Contributed by Anthony Aiello, The Gayle E. Maloney Director of Horticulture and Curator

One of the results of multiple years of plant exploration is the opportunity to find connections in unexpected places. I have been fortunate to work with Michael Dosmann, a colleague at our sister Ivy League institution, the Arnold Arboretum of Harvard University. We have traveled together on expeditions throughout the United States and Asia. In the fall of 2018, our collaboration continued with an expedition to the island of Honshu, Japan, joined by Steve Schneider (Arnold Arboretum), Todd Rounsaville (Polly Hill Arboretum), and Mineaki Aizawa and Tatsuhiko Shibano (Utsunomia University). 

Not only did this two-week expedition allow us to visit six locations where we made 58 collections of 55 different taxa, but more importantly, we gained a deeper understanding of the lessons of biogeography by observing closely-related and similar-looking disjunct species, far from their nearest relatives.  

An extraordinary aspect of this expedition to Japan was observing and collecting species that are closely related to ones that we had seen across China and throughout the U.S.  It was fascinating to get first-hand experience with a trio of species, having previously seen the Japanese counterparts only in cultivation. 

The expedition initially focused on Nikko National Park, located about three hours north of Tokyo, and on our third day in the field, we came across forked viburnum (Viburnum furcatum) growing along the edge of Lake Yuno. With it were massive trees of Japanese arborvitae (Thuja standishii) and hiba false arborvitae (Thujopsis dolobrata), along with thickets of bright-barked Japanese clethra (Clethra barbinervis). Most of the viburnum’s wide, rounded leaves had heavy insect feeding damage, giving it the Japanese common name of mashikari (“always eaten by insects”). Despite this, the growth habit and leaf shape instantly reminded us of two other viburnums seen thousands of miles away: the eastern North American hobblebush (V. lantanoides), and the Chinese counterpart viburnum, V. sympodiale. I had first seen the American species in the Smoky Mountains in 2002 and the Chinese one on a 2005 expedition to Gansu Province.

Throughout this expedition, we made a number of other collections of Japanese species that have familiar North American relatives. There are many examples of the North America-Asia pairings, but fewer examples of the North America-Europe-Asia sets of species. An example of the latter occurrence is beech (Fagus), with the familiar American beech (F. grandifolia) of eastern forests, the majestic cultivated European beech (F. sylvatica), and several less horticulturally known Chinese and Japanese species. While collecting at Lake Yuno, we encountered Japanese beech (F. crenata) growing on a small bluff above the lake.

A few days later, we encountered another impressive beech, Fagus japonica, with a height of some 75 feet, at the University of Tokyo Forestry Department Research Station, in Chichibu (west of Tokyo). In the understory were groves of Japanese pieris (Pieris japonica) and Japanese clethra, and a population of Japanese stewartia (Stewartia pseudocamellia) perched on a steep hillside, elongated and stretching for light among the competing trees. These species are among the most ornamental and desirable of the numerous plants that have come to our gardens from Japan. Seeing them together provided an opportunity to understand their growing requirements, while simultaneously appreciating their beautiful natural arrangements. A choice garden plant, Japanese stewartia is known for its exfoliating bark, large white flowers, and rich autumn leaf colors. 

We 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 in Japan, receiving upwards of 118 inches of annual precipitation. Within this rich, mixed broadleaf-evergreen forest, we made collections from several massive trees of tall stewartia (Stewartia monadelpha), with slick, orange-red bark that grew up to 40 feet tall on steep mountain slopes. The two Japanese species of stewartia highlight another North America–Asia disjunction, reminding us of the mountain stewartia (S. ovata) of the mountainous southeastern U.S., and Virginia stewartia (S. malacodendron), a southeastern coastal representative.

Surrounded by disjunct species–viburnum, beech, stewartia, and maple– we could not help but notice the genera where there is diversity in Asia, but no other temperate counterparts. On one of the days in Wakayama, we also found three species of enkianthus (a genus in the heath family, Ericaceae) related to the more familiar white enkianthus (Enkianthus perulatus). Enkianthus is well-represented by six species in Japan and seven species in China, but with none in North America. It was remarkable to see three distinct species growing together on one mountainside.  Earlier on the trip we collected two other species, bringing our total effort to five of the six Japanese species. 

In contrast to the species diversity of Enkianthus, we made two horticulturally and botanically interesting collections at Wakayama— the wheel tree (Trochodendron aralioides) and Japanese umbrella pine (Sciadopitys verticillata).  Both of these are worthwhile as garden curiosities, but they are taxonomically isolated, so it was fascinating to see these evolutionary “orphans” juxtaposed with those that are species rich with close relatives in both Asia and North America.

If seeing is believing, then this collecting trip to Japan certainly brought me true horticultural religion. The cultural and botanical experiences on these expeditions are priceless; and the ability to see plants growing in their natural environment provides a deeper understanding of their horticultural needs, which we can translate into growing them at the Arboretum.