The Journal of Indian Botany/Volume 2/January 1921/The Distribution of Floras in S. E. Asia as affected by The Burma-Yunnan Ranges
THE DISTRIBUTION OF FLORAS IN S. E. ASIA AS AFFECTED BY THE BURMA-YUNNAN RANGES.
By Capt. F. Kingdom Ward, B.A., F.R.G.S.
I propose first to give a brief account of the flora of the N. E. Frontier of Burma, secondly to indicate its relationships, and thirdly to point out the significance of the N. E. Frontier ranges as regards geographical distribution in S. E. Asia generally.
The N. E. Frontier is formed by a broad arc of very mountainous country sweeping round in a half circle from the eastern end of the Himalaya. Here it forms the watershed between the Brahmaputra and Salween drainage systems. Turning south, it forms the watershed between the Irrawaddy, rising within the curve, and the Salween. My remarks on the flora refer especially to this southern part (Lat. 26° N. Long. 98° 30' E.)
The whole region up to an altitude of 12,000 feet is covered with forest, which is divisible into (i) jungle up to 5,000 feet (ii) temperate rain forest 5,000—8,000 feet (iii) Conifer forest above 8,000 feet.
The jungle is made up entirely of Indo-Malayan forms. It contains species of Dipterocarpus, Shorea, Hiptage, Elaeocarpus, Engelhardtia, Garcinia, Caryota, Calamus and Ficus ; many lianas, such as Mus- saenda ; climbing Aroids, like Raphidophora and Pothos ; tree ferns ; and in the undergrowth many Zingiber aceae, Acanthaceae and Gesne- raceae. There are also many epiphytic ferns and orchids.
The temperate rain forest contains many big trees which rarely possess feebly developed plank buttress roots. Characteristic genera are Sehima, Gordonia, Bucklandia, Quercus, Magnolia, Acer, Rhododen- dron ; also several palm-like Araliaceac, and occasional species of Ficus. Most of the trees are loaded with epiphytic ferns, orchids and Aeschy- nanthus ; and there is a striking development of moss. There are a few lianas of temperate genera — Clematis, Hydrangea, Aristolochia. Undergrowth is not very dense except in deep gullies, it is chiefly bamboo. Small herbs abound, especially Urticaceae, Impatiens, and Strobilanthes.
In the Conifer forest there is a preponderance of Abies, bamboo and Rhododendron emuldrum. Other Conifers, such as the Chinese coffin plank tree {Juniperus sp). and a larch are curiously local ; more widely distributed is a Pseudotsuga. I must also mention here a pine, though it does not belong to this belt, being found on rocky grass-clad slopes between 4000 and 7000 feet ; nor does it form forests, growing with oak and alder trees in open park land.
There are also a large number of shrubs in the Conifer forest — species of Rhododendron, Euomjmus, Bibes, Bubus, Picris, Echinanthus, Bosa, Philadelphus , Deutzia, Hydrangea, Buddleia, and others ; and a few climbers, including Schizandra, Akebia, and Smilax.
In some places, between 9000 and 11000 feet, especially where it is marshy, alpine meadow is developed. Here grow species of Nomocharis, Thalictrmn, Epipactis, Allium, L ilium. Meconopsis, many Compositae, Umbelliferae, etc.
Finally, in the alpine belt are many dwarf Bhododendrons, and shrubs such as Potentilla fruticosa, species of Spiraea, Pyrus, Bcrberis, and bamboo. On the screes and rocks species of Primula, Gremanthodiuvi, Lloydia, Pedicularis, Saxifraga, Gentiana, Gassiope, Androsace-, abound; and there are a few cushion plants.
The most extensively represented order in the mountain belt is Ericaceae. Not only are there over fifty species of Bhododendron alone, but many of them grow socially, covering the summits of the mountains as heather on a Scotch moor. Other well represented genera are Impatiens, Begonia and Strobilanthes at low altitudes ; Bubus, Polygonum, Primula at intermediate altitudes ; Pedicularis, at high altitudes.
But it would give a wrong impression of the flora, did I not mention orders and genera so wealthy in species as Orchidaceae, Gcsneraceae, Leguminosac, Acanthaceae, Zingiber aceae, Banunculaceae Liliaceae, Saxifrag aceae, Bosaceae, Araliaceae, with Arisaema, Chirita, Euonymus and Aeschynanthus.
I now pass on to the question of relationship.
The North East Frontier ranges lie on the flank of the Yunnan plateau to the east, and may be regarded as forming a huge bluff over- looking the low country of Burma and Assam.
Critical examination of the flora shows a mixture of Himalayan, Indo-Malayan, Chinese and endemic forms.
