Land Bridge Travelers Of The Tertiary: The Eastern Asian-Eastern North .

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Land Bridge Travelers of the Tertiary: The EasternAsian–Eastern North American Floristic DisjunctionDavid YihTremains the classic disjunction. It continues tostimulate new scientific papers, with each successive generation applying new research toolsto its mysteries.Recognition of the disjunction began inthe 1750s with botanists making lists of species found in both regions. By the mid-1800sbotanists had collected enough materials tolead them to the astounding conclusion thatthe flora of eastern North America (ENA) hadmore in common with eastern Asia (EA) thanit did with western North America. Most ofPeter Del Tredicihe eastern Asian–eastern North American floristic disjunction is a curious phenomenon that has fascinated botanistsfor over 200 years: the existence of an entirecatalog of species and genera shared by twovastly separated regions and found nowhereelse. It has inspired generations of researchersand given impetus to such fields as biogeography and paleobotany. Scientists now recognizemany different disjunct patterns around theworld, but the eastern Asian–eastern NorthAmerican was the first to be discovered, andENA meets EA: A garden path separates the eastern North American species Allegheny spurge (Pachysandra procumbens, left) from the eastern Asian species Japanese spurge (Pachysandra terminalis).

The Eastern Asian–Eastern North American Floristic Disjunction 15what were once thought to be identical species are now considered congeners (distinctspecies belonging to the same genus), so thedisjunction is more about shared genera thanshared species, and it is now clear that eastern North America has more in common withwestern North America than with eastern Asia.However, it is also clear that eastern Asia andeastern North America have more in commonthan eastern Asia and western North Americaand that a remarkable disjunction phenomenonexists. Today, the list of EA-ENA botanical “disjuncts” (shared taxa peculiar to the two regions)includes about 65 genera, a handful of closelyrelated genera, and a few species (Wen 1999).Most of the genera are temperate; only afew come from subtropical or tropical zones.And most disjuncts are woody plants. Many ofthe herbaceous ones are early-leafing speciesadapted to thrive on the forest floor. Some EAENA disjunct genera that have familiar representative species in the Northeast are listed inTable 1 (Li 1952), along with generalized common names for the species.Disjunct RegionsThe majority of eastern Asian disjuncts growin the Sino-Japanese Floristic Region, whichextends from China’s western Yunnan andSichuan provinces through central, eastern, andmost of southern China to Korea and Japan. Therichest association of disjunct genera occurs incentral China, along the longest river in Asia:the Yangtze (Li 1952). On the American side,the richest disjunct area is along the Appalachian Mountains. The two areas are the onlyinstances globally of the mixed mesophytic forest, one of the most biodiverse temperate foresttypes in the world.Western botanists have reported experiencing a sense of déjà vu in the forests of China.Add to the disjuncts the more wide-ranging species that also occur in both regions, and thelevel of similarity becomes quite high. A recentstudy found that 67% of the seed plant genera in Maine occur on Japan’s Honshu Island(Qian 2002). The similarity was even greater inages past. Several genera that are now endemicto eastern Asia occur in fossil form in NorthAmerica, e.g., the familiar Ginkgo and Metasequoia (dawn redwood), while fossil remains ofSequoia (redwood) and Taxodium (baldcypress),genera now confined to North America, havebeen found in eastern Asia.At present, the similarity is limited by pronounced differences in biodiversity. Disjunctgenera tend to have more species in Asia thanin America. The extreme example is Lindera,with 80 species in eastern Asia but only 2 inNorth America. Indeed, eastern Asia, with its2,753 genera of seed plants, has a biodiversityfar greater than that of eastern North America,which has only 1,230. According to one explanation, the Paleocene forests of both regionswere equally rich in species until severe climatic fluctuations in North America resultedin many extinctions. Another possibility is thatthe complex topography of eastern Asia promoted a greater rate of speciation there due toTable 1WoodyCampsisCaryaTrumpet vineHickoryCatalpa CatalpaCornusGleditsiaHamamelisDogwoodHoney Virginia creeperSassafrasHerbaceousPanaxGinsengPhryma LopseedPodophyllumMayappleSaururus Lizard’s tailSymplocarpusSkunk cabbage

16 Arnoldia 69/3 February 2012Peter Del TrediciPeter Del TrediciThe Sino-Japanese Floristic Region.Manchurian catalpa (Catalpa bungei), left, is native to China, while northern catalpa (Catalpa speciosa), right, isnative to the central and eastern United States and southern Ontario.

