The Malaysian Rhododendrons and the Taxonomic Revolution
David G. Leach
In the era of Victorian conservatories and hothouses the so-called Javanese rhododendrons had a great vogue. The Mauve Decade produced from the arched glasshouses of the wealthy innumerable hybrids which have now passed out of existence and today there are only a few collections in either Europe or America. The species from which they came have remained, by and large, a group of mystery, intriguing and baffling those who have sought a systematic understanding of their distribution and botanical relationships.
The group includes many scaly leaved rhododendrons of extraordinary interest. The "lost" species with the largest flowers in the entire genus, R. toverenae, belongs here. It was discovered in the Horseshoe Mountains of New Guinea by Hunstein in 1884. The fragrant white flowers were seven inches in diameter and about five inches in length. In 1959 the Rev. Mr. Cruttwell of the Anglican Mission collected it on Mt. Dayman and it is once again in cultivation. R. superbum has great five-inch Carnation-scented pink flowers. R. maius and R. archboldianum are also outstanding ornamentals among those with large fragrant whit or pale pink blossoms. Aurigeranum, zoelleri and brassii are conspicuous in the group with yellow to orange flowers. R. konori is highly regarded by Dr. H. Sleumer at the Rijksherbarium in Leiden and it has been successfully grafted on ponticum understock so that it too is now in cultivation.
This whole class of rhododendrons springs suddenly back into the limelight as a result of several expeditions to New Guinea in the last couple of years and the subsequent publication of Dr. Sleumer's masterly classification of the more than 250 species.
It seems unlikely that the influx of new species from recent explorations in New Guinea will contribute to gardens in the northeastern United States, although some may prove to be acquisitions for the mildest parts of the Pacific coastal region. A few originate as low as 400 feet but the majority of the New Guinea rhododendrons come from the middle elevations and sub-alpine forest below 13,500 feet of the high mountains. Perhaps the small group which is found on open grasslands a thousand feet or more below the snowfields, such as R. stonori and R. saxifragoides, which are truly alpine, will be amenable.
The required adaptation is formidable. I scouted some of these Rhododendrons in nature in the Indonesian islands and in Malaya, Cambodia and Viet Nam last spring. In the equatorial climate there is little variation in temperature winter and summer, and almost equal day length the year around. In New Guinea the frost line is between 7,000 and 8,000 feet on some mountains, according to Dr. L. J. Brass of the Archbold Expedition, and frosts occur there every clear morning throughout the year and more generally above 10,000 feet throughout the island. In the warm greenhouse in America these Rhododendrons grow and flower more or less continuously throughout the year.
The recent publication in Indonesia of Dr. Sleumer's monumental study of the Rhododendrons in Malaysia probably foreshadows a complete reshuffling of our whole classification of the genus. It seems likely that we shall once again be thinking in terms of subgenera, sections and subsections in a change that is almost inevitable on the basis of logic and usefulness.
The species collector had better begin thinking right now of Rhododendron also as just one subgenus in a group of subgenera which includes Azaleastrum, Anthodendron and Hymenanthes. There will be some familiar names in the sections and subsections of the revised classification but the fraternity had better brace themselves as well for such strangers as "Choniastrum," "Vireya," "Pseudovireya," "Phaeovireya" and many others that sound equally alien.
In 1958 Dr. Sleumer published The Germs Rhododendron L. in Indochina and Siam in which he first mentioned the discovery in Sumatra of a non-scaly rhododendron in the Irroratum series, a new species subsequently described as R. atjehense, taking its place with Miquel's R. korthalsii on the island. In this same publication Dr. Sleumer demonstrated that elepidote rhododendrons formerly regarded as exclusively southeastern Asian in their distribution extend as well into the Malay Peninsula. Species allied to R. javanicum occur all the way from Indonesia into Viet Nam. Perhaps most surprising of all, the group of which R. vaccinioides is typical covers a vast region from the eastern Himalaya and southwestern China eastward into Malaysia and even into New Guinea, a tremendous sweep of almost 4,000 miles.
All of this changes entirely our ideas of the distribution of Rhododendrons. In The Species of Rhododendron, published in 1930, which has formed the general conception of the genus, the rhododendrons of Viet Nam, Laos and Cambodia were simply omitted, and those of Thailand were passed over lightly. The editor observed, "It has been judged expedient to reserve the Malayan and New Guinea species for future consideration."
Now it is apparent that the Maddenii group extends into North Viet Nam, Laos and Thailand and that a massive reduction in the number of species, from 51 to 22, is in order. Now we know that the beautiful scarlet flowered R. delavayi represents the Arboreum series in both Thailand and Indochina, and that the Fortunei group, through R. serotinum, extends southward and eastward into Thailand, where, incidentally, this species is described as blooming in April. The rhododendrons in the class we now call the Irroratum series acquire six new species which occur in Viet Nam, the Malay Peninsula and Sumatra. Rhododendrons of the Lacteum series are found in Viet Nam. Rhododendrons of the Stamineum series occur in an immense area embracing Burma, southern China, Formosa. Indochina and Thailand. Even a species of the primitive Falconeri group inhabits North Viet Nam. The azaleas of the Obtusum subseries (subgenus Anthodendron, section Tsutsusi, in Dr. Sleumer's treatment) occur in Thailand, Laos, Viet Nam and in the Philippines.
It is apparent now for the first time that there is a gigantic overlap in the distribution of rhododendrons from their two evolutionary cradles on continental Asia and in Malaysia, and that their geographic and taxonomic relationships are quite different than we had supposed.
Dr. Sleumer has followed this revelation with the publication this spring of The Genus Rhododendron in Malaysia in which, for the first time, the so called Javanicum rhododendrons, primarily from Indonesia and New Guinea, and the group which we now know as the Vaccinioides series are gathered into an orderly and logical classification under section Vireya. This monumental work provides a systematic identification of the Malaysian rhododendrons encompassing 261 species and 55 varieties and forms of which 96 series and 29 varieties are newly described from recent collections by members of the Archbold Expedition, the Dutch expeditions and others in New Guinea, and by Eyma in the central Celebes.
It would scarcely be possible to exaggerate the significance of Dr. Sleumer's work. Until the present time more than a quarter of the known species in the genus have been lost in limbo, their relationship unknown and the whole group divorced from the remaining three-quarters of the species which inhabit the earth. Now, at last, we have an ordered identification of the Indonesian and New Guinea species in a system that encompasses all of the world's Rhododendrons. Their relationships are clarified and the whole genus shifts into new perspective in distribution, taxonomy and phylogeny.
For the first time we can say that there are about 925 known Rhododendron species in existence and assign each to its proper place in a botanical classification. A taxonomic revolution is impending and we should welcome the enlightenment that will accompany it.
I can not conclude this brief tribute to Dr. Sleumer's work without remarking on some of the names he has selected for his new species. Among them are R. goodenoughii, R. obscurum and R. perplexum. Dr. Sleumer is a taxonomist with a twinkle in his eye.