I have studied the flora of my home state, Tennessee, for the past 15 years. Initially, these efforts focused on those plants suited for gardens and managed landscapes. In recent years, a heightened interest in ecology led me toward the naturalist’s perspective to include all plants and other organisms functioning as a biological system within their physical environment. This view guides my trail accounts.
I recently spent two weeks in Japan to see my daughter Kate who lives there. I had last visited in March 2010 and enjoyed the stunning display of cherry blossoms (sakura). This trip was timed to see fall color. Kate’s itinerary for us, including five days in Kyoto, a day trip to Nara, and an overnight trek to the mountains of Nikko, focused on the heart and soul of Japan in autumn.
These may seem two disparate paragraphs, yet they share a common bond far beyond the author, a bond that many theorize dates back to the Tertiary up to 65 million years ago. For the past 260 years, botanists have noted certain plants found in both eastern Asia and eastern North America. Botanic explorations around the globe revealed similar plant genera and species, similar ecological niches, and a similar climate in these two continental regions. Floristic similarities were also found in parts of Europe and western North America plus a few other scattered locales.
When the same or closely related organisms are found in two or more widely separated regions it is referred to as a disjunct distribution. There are many examples large and small of interrupted ranges, but the Asia/North America disjunct was the first on a large scale to be recognized and has attracted the interest of scientific notables from Carolus Linnaeus (the father of modern scientific classification) to Asa Gray (Manual of Botany) and Charles Darwin.
Darwin corresponded extensively with Gray and encouraged him to further explore this floristic affinity between the continents. Darwin was nearing publication of On the Origin of Species by Means of Natural Selection (Nov. 1859), presenting his theory that species populations evolve over time. In the same year, Gray published a paper on the “diagnostic characters” of plants collected from Japan by Charles Wright in which he records “observations upon the relations of the Japanese flora to that of North America, and of other parts of the Northern Temperate Zone.”
None of his predecessors had postulated an explanation for such long distance “relations.” Gray did. He used plant fossils, current theories of geological history, and Darwin’s ideas of evolution and common descent to support the concept of a previously extensive flora now disrupted. (Boufford and Spongberg 1983) Twenty years later European botanist Adolf Engler, also using fossil evidence, proposed a wide ranging flora during the Tertiary Period of the Cenozoic Era.
By the start of the Tertiary, seed-bearing plants (angiosperms) had become the dominant terrestrial plant form. Throughout this period spanning 63 million years, continents inched northward and the climate cooled from uniformly warm and tropical to seasonally cool and temperate. Migration pathways over exposed land bridges (Bering to Asia and North Atlantic to Europe) fragmented and disappeared. Continental uplift and development of polar ice caused an epicontinental sea across the North American plains and the Mississippi Embayment along the gulf coast to recede. Broadleaf evergreens retreated southward, deciduous hardwoods (once restricted to high elevations) expanded rapidly to take their place, and conifers colonized high latitudes and upper elevations.
Scientists have theorized that as these changes gradually evolved, a broad belt encircling the northern latitudes favored the development of a rich and widespread flora dubbed the Arcto-Tertiary Geoflora. This flora reached its fullest expression in the Miocene Epoch when the Temperate Broadleaf Deciduous Forest (TBDF) was king. Further geologic and climatic changes would introduce barriers of separation, reduce area coverage of the TBDF, precipitate extinctions, and initiate evolution of distinct species within genera.
Asia and Europe became isolated from North America. The Rocky Mountains rose creating a rain shadow that changed the central plains from forest to prairie and isolating the continent’s western portion from the east. Quaternary glacial advances struck areas unevenly. Europe’s flora was hit hard by extinctions, North America’s was likely reduced particularly in the northwest, and Asia’s escaped much harm.
When early botanists first noticed these continental-scaled plant disjuncts, they identified most of the paired plants as the same species. Further study, however, proved the majority to be distinct, though related, species in the same genus or separate but closely related genera in the same family. Ecological research shows that these paired, distinct species typically occupy similar habitats and plant communities in their home ranges. “The similarities of the forests of Japan, central China, and the southern Appalachians in appearance as well as in ecological associations are in many instances so great that a sense of déjà vu is experienced by botanists from one of the regions visiting the other.” (Boufford and Spongberg 1983)
The disjunct species strongly tend toward certain characteristics. (White 1983)
1. With few exceptions, the plants grow in temperate, moist (mesic) forest environments.
2. Most of the species are woody, broadleaf deciduous plants.
3. Herbaceous species are typically perennial, spring blooming, early leafing ephemerals from rhizomes or tubers adapted to life on the shady forest floor.
