THE FORESTS OF TIBET

Captain F. KINGDON WARD

To the naturalist studying the distribution of life over the land surface of the earth, deserts and high ranges of mountains have a peculiar interest, because, though dry land, they have the same effect as the ocean. They separate faunas and floras which, but for them, would mingle. Thus the Tibetan plateau, embedded in southern Asia, plays the part of an ocean, in so far as it separates floral and faunal regions; it also plays the part of a land bridge in so far as its desert areas are encircled and traversed by great ranges of mountains which act, not only as barriers, but also as carriers between the regions at either end. Tibet, therefore, in its dual role, is apt to confuse the student.

In accounting for distribution as it is to-day an important factor has to be taken into consideration, namely the time factor.¹ Obviously the distribution of plants and animals at any given time is the result of causes working over a long period. How long has this barrier, or that open route, been in existence, and what was the country like before its appearance? In examining the present distribution of animals and plants, these considerations must never be lost sight of.

 

Whereas Tibet is known to the world as a barren and wind-swept plateau, and in effect a desert, I am here concerned only with that part of it which is the exact opposite. About one-tenth of Tibet, far from being a desert, is more or less forested. This forested region is not, however, to-day part of the plateau, but consists of a bewildering maze of mountains, slashed by deep gorges; and though there is abundant evidence to show that most of it at any rate was once part of the plateau, glaciers and rivers have so ploughed it up, that it has now lost any resemblance to a plateau.

The whole area of Tibet is about 750,000 square miles, and the forest region occupies 75,000 square miles, an area as large as Great Britain, though forest is by no means continuous over this large region. The forest region, which we may call the river-gorge country, comprises the whole of south-eastern Tibet, embracing the provinces of Pemako, Pome, and Zayul, with part of Kongbo and the Tibetan districts along the Great Himalayan range east of Bhutan. Northwards and westwards the river-gorge country passes into the outer plateau; southwards into the foot-hills of the Himalaya and associated ranges; eastwards into the more or less forested gorge country of western China.

Before describing the forest region of Tibet, there are certain general considerations which require to be gone into.

The southern face of the Himalaya is thickly forested; but, after crossing the crest line of the main range, no more forest is seen. We are now on the dry plateau. From Kashmir to Bhutan this holds good. But east of the Tsangpo-Brahmaputra, where the mountains apparently change their direction, what happens to the forest belt? Where does the forest line run now? And what is its relation to the mountain ranges?

On examining any atlas of south-eastern Asia the main physical features of the country are easily made out. They are: (i) four major rivers which, rising on the plateau and converging, flow parallel to and close to one another for 200 miles; (2) the Great Himalayan range itself, ending at the transverse valley of the Tsangpo-Brahma- putra; and (3) a series of curved ranges north and east of the main Himalayan range, which appear to follow the courses of the big rivers, separating one from another. In addition to the four rivers which rise on the plateau, a fifth, the Irrawaddy, rises farther south. Of the minor rivers, the only one which concerns us here is the Lohit or Zayul Chu, which for a hundred miles or more flows through the province of Zayul.

The general accepted theory of orthodox geographers is that the Great Himalayan range, after being suddenly upraised in the high peak of Namcha Barwa, on the Assam frontier, makes a hairpin bend to the south-west, parallel to the Tsangpo-Brahmaputra, then sinks down into low ranges of hills forming the eastern boundary of the Assam valley (Patkoi and Naga hills). This hill range continues southwards down the west coast of Burma, and so through the Andaman Islands to Sumatra and Java. The rocks composing this aline- ment are of the same age as the Himalayan rocks, and must have been upraised at the same time.

Previous to this uplift, not only the Himalaya, but the whole of Tibet, and of course the Indo-Gangetic plain to the south, and most of Burma, was beneath the Tethys Sea. At that time, western China, which had been land since Jurassic or earlier times, formed the coast-line of south-eastern Asia. This land was traversed by the Altaids, a mountain system of great antiquity. In what direction the ranges trended cannot be known with certainty, as they have long since been worn down to their roots; but western China, at any rate, was probably a plateau, and the Indo-Malayan mountains trended more or less north and south. However that may be, south-eastern Asia now consists largely of a series of parallel mountain chains trending north and south, gradually curving round to the north-west at their northern ends. They seem to lap round the eastern extremity of the Great Himalayan range, following the courses of the rivers. But the complicated geology of the country between the Tsangpo gorge and the Sal ween has never been unravelled.

