture by the Lloyd Botanic Garden in Darjiling, India.
As for the final species, Tony encountered Viburnum sympodiale in September 2005, while collecting in Gansu Province, China. Although there were no fruits, he and the other explorers gathered leaf tissue and herbarium specimens for future molecular work, and long-term preservation, respectively. This experience caused another déjà vu moment all over again, and the field notes for the collection (NACPEC05-033) acknowledge “this species is very reminiscent of Viburnum furcatum or V. lantanoides.” The team found it in a mesic, mixed deciduous and coniferous forest—much like the habitats preferred by the Japanese and American species—replete with three maple species and a hemlock (Tsuga chinensis), as well as littleleaf boxwood (Buxus sinica), rosy dipelta (Dipelta floribunda), Rosa davidii, and katsura (Cercidiphyllum japonicum). Despite four attempts, the Arnold Arboretum has only had this species in cultivation once: cuttings from the Royal Botanic Garden, Edinburgh, which rooted and survived on the grounds for eight years. Three seed acquisitions were never successful.
If the early work of plant explorers enabled discoveries pertaining to biogeography, like Gray’s initial articulation of disjunct species, then the slow acquisition of other disjunct species (including Viburnum nervosum and V. sympodiale) makes us wonder what breakthroughs current and future plant exploration will enable. Aside from Viburnum furcatum, our expedition to Japan yielded a number of other collections of species that have familiar North American relatives. Among the most exciting was a collection of Stewartia: another genus with members that only occur in eastern North America and eastern Asia. At the University of Tokyo Forestry Department Research Station, in Chichibu, west of Tokyo, we encountered a population of Japanese stewartia (S. pseudocamellia) perched on a steep hillside, the trunks elongated, stretching for light among an overstory of impressive beeches (Fagus japonica). This stewartia, known for its exfoliating bark, large white flowers, and rich autumn leaf colors, is among the most ornamental plants that have come to our gardens from Japan. We were excited to make this collection because, despite its common name, our collecting companion Todd Rounsaville confirmed that the only documented, wild collections of Japanese stewartia in the United States are of Korean origin. Our collection represents an infusion of novel Asian material into cultivation.
We also collected a second species of Stewartia at the last location of the expedition, the Hokkaido University Forestry Station in Wakayama Prefecture on the Kii Peninsula, the southernmost tip of Honshu. This mountainous region is among the wettest areas of Japan, receiving upwards of 118 inches (300 centimeters) of annual precipitation, which we experienced during a continuous downpour—one of the wettest collecting days of our collective careers. We made collections of Stewartia monadelpha, aptly named the tall stewartia, which grew to about 40 feet (12 meters) tall, with trunk diameters of nearly 18 inches (45 centimeters)—showcasing the slick, orange-red bark.
Of course, we made other collections of disjuncts on the trip, including green- or snake-barked maples (Acer micranthum and A. tschonoskii) that are within the same section or clade as our familiar North American moosewood (A. pensylvanicum). We also couldn’t help but notice the genera without disjunct representatives. On the rain-soaked day in Wakayama, we collected three species of Enkianthus (E. cernuus f. rubens, E. nudipes, and E. sikokianus), a genus in the heath family (Ericaceae), which is represented by six species in Japan, seven in China, but none in North America. It was remarkable to see three distinct species growing together on one mountainside. Earlier on the trip, we collected E. campanulatus and E. subsessilis, bringing our total to five of the six Japanese species. We also made two horticulturally and botanically interesting collections at Wakayama: wheel tree (Trochodendron aralioides) and Japanese umbrella pine (Sciadopitys verticillata), which both have a peculiar taxonomic standing. Wheel tree has only one other member in its family and order (Trochodendraceae and Trochodendrales, respectively), and the umbrella pine is the lone representative of its family (Sciadopityaceae).
Importantly, through collection we’ve brought these species—long-lost cousins (as well as evolutionary orphans)—together in common gardens, as living plants, to observe them in cultivation. And like our pursuit of these four déjà vu viburnums, the work as plant explorers continues. Gray used collections of herbarium specimens as inspiration to make daring hypotheses about biogeography. In that same vein, as plant explorers and curators we build collections to inspire future scientists to make new discoveries using not herbarium specimens and DNA samples (important as they may be) but living organisms. It is a long-game we play, however, for a century after the last species in the quartet was described, it has nearly evaded our cultivation. But, with dogged determination, hopefully we (or our successors) will achieve the perfect ensemble: a full quartet growing and performing together for audiences to enjoy and to study.
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Park, B., M. Sinnott-Armstrong, C. Schlutius, J.-C. P. Zuluaga, E. L. Spriggs, R. G. Simpson, E. Benavides, M. J. Landis, P. W. Sweeney, D. A. R. Eaton, and M. J. Donoghue. 2018. Sterile marginal flowers increase visitation and fruit set in the hobblebush (Viburnum lantanoides, Adoxaceae) at multiple spatial scales. Annals of Botany, 123: 381–390.
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Anthony S. Aiello is the Gayle E. Maloney Director of Horticulture and Curator at the Morris Arboretum of the University of Pennsylvania in Philadelphia. Michael S. Dosmann is keeper of the living collections at the Arnold Arboretum.
The authors thank Michael Donoghue, Brian Park, and Patrick Sweeney for their conversations in the preparation of this article and use of images, as well as our collecting companions during our expedition to Honshu in 2018 for their friendship and collaboration.