Courtesy of David CreechNancy RoseThe Eastern Asian–Eastern North American Floristic Disjunction 17Robert MayerShowy flowers are a feature shared by Chinese trumpet creeper (Campsis grandiflora), left, and the familiar trumpet creeper(Campsis radicans) of North America, right.Lindera obtusiloba (seen here in fall color at the Arnold Arboretum) is one of the many Lindera species native to eastern Asia.the abundance of varied habitats and naturalbarriers that could allow different populationsof a species to evolve separately (Sargent 1913;Qian and Ricklefs 2000). The EA–ENA disjunction is now recognized not only among plants,but among taxa of fungi, arachnids, millipedes,insects, and freshwater fish, as well (Wen 1999).But botanists can take credit for being the firstto notice and document the phenomenon.A Theory BloomsThe earliest hint came in the 1750 dissertationof Halenius, a student of Linnaeus. It mentionsnine species found both on Siberia’s Kamchatkapeninsula and in North America, includingmembers of genera familiar to New Englandbotanists: Asplenium, Lycopodium, Anemone,Heuchera, and Spirea. Commercial exploitationof the phenomenon had already begun. PèreLafitau, a French Jesuit, had discovered American ginseng (Panax quinquefolius) growing nearMontreal in 1716, and French Canadians wereneglecting their farms in the rush to collect wildginseng for export to China (Kingsford 1888).In 1784, C. P. Thunberg, a Swedish botanist,included in his Flora Japonica twenty speciesfirst described for North America (Boufford andSpongberg 1983). The following year, the Italian botanist Luigi Castiglioni began a two-yearsojourn in the United States. Castiglioni’s mission was to bring back useful seeds to Italy, andhe is credited with introducing to continentalEurope such trees as black locust (later to showinvasive tendencies), catalpa, and arborvitae. In1790 he published his Viaggio negli Stati Unitidell’ America, with elegant botanical drawingsof such American sights as the franklintree,already rare in the wild and soon to be extinctoutside of cultivation. It also contains the firstexplicit discussion of the floristic similarity ofeastern North America to Japan, the only partof eastern Asia for which published floras werethen available. Overlooked by nearly all whowould later treat the disjunction, he receivedscant credit for his role in its discovery (Li1955). Brief comments in the work of Pursh andthen Nuttall reveal little beyond an incipientrecognition of the disjunction, though Nuttall

February 2012Michael DosmannNancy Rose18 Arnoldia 69/3Peter Del TrediciPeter Del TrediciThough their flowers look similar, Chinese witchhazel (Hamamelis mollis), left,blooms in late winter or very early spring while common witchhazel (Hamamelisvirginiana), right, blooms in late fall or early winter in eastern North America.Cultivated Chinese ginseng (Panax ginseng), left, and a fruiting specimen of American ginseng (Panax quinquefolius), right.