The preponderance of an Indo-Malayan flora at low altitudes is easily understood, since these ranges penetrate the Indo-Malayan region to the south and west. The point to notice is that it is soon lost ; it does not extend eastwards. In the Brahmaputra valley, the Indo-Malayan flora gets as far as 30° N. in the Irrawaddy basin to about 29°, in the next valley to the east — the Salween, to about 28°, but so impoverished as to appear scarcely dominant. In the Mekong valley however it only penetrates to about the twenty-third parallel, i.e., to the tropic, and now it is sadly stricken. Eastwards of the Mekong this Indo-Malayan flora spreads over the peninsulas of S. E. Asia, south of the tropic, and creeps north- wards up the China coast again almost as far as the Yangtze in latitude 30,° but only as a narrow belt. However, there is a wide divergence between the Indo-Malayan flora of Fokien and that of the Irrawaddy basin in the same latitude, owing to the N. E. Frontier barrier and the intervening plateau of Yunnan, which has prevented any transference of species across from west to east.
The Himalayan element is very strongly represented at interme- diate altitudes, less so in the alpine region, though the connection is well seen there too.
Now this flora cannot have made its way straight across the Assam and Irrawaddy valleys from the broken end of the Himalaya. It must have come right round the arc from the north ; and we naturally ask : How did it cross the gorge of the Dihang, which even now separates the eastern Himalaya from the N. E. Frontier belt ?
How can the seeds of alpines and sub-alpines, which are rarely constructed for long distance travel, cross such a chasm ? I need only instance such plants as Mcco?iopsis Wallichii, Isoyyrum adianti- folium and Lilium Thompsonianitm. Such plants as actually have seeds adapted to long distance travel, e.g. Podophyllum Emodi, Lilium giganteum and Rosa sericea, are found much further east than the N. E. Frontier ranges, extending into Kauru, Sbeuri, and often right across China to Japan.
Two possible explanations suggest themselves to account for this strong Himalayan element on the N. E. Frontier.
(i) That the whole region of southern and eastern Tibet enjoyed originally a temperate climate, and was covered by a uniform flora. When desert conditions began to prevail in Tibet, owing to the uplift, in Eocene times, of the Himalaya, and the N. E. Frontier ranges, the flora withdrew to these mountains, and was subsequently separated into three great blocks — Himalaya, N. E. Frontier, and western China— by the formation of the river gorges. This theory requires the flora to be of Pre-eocene age, but that is not impossible.
(ii) That there was previously a continuous Sino-Himalayan range, as it may be called, stretching across the present headwaters of the Irrawady and binding the Himalaya to the great China divide which passes through Kauru and Sbeuri ; and that this range was breached by the uplift of the N. E. Frontier ranges since Eocene times. But there is no proof at present that the N. E. Frontier ranges are younger than the Himalaya uplift though they certainly are features of original structure, and not, as Kropatkin believed, pidges separating grooves cut in the Tibetan plateau by the rivers, In other words, the breach in the Sino-Himalayan range is not the passive work of rivers cutting their way back, but is a far more formidable gap resulting from an active uplift of ranges whose axes cut across a possibly older uplift.
But whether we hold that this Sino-Himalayan flora was once more widely spread over Tibet at a time when that country had a more genial climate, as suggested in the first alternative ; or whether we hold that it merely crept across from Kauru to the Himalaya along a continuous range, prior to the uplift of the N.E. Frontier ran- ges, subsequently surging southwards through the breach they made, as in the second alternative ; we perceive that many species, now quite isolated, have survived unchanged through a long period, since they are now found in places as far apart as the Himalaya, N.E. Frontier and western China, to say nothing of Japan and N. America.
I say nothing as to the direction taken by the stream of flora, beyond the manifest fact that the alpine flora of the Htawgaw Hills (Lat. 26° N., Long. 98° 30 E.) has come neither from the east, nor from the west, but straight down the ranges from the north.
No doubt the flora of the great Asiatic divide has swayed back- wards and forwards, but in the main it is generally believed to have originated within the Arctic circle, and to have moved south- westwards into Asia and south-eastwards down the Atlantic coast of N. America, with the glaciation of the North. So much for its intro- duction into Asia ; but its movements after that are another matter, altogether. That hypothesis sufficiently accounts, in a broad way, for the relationship existing between the floras of the Himalaya, Japan and North America ; it by no means accounts for the relationship existing between the floras of the N. E. Frontier, the Himalaya, and Western China.
The Chinese element in the flora is also conspicuous, but a dis- tinction must here be made between the two sources from which it has been derived. These are : —
(i) The valleys to the immediate south and east; (ii) the high ranges of far western China, which, thrusting southwards from the main body hold in their grasp much of Yunnan. These sources must be carefully distinguished.