4. Many of the disjunct plant families are older from an evolutionary standpoint. Their flower structure is more primitive than other existing flowering plants (ranalian complex). These families include Magnoliaceae – Magnolia; Ranunculaceae – Buttercup, Meadow-rue, Liverleaf, Clematis, Black Cohosh, and Dolls Eyes; Annonaceae – Pawpaw; Lauraceae – Sassafras and Spicebush; Calycanthaceae – Sweetshrub; Berberidaceae – Barberry, Blue Cohosh, Umbrella Leaf, May-apple; Menispermaceae – Canada Moonseed; and Nymphaeaceae – Water Lily.
Research on disjunct species in the Great Smoky Mountains National Park showed these plants “concentrated in mesic low- to mid-elevation forests.” (White 1983) White noted that these habitats are already richly diverse and the disjunct species increase that community diversity. This temperate forest type, called mixed-mesophytic and known as the “cove” forest in the Smokies, is characterized by greater species richness than any other forest type in temperate climes. The Appalachians and central China are the only areas globally where mixed-mesophytic forests currently exist. (Yih 2012)
Approximately 65 genera have been identified as disjuncts appearing only in eastern Asia and eastern North America. Wofford (1989) listed 46 of them as occurring in the Blue Ridge of Tennessee. The list includes Aletris (Colicroot), Melanthium (Appalachian Bunchflower), Tipularia (Cranefly Orchid), Buckleya (Pirate Bush), Pyrularia (Buffalo Nut), Liriodendron (Tulip Poplar), Adlumia (Allegheny Vine), Astilbe (False Goat’s-beard), Hamamelis (Witch Hazel), Cladrastis (Yellowwood), Pachysandra (Allegheny Spurge), Stewartia (Mountain Camellia), Panax (Ginseng), Pieris (Mountain Fetterbush), Halesia (Silverbell), Campsis (Trumpet Creeper), Phryma (Lopseed), and Mitchella (Partridgeberry).
For the 26 genera additionally found in western North America, more than 90% occur in Tennessee’s Blue Ridge. (Wofford 1989) These include Tsuga (Hemlock), Clintonia (Bluebead Lily), Disporum [Prosartes] (Nodding Mandarin), Trillium, Calycanthus (Sweetshrub), Dicentra (Dutchman’s Breeches), Aristolochia (Dutchman’s Pipevine), Mitella (Bishop’s Cap), Tiarella (Foamflower), Rubus (Blackberry), Rhus (Sumac), Geranium, Aralia (Devils-walkingstick), Leucothoe (Doghobble), and Menziesia (Minnie Bush).
Adding Europe to the mix, the four regions share at least 20 genera such as Erythronium (Trout Lily), Veratrum (False Hellebore), Ostrya (Hophornbeam), Clematis, Cimicifuga (Bugbane/Black Cohosh), Hepatica (Liverleaf), Asarum (Ginger), Rhododendron, Philadephus (Mock Orange), Platanus (Sycamore), Waldsteinia (Barren Strawberry), Cercis (Redbud), Staphylea (Bladdernut), and Aesculus (Buckeye). (Wofford 1989)
Western North America and the Southern Appalachians share 150 genera including Xerophyllum (Turkeybeard) and Dirca (Leatherwood). (Wofford 1989) I have photographed so many of these plants during my Smokies hikes!
DNA studies have enabled botanists to more accurately scrutinize the evolutionary development of these plant species (phylogeny), and they are finding a mixed bag of information. It paints a far more complex history that “may have involved multiple historical events at very different geological times in different genera.” (Xiang, Soltis and Soltis, 1998) In some instances, genetic work reveals that paired disjunct species are not as closely related to each other as to other relatives indicating continued species evolution and diversification after separation.
Some believe Asia was at or close to the center of origin for flowering plants. It has far richer plant diversity, more genera and species, than eastern North America. Asian temperate forests contain approximately three times as many tree species as similar forests in North America. (Guo 1999) Xiang, Soltis, and Soltis (1998) state “eastern Asia, with its 2,753 genera of seed plants, has a biodiversity far greater than that of eastern North America, which has only 1,230.” They cite the “extreme example” of Lindera (Spicebush) [east Asia has 80 species, eastern North America three] and assert this could be attributed to Asia’s “complex topography…promot[ing] a greater rate of speciation due to the abundance of varied habitats and natural barriers that could allow different populations of a species to evolve separately.”