undertook to note the geographical distributions of North American genera. It remainedfor Asa Gray, the preeminent American botanist of the nineteenth century, to focus attention on the disjunction, bringing it to the noticeof the wider scientific community in a seriesof articles beginning in 1840 and spanningnearly 40 years.Charles Darwin, who began an extensive correspondence with Gray in 1855, encouragedhim to study the global distributions of theNorth American flora. In his second letter toGray he wrote, “The ranges of plants, to theeast and west, viz. whether most are found inGreenland and Western Europe, or in E. Asiaappears to me a very interesting point as tending to show whether the migration has beeneastward or westward” (Darwin 1855). In 1859,after studying new collections from Japan, Graypublished his classic “Diagnostic Characters”paper that included a list of 134 species sharedby eastern North America and Japan. On thelist were such northeastern plants as bluecohosh (Caulophyllum thalictroides), fox grape(Vitis labrusca), ditch stonecrop (Penthorumsedoides), honewort (Cryptotaenia canadensis),hobblebush (Viburnum lantanoides), wild ginger (Asarum canadense), red trillium (Trilliumerectum), large twayblade (Liparis liliifolia),and rose pogonia (Pogonia ophioglossoides). Allwere later shown to be distinct from their Asiancounterparts—belonging to the same disjunctgenus, but different species. The two ferns onGray’s list, sensitive fern (Onoclea sensibilis)and cinnamon fern (Osmunda cinnamomea),turned out to be too widespread globally toqualify as disjunct species (Li 1952). In additionto comparing the flora of Japan to various otherregions, Gray’s “Diagnostic Characters” contains an extended discussion of the EA-ENA disjunction. “It will be almost impossible to avoidthe conclusion,” he writes, “that there has beena peculiar intermingling of the eastern American and eastern Asian floras which demandsexplanation” (Gray 1859). It was the eve of theappearance of Darwin’s Origin of Species, during a ferment of interest in the natural world,and there was no shortage of theories on suchtopics. “Schouw’s hypothesis” held that therehad been multiple geographic origins of manyspecies. At a time when naturalists were strug-Courtesy of Tom Barnes, University of KentuckyThe Eastern Asian–Eastern North American Floristic Disjunction 19Rose pogonia (Pogonia ophioglossoides), seen here,is native to bogs in much of eastern North America.Pogonia japonica (formerly listed as P. ophioglossoidesvar. japonica) is a very similar-looking species nativeto Japan and parts of China.gling to reconcile scientific rigor with cherishedbeliefs, Gray was also conversant with suchhybrid approaches as Maupertius’s “principle ofleast action,” according to which it was “inconsistent with our idea of Divine wisdom thatthe Creator should use more power than wasnecessary to accomplish a given end” (quotedin Gray 1859). By applying this principle, onecould argue (without sacrificing piety) that oncecreated, the far-flung species had migrated ontheir own, rather than requiring further divineintervention. With characteristic grace, Graygave dispassionate consideration to all points ofview. J. D. Hooker, the prominent British botanist and a close friend of Darwin, had recentlyproposed, in relation to southern-hemispheretaxa, “the hypothesis of all being members ofa once more extensive flora, which has brokenup by geological and climatic causes” (quoted in

20 Arnoldia 69/3 February 2012Boufford and Spongberg 1983). In the end, Grayapplied a similar hypothesis to the Asian andAmerican floras. With various refinements, itremains in effect to this day.The Ongoing PuzzleNancy RoseWith the general adoption of cladistics in thelatter part of the twentieth century and rapidadvances in molecular genetics, new tools haveemerged for studying the disjunction. Most scientific papers from the last twenty years usemolecular data and focus on a single disjunctgenus. There are several sorts of molecularlevel data to choose from (the most popularhas been ITS—short for sequences of internaltranscribed spacer regions of nuclear ribosomalDNA). Though their relative merits are stillbeing assessed, the information they yield pertains not only to phylogeny (how disjuncts arerelated in terms of evolutionary descent), butalso to dating divergence times and inferringpathways and directions of migration.Often the genetic analyses match nicely withthe prior work of traditional taxonomists. Thegenus Sassafras, for example, is monophyletic.That is, its three species constitute a clade.The eastern North American S. albidum is“sister” to the smaller clade made up of its twoeastern Asian counterparts. Molecular datasubjected to statistical methods put their intercontinental divergence time at around 15 million years ago (Nie et al. 2007). Sassafras alsoillustrates several frequent patterns; diversification in one or both continents followingthe time of separation is common, and disjunctgenera tend to have more eastern Asian thanAmerican representatives.The upshot of all the investigations intogeology, the fossil record, climate studies,taxonomy, and the molecular clocks and phylogenetic analysis of modern genetics is stilla necessarily tentative picture of the disjunction’s history. But there is agreement on thebroad outlines. Most scientists do not considerAsa Gray listed sensitive fern (Onoclea sensibilis) as a disjunct species, but it was later determined simply to be avery globally widespread species.

The Eastern Asian–Eastern North American Floristic Disjunction 21Peter Del TrediciPeter Del TrediciNative ranges of Sassafras species in eastern North America and eastern Asia.Foliage of Sassafras albidum (left), a familiar native tree in much of the easternUnited States, and of S. tzumu (right), native to China.Sassafras albidumS. randaienseS. tzumuThe entire genus Sassafras,which constitutes a clade.The eastern North AmericanS. albidum is “sister” to thesmaller clade made up of itstwo eastern Asian counterparts, S. tzumu andS. randaiense.

February 2012Chris Evans, River to River CWMA, Bugwood.orgMichael Dosmann22 Arnoldia 69/3Autumn foliage of kousa dogwood (Cornus kousa), top, from Asia, and flowering dogwood (Cornus florida),bottom, from North America.

