Dipterocarps, trees dominating the forests of tropical Asia

Yang trees along the Chiang Mai-Lamphun Road.

Yang trees along the Chiang Mai-Lamphun Road.

Teak was formerly a major component of the deciduous forests throughout Northern Thailand and Burma. However, in the second half of the 19th century, the extensive exploitation of the teak forests became a crucial factor in the political developments of the region. By the 1960s, teak had been logged out almost completely in its natural state. At present, virtually all ‘teak forests’ are plantations of fairly young trees. This all may suggest that such forests were the region’s dominant natural tree cover. Actually, they were not — they were restricted to the lowlands and moister lower hill slopes.

The trees of a family called dipterocarps (Dipterocarpaceae) were then as prominent as teak, and they still flourish in today’s forests. For the local economy, they were of great importance. Some species supplied excellent timber, while resins were extracted from others. In the north, they are a common element in evergreen and mixed deciduous forest and may be dominant in dry forests. What’s more, a number of others that don’t grow here, such as the well-known meranti (Shorea curtisii), are major components of the tropical rain forests of the Southern Thailand, Malaysia and Indonesia. Dipterocarps often dominate such woodlands in Asia, but they are completely absent from tropical Africa and America.

They are named after the two-winged (di-ptero) seeds (carps) of the representatives of the genus Dipterocarpus — which are somewhat similar to those of the maple but usually larger. However, others included in this category may have smaller, five- or six-winged seeds. Many species can grow to a respectable height of fifty metres and have a straight trunk without branches up to a height of 30 to 35 metres. Some of the largest specimens are venerated by the local inhabitants.

Best known in Chiang Mai are the trees of Dipterocarpus alatus (in Thai called yang na / ยางนา), such as the giant one next to the City Pillar Shrine in the compound of Wat Chedi Luang. They are also the majestic specimens in Saraphi district flanking Highway 106 to Lamphun.

The former was planted in 1796, when King Kawila re-established Chiang Mai after the city had been deserted for decades following endemic warfare. Its crown has suffered a lot from storms and old age during the past decade. Before, it was truly impressive and its huge branches supported many bees’ honeycombs. The trees in Saraphi, though, were not planted till early in the twentieth century — in a picture of the road in dr Neis’ travel account from 1884 they were still conspicuously absent.

Their Thai name refers to the resin (yang) that in the past was extracted from their trunk. Therefore, they are sometimes called ‘rubber trees’, but those in the rubber plantations of the South belong to the exotic (introduced) species Hevea brasiliensis (in Thai called yang para), a member of the unrelated family of Euphorbiaceae.

In the wild, Dipterocarpus alatus is rather uncommon, but the similar Dipterocarpus turbinatus (yang daeng / ยางแดง) and Dipterocarpus costatus (yang pai / ยางปาย) are widespread and may be dominant in evergreen forests at elevations from 500 to 1200 metres.

The two trees towering above the forest canopy in the lovely water colour by M.R. Smansnid Svasti on the cover of A Field Guide to Forest Trees of Northern Thailand are yang pai and yang daeng. You can see many of the former on Doi Suthep at about the same altitude as Wat Phra That Doi Suthep. Huge specimens of the latter, many of them once part of the natural forest cover, tower above the road to Pai near the village of Sop Poeng. In the past this species was extensively exploited for extracting its resin, as noted in the following description by German botanist Carl Gustav Hosséus who roamed the forests of the North from 1904-1906.

‘If one enters such a forest, the splendid Dipterocarpus stem loses a lot of its aura, because one metre above the ground, in almost all older trees, a deep, black hole has been burned and the spots coated with soot reach a few metres above this. Here the locals use their much loved robber system. Usually in the dry season they dig out the stem so that an oil reservoir is created at the bottom of this niche. Because the oil thickens through contact with the air the locals light a fire on the ground so that the no longer fluid tree-oil becomes liquid again. They collect it then in tin vats. The tree-oil collected in this manner is especially used in the interior of the country and is barely suitable for export.’

