Rhododendron Subsection Saluenensia
Melva N. Philipson
Botany Division, DSIR, Lincoln
Christchurch, New Zealand
Reprinted from Bulletin #69 The New Zealand Rhododendron Association
The revised classification of Rhododendron has brought a number of changes not only to the framework of the system, but also to some units within the groups which were formerly known as series. The series are now known as subsections and it is about the subsection Saluenensia (formerly series Saluenense) that I wish to speak today. Dr. James Cullen, Deputy Director of the Royal Botanic Garden, Edinburgh, has been responsible for the revision of the lepidote or scaly-leaved rhododendrons (except for the subsection Lapponica) and the following is an account of the information contained in his revision of the subsection.
In 1919 Dr. Otto Stapf of Kew published the first description of R. calostrotum and wrote "the affinities of this group are so close as to be bewildering and one wonders how the tangled skein of forms will be unraveled". Hutchinson recognised 11 species in his account of the group in "The Species of Rhododendron" published in 1930, and Davidian, in a revision in 1954, reduced this number to 8, one of which he divided into 2 varieties. Both he and Hutchinson included R. fragariflorum in the series Saluenense but Dr. Cullen excludes it altogether and places in a subsection of its own, subsection Fragariflora. In the latest work Dr. Cullen has reduced the species to 2, one divided into 4 subspecies, and the other into 2. I would like to tell you how he came to this conclusion. I feel that here is a good example of the difficulties of a revision, and of the challenge constantly faced by taxonomists when attempting to reduce the complexities of natural populations into entities which will fit the straight-jacket of man-made classification.
For an understanding of the species as they occur in nature, botanical studies must be made on specimens collected in the wild. It is most convenient to use dried herbarium specimens which retain all the necessary botanical characters. Each specimen is subjected to careful morphological analysis; distributional, altitudinal and ecological ranges are plotted and any other relevant information incorporated which will throw light on the interrelationship of forms.
From observations such as these, it was deduced that there were indeed only two groups which were reasonably distinct in their form, and in the territory they occupied. The 2 basic groups in this case were calostrotum and chameunum.
One group with which the name calostrotum has been associated occurs in China to the west of latitude 99 degrees, while the other, with which the name chameunum is linked, occurs to the east of it. In addition, calostrotum has been recognised as occurring as 2 variants, one in the north of the range and known as riparium and the other in the south, which is the more typical form of calostrotum but occupies a much more restricted area.
Complications have arisen because the 2 main groups calostrotum and chameunum overlap their ranges around 98 degrees - 99 degrees E. and 28 degrees - 29 degrees N. In these regions many intermediates have been collected, some from apparently quite stable populations, while others are quite variable and occur only in mixed populations, strongly suggesting that natural hybridization is active in such situations. In addition, these populations in the area of overlap show their own ecological and altitudinal preferences. The problem was how to interpret such a complex situation.
R. chameunum has a wide distribution east of latitude 99 degrees; it also has an additional range up to 15-16,000 ft., often near the limit of vegetation. It is variable in height and leaf size and these characters are directly related to altitude. The smallest leaved prostrate plants with bristly branchlets are familiar in horticulture as prostratum, but the collections show every transition may occur, from the very small-leaved prostrate form to the erect shrub with leaves 1" long, which we know as chameunum. The name "chameunum" is derived from the Greek "chameunos" which means "lying on the ground", obviously alluding to the high alpine prostrate plant, not to the plant of lower altitudes which may reach 2 ft. in height. Both have bristly branchlets, leaf stalks and midribs, and saucer-shaped flowers of crimson-purple. In other words, prostratum is just a high alpine form of chameunum, and cannot be maintained as a separate species in the formal botanical classification.
In contrast, the calostrotum group is devoid of the bristle-like hairs of chameunum, although these may be present on flower stalks and on leaf margins to varying degrees. As already mentioned, this westerly group occurs in 2 forms, one to the north and more widely spread than the other, and known as riparium. The form to the South occupies a limited area and is the one we usually know as calostrotum, with dull green leaves, matte on their upper surface and with pale reddish-brown undersurfaces.
The forms collected in the area of overlap appear to be intermediate in their characters between the sub species present in these areas and some to have become established as true-breeding populations. They have therefore been interpreted as stabilised hybrids, occupying distinct ecological niches. Dr. Cullen has ranked them as subspecies.
In a relatively small area in the north of the distributional area is the form which has been known as saluenense, and is believed to be a hybrid between the only other 2 species which occur in this area. As frequently happens, this first collected and described member of the group is not the most widespread and common. Its name, however, must be retained. The whole group therefore is known as R. saluenense and 2 forms of it are recognised, the widespread R. saluenense subspecies chameunum, and the very local form in the north, believed to be a stabilised hybrid between R. calostrotum subsp. riparium and R. saluenense subsp. chameunum and known as R. saluenense subsp. saluenense.
Because chameunum occurs in such variation, from the upright small shrub to the prostrate high alpine form, and hybridises with riparium in the area of overlap, one might expect to find other intermediates in this same area. And indeed they do occur. Over the years they have been given names, such as R. nitens and R. calciphilum. Dr. Cullen believes them to be intermediate between the high alpine forms of chameunum (prostratum) and riparium, but because their occurrence is sporadic and they appear in mixed populations, he believes them not to be stabilised, and therefore does not recognise them formally.
Further south, subsp. calostrotum replaces subsp. riparium and in the well-defined areas where R. calostrotum subsp. calostrotum and R. saluenense subsp. chameunum overlap, another set of intermediates occur which here appear to be stabilised, and are formally recognised as subspecies. One form R. saluenense subsp. riparioides is interpreted as being intermediate between subsp. chameunum and subsp. calostrotum. R. saluenense subsp. keleticum is believed to be intermediate between subsp. calostrotum and subsp. chameunum (prostratum) and is found only in their area of overlap at the highest altitudes. We have known the small leaves, prostrate form as R. radicans. It is like R. prostratum (now R. saluenense subsp. chameunum) without the bristles on the shoots and leaf stalks but it can vary to a semi-prostrate shrub up to 1 ft, the form to which the name R. keleticum was given in 1920. The name radicans given in 1922 is therefore a later name given to a form of the same plant and must be discarded (or 'sunk' as we say), and the name keleticum prevails.
This then is the latest interpretation of that "twisted skein" of which Dr. Stapf wrote in 1919. Dr. Cullen has unraveled it using information based on distributional data recorded by the plant collectors, and careful analysis of the characters of the plants themselves. The group as a whole is a good example of the variability of natural populations and the problems these variations give the taxonomist in attempting to understand and define them. As a result of this work a few well known names have disappeared from the formal classification (though they will be retained in some form in horticulture), and a few new names have appeared. While such changes can be of temporary inconvenience to the horticulturist they are an indication of our better comprehension of the complex plant populations in nature whose elements provide us with so much delight in our gardens today.