HOLLOWS, PEEPERS, AND HIGHLANDERS: AN APPALACHIAN MOUNTAIN ECOLOGY

7. The Asian Connection

45 7 The Asian Connection H ere’s a claim that may surprise you: The forests of eastern Asia and southern Appalachia are so similar that if you were swept from one to the other, you would be hard pressed to tell them apart. These regions share many similar plants and some animals , even though intervening areas host vastly different organisms. Living things with fractured geographical distributions are termed disjunct. In this chapter we explore this unexpected connection: the Asia-North America disjunction. The disjunction, usually at the genus and species levels, involves more than fifty genera of Appalachian plants that are restricted to eastern North America and eastern Asia and, except in fossil form, are absent in between. Among some genera, several Appalachian species are geographically separated from Asian relatives : hickory, tulip poplar, sassafras, yellowwood, coffee-tree, silverbell , witch hazel, and stewartia. Within other tree groups, such as the maples, dogwoods, persimmons, hollies, sumacs, snowbells, Hercules-clubs, and sweetleafs, the disjunctions affect only certain pairs of related species. Two observations are telling: (1) many disjunct genera contain few species, and (2) all are confined exclusively to eastern North America and eastern Asia. If not for these two facts, we might have dismissed the separations as due to random chance alone. This frees us to pursue a more interesting explanation. Among trees, two pairs of related species, tulip poplar and Carolina hemlock, present particularly striking disjunctions. There 46 are only two species of tulip poplars, one in China and one in the United States. Likewise, the closest relative of Carolina hemlock, a local tree peculiar to the southern Appalachians, is in eastern Asia. In both the tulip poplar and hemlock genera, each North American species differs only slightly from its Asian twin. The understories and ground covers of the two regions also share many plants. More than two-thirds of the total orchid genera of temperate North America have representative orchid species in eastern Asia. At least eight of these genera consist of disjunct species pairs. In contrast, not a single orchid genus grows exclusively in North America and Europe. Similar Asia-North America disjunctions are also known for ferns, ginseng, mayapple, Jack-in-the-pulpit, skunk cabbage, lichens, and mosses. Let’s focus on two small regions of China and North America. The floras of the eastern Chinese province of Hubei and our Carolinas share many families and genera. About 75 percent of their plant families are represented by at least one species in each region. Mimicking the continental pattern, many of the state and provincial disjunct species are closely related. In addition to likenesses, the floras of these two areas differ in that each has evolved some of its own endemic plant species. Although less common, some animal distributions are also disjunct . The copperhead has a species in the same genus in southeastern Asia. The paddlefish, hellbender, and alligator also have disjunct Asian relatives. So, here we have a paradox: Eastern North America was most recently continuous with the continents of Europe and Africa, yet Appalachia is floristically more similar to eastern Asia. The most widely accepted theory to explain this puzzle suggests that the isolated plants remain as survivors of an ancient circumpolar plant community that died out in Europe, western Asia, and western North America. The Beringian land bridge linked Russia and Hollows, Peepers, and Highlanders 47 Alaska beginning in the Miocene epoch, about 9.25 million years ago. This timing precluded extensive migrations between Asia and America via Beringia before the Miocene. Yet one-half to two-thirds of the Chinese plant species originating in the late Cretaceous, 70 million years ago, and one-fourth to one-half of the plants arising in the late Eocene, 40 million years ago, are nearly identical to their nearest relatives in North America. The fossil record shows only a few Chinese species similar to those of North America after the Eocene. Therefore, today’s disjunct plants must have been continuous between eastern Asia and eastern North America long before land connected Russia and Alaska. The floras must have connected via Europe. The disjunct genera of eastern Asia and eastern North America must represent remnants of ancient plants that were once broadly distributed throughout the Northern Hemisphere. The two floras remain similar today presumably because neither site experienced extensive Pleistocene glaciation and both have shared a similar climate . Not only do these two regions have...