China’s flora, not extensively explored and cataloged until the late nineteenth and early twentieth centuries, was of particular interest for its diversity of woody species. Botanists were not the only people intrigued with Asia’s botanical riches. Horticulturists, noting the similarities in climate and plants, accurately estimated the likely success for cultivation of Asian species in North America as ornamentals. Charles Sargent, first director of the Arnold Arboretum (Harvard University), raised seeds sent to him from China and pushed for the collection of more specimens including living plants. (Boufford and Spongberg 1983)
This climatic, floristic, and ecological affinity has a distinct downside. Asian plants grown in North America may find the local accommodations much to their liking. Freed from evolved competition and predation (insects and disease) back home, their new digs pose little impediment to growth and spread. North American plants, fighting their own battles with native competitors and predators, are quite unused to the foreign competition and have yet to evolve effective means to fight back. Many lose ground to aggressive exotic species which then take over habitats, drastically diminishing diversity. Other organisms dependent on the displaced native species lose too.
In Tennessee, 135 species are listed as known or possible exotic invasive plants. Nearly half, 60 species, are native to Asia and the overwhelming majority are woody (trees, shrubs, or vines). Many other species are generally ascribed to “Eurasia.” Among the most serious pest plants, 18 of the 26 species ranked as a Severe Threat in Tennessee are from Asia. Sixty-six species on the invasive list may be found in nurseries for cultivation, and 40 of them are from Asia.
The same pairing of remarkably similar plants occurs with some of the invasive species: Burning Bush (Euonymus alatus) and Hearts-a-bustin’ (E. americanus), Japanese Wisteria (Wisteria floribunda) and American Wisteria (W. frutescens), Japanese Honeysuckle (Lonicera japonica) and Trumpet Honeysuckle (L. sempervirens), Sweet Autumn Clematis (Clematis terniflora) and Virgin’s Bower (C. virginiana), Japanese Meadowsweet (Spiraea japonica) and Virginia Meadowsweet (S. virginiana), Asian Bittersweet (Celastrus orbiculatus) and American Bittersweet (C. scandens).
The disjunct phenomenon is not limited to plants. Similar species have been discovered among fungi, arachnids, millipedes, insects, and freshwater fish. (Yih 2012 [Wen 1999]) Experts continue to research, debate, and refine the interpretation of data collected to more fully illuminate this fascinating connection.
Information for this blog post was derived from the following scientific papers, articles, and Web sites. If my limited scientific study has led to any misrepresentations, I take full responsibility for these errors.
Boufford, D.E., and S.A. Spongberg. 1983. Eastern Asian-eastern North American phytogeographical relationships: A history from the time of Linnaeus to the twentieth century. Annals of the Missouri Botanical Garden 70:423-439.
Graham, Alan. 1993. History of the Vegetation: Cretaceous (Maastrichtian) – Tertiary. Vegetation and Climates of the Past, Flora of North America. floranorthamerica.org
Guo, Qinfeng. 1999. Ecological comparisons between Eastern Asia and North America: historical and geographical perspectives. Journal of Biogeography 26:199-206.
White, Peter S. 1983. Eastern Asian-eastern North American floristic relations: The plant community level. Annals of the Missouri Botanical Garden 70:734-747.
Wofford, B.E. 1989. Floristic elements of the Tennessee Blue Ridge. Journal of the Tennessee Academy of Science 64/3:205-207.
Wood, Carroll E. 1972. Morphology and phytogeography: The classical approach to the study of disjunctions. Annals of the Missouri Botanical Garden 59:107-124.
Xiang, Qiu-Yun, D.E. Soltis, P.S. Soltis. 1998. The eastern Asian and eastern and western North American floristic disjunction: Congruent phylogenetic patterns in seven diverse genera. Molecular Phylogenetics and Evolution 10:177-190.
Yih, David. 2012. Land bridge travelers of the Tertiary: The eastern Asian-eastern North American floristic disjunction. Arnoldia 69/3:14-23.
Radford University, Biomes of the World and Arcto-Tertiary Geoflora