As regards the first, this flora extends eastwards no further than the Salween valley, and northwards no further than about the 26th parallel ; that is, it embraces the basin of the Shiveli river. It may be called the Burma-Yunnan area, and is characterised by many identi- cal species of 'Rhododendron, by the Candelabra Primulas, many Gelastraceae, Gesneraceae, etc. This flora is highly endemic. The typical Chinese flora is confined to higher altitudes, and includes many alpines such as Polygonum Forsterii, Primula serratifolia, Cassiope palpebrata, besides sub-alpines such as Primula sonchi folia, Nomocharis, etc.
But with the flora of Yunnan east of the Salween, apart from the high ranges which thrust long processes southwards from the north- ern mountain masses, the North Bast Frontier flora seems to have nothing in common.
Endemic species are found especially in the alpine region, and to a lesser extent at intermediate altitudes. Amongst the former may be mentioned species of Rhododendron, Primula, Cremanthodinm etc., amongst the latter the monotypic genera Becsia and Sporoxia, species of Strobilanthes, Codonopsis and various Gesnerads.
Those plants which are both Himalayan and Chinese are widely distributed across eastern Asia, often occurring in Japan as well and sometimes even in N. America, (e.g., Clitoria mariana). On the other hand there are plants ranging from the Himalaya into western China, which are not found on the N. E. Frontier, (e.g., Isopyrum grandiflorum).
Plants from what I have called the Burma-Yunnan area are not found in the Himalaya. This area is simply the splayed out ends of the North East Frontier belt, tailing off southwards into the Indo- Malayan region; it has carried this flora southwards into the Indies, where for instanco Primula imperially is found on the mountains of Java.
But many plants of the Himalayan foot hills, representing the Indo-Malayan element, are found in the Burma-Yunnan area.
Negative results in the alpine belt are as valuable as positive results. I have insisted on the relationship of the alpine flora to that of the Himalaya and western China ; but what shall we say to the complete absence of such typical genera as Aquilegia, Tncarvillea, Primulas of the Amethystma section, and Pinguicula alpina ; and to the extreme poverty of such as Meconopsis, woolly Compositae, alpine Leguminosae, Berber is, Cotoneaster , Lonicera, etc.? In view of the large number of Rhododendrons, especially of the dwarf groups, we are, forced to the conclusion that if it is simply a question of altitude — and the highest peaks are little over 13,000 feet — then altitude affects plants both absolutely and relatively. In other words, Rhodadendrons flourish within 1,000 feet of the snow line, wherever the snow line may be ; but Meconopsis for instance, flourishes only above 15,000 feet.
We are now in a position to appreciate to some extent the part played by the North East Frontier belt as regards distribution in South East Asia generally. In the main it clearly separates the Indo-Malayan flora from the Chinese flora. But at the southern end of the belt, there is an inter- polated flora, — endemic to a high degree, but related also to the Indo- Malayan on the one hand, and the Yunnan plateau on the other — occupying the Burma-Yunnan area.
Striking, however, as is this North East Frontier belt as a barrier, it is even more so as a bridge between the Himalayan ranges to the west and the Chinese ranges to the east. Despite their complete separation, the two former areas possess closely related floras.
There has been something of a volt face as regards the Himalayan and Chinese floras. Formerly they were believed to be almost iden- tical. Exploration in western China showed that this was not the case, and lately the tendency has been to exaggerate the differences, which after all turn largely on our conception of species. The North East Frontier belt is the link between the two, and their relationship will be better appreciated as the flora of the former is investigated.
The three areas, Himalaya, North East Frontier and western China form a sort of letter Y, the three limbs of which, each composed of parallel ranges, are separated from one another, but connected independently with the desert plateaux of central Asia ; and it will probably be found that while the easternmost range of the N.E. Frontier belt shows a closer relationship with the western China limb, the westernmost range shows a closer relationship with the Himalayan limb.
If our conception of a Sino-Himalayan range, subsequently breached, is correct, then clearly a Sino-Himalayan flora reached from Nepal to Sheuri before it appeared on the N.E. Frontier ranges to the South. The subsequent uplift of these ranges, breaching the Sino-Himalayan range, would account for the presence of both Himalayan and north-west China plants so far south, derived from the broken ends of the main range.
To sum up, the N. E. Frontier belt, and more especially the Mekong-Salween divide, is primarily a barrier, botanical and zoo- logical, marking the eastern limit of the Indo-Malayan, or Oriental region, for at least 750 miles.
Secondarily it is, or has been, connected in the north with the Himalayan ranges on the one hand, and with the great China divide on the other, serving both to keep them apart and to link them up to a common centre.