Distant Relatives

In the summer of 1872 — the year Grant defeated Greeley for a second term and the Credit Mobilier scandal broke; the year Luther Burbank developed his eponymous potato and Aaron  Montgomery Ward invented mail-order merchandising; the year General Meade, the victor of Gettysburg, died and Rasputin, Bertrand Russell, and Calvin Coolidge were born — the botanist Asa Gray came out to California to see the redwoods and giant sequoias.
Gray was duly impressed, as most travelers were: “No account and no photographic representation . . . gives any adequate impression of their singular majesty,” he later told a scientific audience. He was a bit dubious about the habit of naming the trees after notables like Grant and Sherman: “Whether it be the man or the tree that is honored in the connection, probably either would live as long, in fame and in memory, without it.”
But his response was not just aesthetic. Gray, like his contemporaries Darwin and Wallace, saw patterns in space and time that began to make sense of the distribution of living things. The redwoods formed part of a particularly intriguing one. The two species had limited ranges in the West, and their nearest relatives were a continent and an ocean away: the bald cypress of the Southeast and the Chinese swamp cypress. (This, of course, was years before the discovery in southern China of the dawn redwood, previously known only from fossils.)
He was aware that other groups of plants had a similar disjunct distribution: the Pacific yew, of Taxol fame, and the Torreya or California nutmeg, each with relatives in East Asia and in one river drainage in the Florida panhandle. Gray had earlier been struck by the similarity between the floras of China and Japan and the eastern United States. The relationship between California and the Pacific Northwest and the other two areas, while not as strong, was suggestive enough to call for an explanation.
Later botanists added a number of taxa to the list of plants with such patterns of occurrence. Rhododendrons reach their peak of diversity in East Asia, but have two species on the West Coast and two in the Appalachians. Spicebush, with its wine-barrel scent, has representatives in the southeastern US, Oregon and California, and China. More: the delicate fawn lilies, the wild ginger that covers forest floors, the trilliums, the Clintonia lily with its alarming purple berries, the bleeding-hearts, buckeye, redbud, and mock orange.
Even within widespread groups like the oaks, botanists can detect East Coast/West Coast/East Asian affinities. Up in the Klamath Ranges there’s the scrubby Sadler or deer oak, whose next of kin are in East Asia and eastern North America. Some closely related species occur in two of the three regions. The western vine maple is more like Japanese maples than other North American maples. China has an abundance of tanoaks and chinquapins, otherwise found only in California and Oregon.
The recently discovered Shasta snow-wreath, an odd member of the rose family, illustrates another pattern: its only near relative is native to Alabama.
What does all this mean, though? In Asa Gray’s time, it was widely accepted, even by scientists like Louis Agassiz, that plants grew where they grew because that’s where God wanted them. Gray, Darwin’s staunchest American defender, was not comfortable with this notion of the Divine Landscaper, or with biogeographical arguments that invoked the lost continent of Atlantis. The exploration of the fossil record gave a historical dimension to botany and suggested that the distribution of some of these plants had once been much wider.
Fossils very much like the living sequoia and bald cypress had turned up in Greenland and Norway as well as in the American Rockies. Paleobotanists were finding other surprises: ginkgos, surviving only in China, once flourished in California and Europe. Most of these remains were from the last 65 million years, but some appeared to date back to the time of the dinosaurs. The picture emerged of an ancient temperate-zone forest girdling the Northern Hemisphere and extending much farther poleward than any tree grows today.
Thirteen years before his California visit, reflecting on the flora of Japan, Gray had written: “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.” He thought, correctly, that the glacial era had been one such vicissitude. But it was only the most recent of the changes that reduced the old Tertiary forest to its present remnants. In North America, the rise of the Sierra Nevada and the Rockies, together with a global drying trend, transformed the forests of the continental center into savanna, then into grassland. Then, as the world grew colder, the glaciers pushed the vegetation of Europe, trapped by the Alps and the Mediterranean, into oblivion and that of North America into southern refugia.
Those refugia still have a distinctive character. The Klamath region is richer in what botanists call “Tertiary relicts” than any other part of the West.
Peter Raven and the late Daniel Axelrod, in their Origin and Relationships of the California Flora, describe the climate there as very much like that of pre-Ice Age times: “The precipitation season is longer than elsewhere in California, there is more summer rain, winters are not excessively cold, temperatures are more moderate in summer, and drought stress is lower…than in the Sierra.” And the glaciers never reached there. Hiking in those mountains is a kind of time travel for pedestrians.
Other botanists have pointed out parallels between the Klamaths and the southern Appalachians, another haven for Tertiary relicts. The Appalachians are also old mountains that escaped glaciation and maintained a moist, moderate climate. If anything, they were a more secure home for plants driven south by the glaciers; trees like sweetgum, sassafrass, magnolia, and persimmon survived there, but died out in the Pacific Northwest. Too bad; it would be nice to have persimmons growing wild in Shasta County. The Shui-Hsu Valley in central China, home of the dawn redwood, is another such pocket with wet summers, mild winters, and a rich relictual flora.
We now know that one piece of the puzzle denied to Gray was plate tectonics. North America and Eurasia were once part of a supercontinent called Laurasia, the northern counterpart to Gondwanaland. As the widening of the North Atlantic some 50 million years ago broke up a pan-Northern Hemisphere plant community, the ancestral Arcto-Tertiary flora began to evolve into the distinct but closely related species we see today.
Asa Gray ended his talk on “Sequoia and its History,” delivered in Dubuque, Iowa in August 1872, with a striking metaphor for the unfolding of the evolutionary process in the sweep of geologic time: “Organic nature — by which I mean the system and totality of living things, and their adaptation to each other and to the world — with all its apparent and indeed real stability, should be likened, not to the ocean, which varies only by tidal oscillations from a fixed level to which it is always returning, but rather to a river, so vast that we can neither discern its shores nor reach its sources, whose onward flow is not less actual because too slow to be observed by the ephemerae which hover over its surface, or are borne upon its bosom.”

Comments are closed.