The Eastern Asian–Eastern North American Floristic Disjunction 23long-distance dispersal to have played muchof a role. The prevailing view is that mostdisjuncts are remnants of genera that wereonce widely distributed in the northern temperate zone during the Tertiary period. Thesebroad distributions in the northern hemisphere were made possible by recurring landbridges. Bering land bridges connecting Asia toNorth America were present at several timessince the Mesozoic era. North Atlantic landbridges connected North America to Europevia Greenland beginning in the early Tertiary,and by the mid-Tertiary, Europe and Asia wereconnected by a land bridge along the TethysSeaway. After the establishment of the northern Tertiary flora, the formation of the RockyMountains brought profound changes in climate and rainfall patterns, causing the genera to disappear from western North Americaduring the late Tertiary and Quaternary. During the Quaternary glaciations, they were alsoextirpated from Western Europe.Drawing on fossil, geologic, and climaticevidence, B. H. Tiffney proposed five differenttime periods during which migrations over theland bridges may have occurred between thetwo regions (pre-Tertiary, early Eocene, lateEocene-Oligocene, Miocene, and late Tertiaryto Quaternary), with different types of plantsfeatured in each migration (Tiffney 1985). Amultiple-origins view is also supported bymolecular evidence. Molecular clock data fromCornus, Boykinia, and Calycanthus suggestthat the disjunction could have involved multiple events at different geological times indifferent genera (Xiang et al. 1998).Ultimately, the EA-ENA disjunction is partof a broader picture that will occupy biogeographers for years to come. Studies of northernhemisphere intercontinental disjuncts point tocomplex biogeographical relationships amongtaxa in five major regions, including not onlyeastern Asia, eastern North America, andwestern North America, but western Asia andsoutheastern Europe as well. Darwin’s desire todetermine whether migration happened “eastward or westward” has grown into a multifaceted field of study.Cited WorksBoufford, D. E. and S. A. Spongberg. 1983. Eastern Asian–Eastern North American PhytogeographicalRelationships—A History from the Time ofLinnaeus to the Twentieth Century. Annals ofthe Missouri Botanical Garden 70: 423–439.Darwin, Charles. 1855. Letter to Asa Gray, June 8. InFrancis Darwin, ed. The Life and Letters ofCharles Darwin, vol. 2. 1959. New York:Basic Books.Gray, Asa. 1859. Diagnostic Characters of New Speciesof Phænogamous Plants Memoirs of theAmerican Academy of Arts and Sciences, NewSeries 6(2).Kingsford, William. 1888. The History of Canada. London:Trübner & Co.Li, Hui-Lin. 1952. Floristic Relationships between EasternAsia and Eastern North America. Transactionsof the American Philosophical Society 42(2):371–429.Li, Hui-Lin. 1955. Luigi Castiglioni as a Pioneer in PlantGeography and Plant Introduction. Proceedingsof the American Philosophical Society 99(2):51–56.Nie, Z.-L., J. Wen, and H. Sun. 2007. Phylogeny andbiogeography of Sassafras (Lauraceae) disjunctbetween eastern Asia and eastern NorthAmerica. Plant Systematics and Evolution 267:191–203.Qian, Hong. 2002. Floristic Relationships betweenEastern Asia and North America: Test of Gray’sHypothesis. The American Naturalist 160(3):317–332.Qian, Hong and Robert E. Ricklefs. 2000. Large-scaleprocesses and the Asian bias in species diversityof temperate plants. Nature 407(6801): 180–182.Sargent, Charles S. 1913. Introduction to E. H. Wilson’sA Naturalist in Western China. New York:Doubleday, Page & Co.Tiffney, B. H. 1985. Perspectives on the origin of thefloristic similarity between eastern Asia andeast North America. Journal of the ArnoldArboretum 66: 73–94.Wen, Jun. 1999. Evolution of Eastern Asian and EasternNorth American Disjunct Distributions inFlowering Plants. Annual Review of Ecologyand Systematics 30: 421–455.Xiang, Qiu-Yun, Douglas E. Soltis, , and Pamela S. Soltis.1998. The eastern Asian and eastern andwestern North American floristic disjunction:Congruent phylogenetic patterns in sevendiverse genera. Molecular Phylogenetics &Evolution. 10(2): 178–190.David Yih, Ph.D., is a musician, writer, and naturalist.

first described for North America (Boufford and Spongberg 1983). The following year, the Ital-ian botanist luigi castiglioni began a two-year sojourn in the United States. castiglioni's mis-sion was to bring back useful seeds to Italy, and he is credited with introducing to continental Europe such trees as black locust (later to show

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