There is another possible explanation. Whatever the age of the rocks east of the Tsangpo gorge—and they are probably long pre— Eocene-they may have been caught up in the earth movements which caused the Himalayan uplift, given a new direction, and become welded to the new ranges, which were being upheaved from the Tethys. To speak of this reconditioned range (if it exists), whose materials are of great antiquity, as an eastward extension of the Himalaya, a comparatively recent uplift, is possibly unorthodox; but it certainly is a great convenience.

So far as geological and topographical evidence go, this eastward extension into China actually exists; but the evidence is admittedly scanty.¹ The appearance of the country is easily accounted for on this hypothesis—and it is only an hypothesis to explain certain facts. The real point is that the distribution of animals and plants in southeast Asia is what one would expect to find if the Himalaya, or any other barrier, extended eastwards from the Tsangpo gorge to the Pacific Coast. The present system of mountain alinements and river gorges would not account for this distribution, but on the contrary would make it appear haphazard. The Himalayan extension theory is simply an hypothesis which fits the facts, admittedly few. It is by no means proved. Whether geologists or geographers, in a more restricted sense, have the better right to define a mountain range, is a matter of opinion. The age of the rocks is not now the question; it is the forces which have acted upon them that concern us here. But the naturalist cannot fail to notice that the country between the Tsangpo gorge and the Yangtze has, since Pliocene times at least, acted as though it were a great barrier, comparable to and continuous with the Himalaya; and to a less extent as a carrier, or bridge, between the Himalaya and China.

This answers the questions I asked at the beginning—where does the forest line run here, and what is its relation to the mountain ranges? The country to the south and south-west is forested; northwards the forest rapidly dwindles, from the Tsangpo eastwards as far as the Mekong river, beyond which the forest line turns north-east- wards. The influence of rain-bearing winds from the south-east is now beginning to make itself felt, hence the stopping of the southwestern winds from the Indian Ocean is offset by rain from another quarter.

 

A second point of interest is the remarkable resemblance between the extreme north-western end of the Himalaya and the south-east end, as pointed out by Professor Mason, a resemblance by no means superficial, though how fundamental is not known.¹ Professor Mason was, however, seeking a parallel between the underlying structure of the two ends of the Himalaya, since he referred to Wadia's work on the Himalayan Syntaxis in the north-west. I want here to stress only the visible physiographical resemblance.

The Tsangpo-Brahmaputra river, after leaving the plateau, crashes through an immense gorge, beneath the high Himalayan peak Namcha Barwa (25,445 feet). Similarly, the Indus flows in a terrific gorge, passing half round the great peak Nanga Parbat (26,660 feet). Both rivers make knee-bends close to their respective high peaks. Opposite Namcha Barwa, north of the Tsangpo, is the peak of Gyala Peri, 23,460 feet; north of the Indus, opposite Nanga Parbat, is Rakaposhi, 25,550 feet. This latter peak is on the Haramosh 'ridge^’. According to Burrard, Gyala Peri stands on the Assam Himalaya.² The evidence is against such a conclusion, and it seems to the present writer that it stands on a ridge comparable to the Haramosh ridge, not directly connected with the Great Himalayan range; or perhaps on the so-called 'Ladakh range'. Burrard further says that there is a gap of sixty-five miles in the Ladakh range north of Ghomo Lhari, drained by the Nyang Chu. I have previously given reasons for believing that the so-called Ladakh range in longitude 90° E. is cut through by the Tsangpo, ‘just as in the west it is twice cut through by the Indus.^’ If so, that is yet another point of resemblance. Finally, the Indus is joined by the Gilgit river at the great bend, just as the Tsangpo is joined by the Nagong river (or Po- Tsangpo, as opposed to the Kongbo-Tsangpo, as the main river is called); and higher up, the Indus is joined by the parallel stream of the Shyok river, which has its counterpart in the east in the Gyamda river. Again, the Shyok has its origin in the great glaciers of the Karakoram range to the north; it is pertinent to ask whether the Gyamda river rises in a range of comparable height north of the Tsangpo. Thus in its topographical features, the western and eastern ends of the Great Himalayan range closely resemble one another, and this may, or may not, extend to an underlying likeness of structure.

