Fossil Records of Rhododendrons
H. P. Hansen
Dean Graduate School
Oregon State College
The following are excerpts from "Pliocene Floras of California and Oregon," by Chaney, Condit, and Axelrod, Carnegie Inst., Wash. Pub. 553, 1944.
"This genus is known in the fossil record of North America from only two occurrences, one in the early Tertiary of Alaska (Hollick, 1936. p. 160, pl. 99, fig. 4) and the other in the Trout Creek beds of eastern Oregon (MacGinitie, 1933, p. 64, pl. 7, fig. 4). The Trout Creek specimens were identified tentatively as Rhododendron, and no specific name was applied. MacGinitie noted its resemblance to modern species from southwestern China, but discussed its relationships no further.
"The present specimen is very similar to the Trout Creek fossil, differing from it in being somewhat narrower and having apparently fewer secondaries. The University of California Herbarium contains several hundred sheets of various species of Rhododendron from the provinces of Szechwan and Yunnan in southwestern China and the eastern border of Tibet. Among these are several species which correspond closely to the present material, notably R. floccigerum Franchet, R. poecilodermum Balfour f. and Forrest, and especially R. rockii Wilson, as represented on sheet 381231. Most of the rhododendron leaves, even of the species cited, are broader than the fossils, although they approach closely the same proportions. All the other characters of the leaves in the majority of the species of this genus, however, are closely matched in the fossils. Both have entire revolute margins, the same arrangement of the secondaries, a thick midrib, and closely netted ternaries. In addition, several of the fossils have a thick coating of carbonaceous matter which may represent the remains of the well developed covering of hair found on the lower surface of most rhododendron leaves. One of the fossils consists of a part of a branch with 11 fragments of alternately disposed leaves attached to it. It is quite possible that the Trout Creek and Buchanan Tunnel fossils represent the same species, even though the latter are consistently narrower than either the Trout Creek or the living material with which both have been compared.
"The modern Chinese rhododendrons appear to be distributed at high altitudes, centering at 9000 to 12,000 feet in Yunnan, Szechwan, and adjacent Tibet. Unfortunately no information about the elevation at which R. rockii occurs is available, but presumably it, too, grows in the same places. One American species, R. maximum Linnaeus, from the Appalachian Mountains, approaches the fossils in general character. This species is distributed from Nova Scotia to North Carolina, commonly along streams at elevations up to 3000 feet. The fact that one of the fossils is a stem with 11 leaves attached indicates that it probably was not carried far before entombment, and hence grew close to the site of deposition."
"Rhododendron californicum Hooker is a regular associate of the redwood forest on the north coast of California, but its fossil equivalent has not been recorded in the Arcto-Tertiary Flora of the western United States. Four leaf bases in the Neer's Hill florule show close relationship to R. californicum, but are too incomplete to warrant description as a fossil species."
Rhododendron californicum ranges from Washington southward into California with the redwood forest, and is clearly an indicator of mesic conditions. At the south it is limited largely to seaward-facing canyons in the redwood forest, but northward it ranges inland with the forest where rainfall is heavy and fogs are frequent.
"Many fossil species of Rhododendron have been described from the Tertiary of Europe, but R. sierrae Condit from the Table Mountain flora and the present specimens comprise the only described records of the genus in the Tertiary of western North America. The Table Mountain species, which has Asiatic rather than American affinities, differs from the Sonoma species in its long, slender outline. Rhododendron idahoensis H. V. Smith (1939) from Thorn Creek apparently is fagaceous; its venation is totally different from that shown by Rhododendron. One of the Thorn Creek species, however, Arbutus opaca H. V. Smith (1941), may be Rhododendron; judging from the figure and the description, it compares closely with the modern R. californicum.
"The high concentration of Rhododendron species in China today suggests that the modern American species of the genus may be relicts of the element now restricted to Asia (Cercidiphyllum, Ginkgo, Glyptostrobus, Koelreuteria, Pseudolarix, Trapa) which was abundant at middle latitudes in North America during the Middle Tertiary."
As suggested in the literature the genus Rhododendron has its center of distribution in China from which it spread into North America, probably via Alaska and thence south and east.
At what time R. macrophyllum as of today became a distinct species is theoretical. It could have survived on the Oregon Coast throughout the Pliocene and Pleistocene to the present, as far as the physiographical and geological history of that area is concerned. This would indicate a possible age of three to seven million years for it or its immediate ancestor. There are two species of rhododendron in Alaska, R. camtschaticum and lapponicum, the latter being circumpolar. The northernmost range of R. macrophyllum is the headwaters of the Skagit River in southern B. C.