 

Neither structurally nor topographically has the Great Himalayan range been traced west of the Indus; on the contrary, if geological continuity is the test of a mountain range it bends to the south-west, along the Indus. The same is true of the eastern Himalaya in Assam, though with this important difference. Even the geologists admit that the Tsangpo-Brahmaputra crosses the Himalaya, since the Naga hills lie to the south of it. The present writer, however, maintains that geological structure, which is invisible to the ordinary field geographer, is not the only criterion of a mountain range.

It may now be asked why, if the two ends of the Himalaya are so alike in outward form, the forested regions of Tibet are confined to the eastern end. Not only is there no forest in western Tibet, but that region is indeed peculiarly sterile. The main reason is, of course, that western Tibet is entirely cut off from the rain-bearing winds, which do not reach so far inland. It is surrounded by deserts and by high ranges of mountains, and is moreover twice as far from the ocean as is eastern Tibet. That would matter little, but for the fact that the monsoon on that side is less strong than it is from the Bay of Bengal.

A second reason is to be found in the fact that only one great river, the Indus, pierces the mountain ring at the western end of the Himalaya, while three, the Tsangpo-Brahmaputra, the Salween, and the Mekong, pierce it at the eastern end. Besides these three there are the broad passages of the Irrawaddy and the Dibang for the rain-bearing winds to follow. Finally it must be remembered that even in the south-east the forested parts of Tibet lie south of the plateau proper, and therefore south of the main containing wall against which the winds strike. The river gorges which lead up to the plateau merely allow a forest climate to penetrate on a very narrow front, so that we do find forest creeping in through the breach. But once behind the main containing wall of the plateau it rapidly dwindles and disappears.

Here I want to emphasize once more the fact that at the eastern end of the Himalaya the ranges act, from a biological point of view, as though they were continuous from the Tsangpo to the Yangtze, and had been cut through by a series of transverse gorges. It is to the presence of these gorges and mountains that Tibet owes its forests, south and west of the rain screen. The forest belt stretches roughly east and west between the meridians 92⁰ and 98° E. and the parallels 28° and 31⁰ N. Since parts of Tibet lie so low as 4,000 feet, we have to consider a series of belts from 4,000 to 14,000 feet, the kind of forest, whether sub-tropical or temperate, evergreen or deciduous, depending on the altitude, or more correctly, upon the rainfall and its seasonal distribution.

The forests of Sikkim are familiar to many Himalayan travellers, and may be taken as typical of the Tibetan forests; though it is not so generally known that very similar forests extend eastwards at least as far as China, through Bhutan, Assam, a part of Tibet, and Burma.¹

Between 2,000 and 6,000 feet altitude this forest is of the hill jungle, more or less evergreen, type, with few coniferous trees, and those generally scattered. Such forest prevails on the outer ranges, up to the altitude indicated. In Tibet it occurs only in the Tsangpo- Brahmaputra valley, just above the Assam frontier. The Lohit valley is clothed with similar forest, south of the Tibet frontier. Beyond that frontier, in the province of Zayul, where the climate is drier, a very different type of forest quickly replaces the hill jungle of the moister region. This drier type is composed almost entirely of Pinus Khasia; and within the Pinus zone the shadier gullies are filled, not with hill jungle, as one might perhaps have expected, but with a much more temperate type of forest, which may be called temperate rain-forest.

Even so high as from 5,000 to 6,000 feet, if the climate is sufficiently wet, the hill jungle still has a sub-tropical appearance, due largely to the presence of bamboos in variety, and of palms, especially the climbing sort.² Screw-pines (species of Pandanus), bananas, epiphytes, and large climbing plants also add to the effect. It is not, however, my intention to analyse the various types of forest in detail, but merely to draw attention to them. The lower forest belt of hill jungle is inhabited by various tribes, mostly of Tibetan origin— Abors, Mishmis, Kachins, and others; but very few real Tibetans live so low down as this. The Tibetan province of Pemako, where hill jungle is well developed, is inhabited by tribes more or less Tibetanized; but the pure Tibetan does not thrive below 10,000 feet altitude.

On the outer slope of the Himalaya, above the zone of hill jungle, between 6,000 and 9,000 feet, is the zone of temperate rain-forest. In Sikkim the forests above Gangtok, with their magnolias, oaks, maples, and rhododendrons, are sufficiently familiar nowadays to need no further description. Here the atmosphere is as damp as in the lower zone, but much lower temperatures prevail, and the temperate rain-forest is at least half deciduous. This zone also extends right across from the eastern Himalaya to the Salween. East of the Sal ween the climate is drier and colder, and the limiting line of temperate rain-forest runs south-eastwards into Yunnan.

 

Again the only part of Tibet where true temperate rain-forest occurs is in the Tsangpo valley. In Zayul, in the Lohit valley where pine forest replaces hill jungle, one might reasonably expect the next higher zone to be exceptional too. In fact, it is only the pine forest at the bottom of the valley which is abnormal. As one ascends out of the valley the atmosphere grows rapidly moister, and there is little if any difference between the temperate rain-forest of Sikkim and the temperate forests of Zayul. Here too we find magnolias, oaks, maples, hollies, and other familiar trees. The distinction, in fact, is not worth stressing, although a few species are lacking from the drier forests.

Above the temperate forest zone is a still more northern type of forest, which we may call Rhododendron-Conifer forest. There is here not the variety of trees we are accustomed to see in the temperate forests. At about 9,000 feet coniferous trees begin to predominate, till at 11,000 feet there are very few deciduous trees. Forest of this type is found in the valleys of all the eastern Tibetan rivers, but again, as far as Tibet itself is concerned, only in the valley of the Tsangpo, and that of its big tributary, the Nagong river, and in Zayul.

The last zone of Tibetan forest has no exact counterpart on the Himalayan ranges. As the climate gets rapidly drier after crossing the main range, the forest abruptly ceases. The Tibetan plateau is devoid of forest, as any one who has crossed the Tang La on the road to Gyantse knows. But at the eastern end of the Himalaya, thanks to the great transverse valleys, the change is not so abrupt. The temperate forest of the Tsangpo gorge gives place to pine forest at 9,000 feet, as the plateau is approached. The upper reaches of the Nagong valley are clothed with forests of Picea, groves of which are even found in the Salween gorge. As the valleys become higher and the country still more arid, jumper takes the place of Picea. In the Tsangpo valley, patches of juniper are found almost as far west as Lhasa; but this is not forest. Trees, chiefly elm, willow, and fruit- trees, are found in the villages in the valleys up to 12,000 or 13,000 feet; and thickets of Hippophae by the side of streams. But, again, these do not constitute forest, and it is substantially correct to say that forest is found only in the extreme south-east, in the immediate neighbourhood of the great rivers, and on the dividing ridges. If one were to draw a line through Shugden Gompa, roughly south- eastwards, and to prolong it to the north-west, then one might say that the forested parts of Tibet lay to the south-west of that line; to the north-east of it there would be no forest until the Yangtze is crossed.

It might be claimed that this country to the south-west is not Tibet at all. It has lost its plateau-like appearance, though it is obvious that much of it once formed part of the plateau. It is not for the most part inhabited by true Tibetans. Nevertheless it is politically part of Tibet. It differs from the outer plateau in degree only; the rivers are cut deeper, in wider troughs; the mountains are higher, and by contrast with the abysmal gorges look much higher. The forested slopes contrast strangely with the bare mountains of the plateau. But it is only a part of the plateau which has been more corroded, and the debris more completely removed, than elsewhere. It is a part of the plateau in the third stage of disintegration.

I have drawn attention to the forested region of Tibet because it is in many ways the most fascinating part of the country. Not much is known of the composition of the forests themselves; in the alpine regions, above the tree line, is a wealth of alpine flowers. Much remains to be learnt about the animals, birds, and insects of this region, which is pre-eminently the happy hunting-ground of the naturalist. Above all, it would be interesting to hear what the geologists have to say about this country from Namcha Barwa eastwards. There are years of patient work to be done yet before we shall be in a position to say definitely how the mountain folds run.

⇑ Top