Locally this yang-oil was of great importance. It was burnt for lighting, and was the oil base for ink, paints, lacquer and varnish. Boats were caulked with the resin, and it was also applied to bamboo as a preservative. It was frequently used in traditional medicine, as well. Additionally, the oil could be used as a fuel. In one year, a tree of average size could furnish 20 – 30 litres of oil. Such tapping and burning of the trees ultimately leaves them vulnerable to insect infestation and storms, and most will end up fallen dead to the ground.

Even more common and numerous are Dipterocarpus obtusifolius (yang hiang/ยางเหียง) and Dipterocarpus tuberculatus (tueng/ตึง or yang phluang/ยางพลวง), and the related species Shorea siamensis (rang/รัง) and Shorea obtusa (teng/เต็ง) which produce five-winged, and six-winged seeds. In the North, these four species dominate the very common dry dipterocarp forests, but none of them reaches the height or acquires the stately trunk of the yang daeng or yang pai. The tueng is especially numerous and, in fact, is thought to be the most common tree species in Chiang Mai province. Because of this, as early as 1900, botanists distinguished a vegetation complex which they named ‘eng forests’ (eng being its Burmese name). As these grow on poor laterite soils, they were also called laterite forests, but nowadays they are usually classified as dry dipterocarp.

You will easily recognize the common tueng on the lower slopes of Doi Suthep after you have crossed the Huai Kaeo (Crystal Stream). The tree has a rather short trunk and characteristic gnarled and twisted main branches bearing large leaves. In the past, it was also extensively tapped for its resin. The dried leaves are still widely used for thatching roofs, while green ones were formerly a popular wrapping material in the markets. They are also still used in agriculture, for instance to protect young strawberry plants.

In the hot season, many dipterocarps drop their leaves and become completely bare for some weeks. Before shedding, the leaves of the rang in December and January turn orange and bright red, while those of the teng become yellow, contributing much to the “autumnal” colours of that season. Often fires rage through these forests, but species like the tueng and rang with their thick corky bark are very tolerant of them. In February, a dry dipterocarp forest with its bare branches and torched undergrowth is a far cry from the lush green ‘tropical rainforest’ foreigners have in mind when visiting the north. In areas with annual forest fires — mostly ignited by man — these species may therefore gradually have replaced less fire-tolerant species.

Late March and early April,  these forests burst into live. Even before the first rains the tueng is unfolding its bright green leaves while the bright red seeds are ripening. And after the first rains, grasses begin to sprout, and many plants such as gingers and the lovely dok din (a small root parasite) start flowering on the torched forest floor. Many orchids that cover the branches of the tueng and other trees of this forest type may flower as well.

Another rather common, though evergreen, dipterocarp is the takhian thong/ตะเคียนทอง (Hopea odorata) which may grow to a height of fifty metres. Large specimens, however, are these days rare because most have been logged for their excellent wood, known in the timber trade as ‘ironwood’. It was the one of choice for making the racing-boats stored in many temple compounds. These are made of solid dugout logs, and ironwood is preferred to teak as it is more flexible and less susceptible to splitting. The royal barges, made of single trunks of the takhian thong, are said to be the largest dugouts in the world. Near Wat Dusitaram in Bangkok’s Bangkok Noi district, what’s more, three such ancient, uncompleted examples are stored in the Sala Rua Boran, the longest reaching 42.10 metres with a width of 1.25 metres.

Sadet Mae Takhian Thong, the spirit who is said to live in these trees, is offered incense, flowers, sweets, and fruits by the locals. As these numphs are believed to be especially powerful, the felling of a takhian thong and the hewing of a racing-boat are traditionally prededed by elaborate placatory rituals.

Of the numerous other dipterocarps, the phayom/พะยอม (Shorea roxburghii), a large tree with much-valued timber which may reach a height of 40 metres, is also worth a mention. In the wild, it has become scarce due to logging pressure. In February, it is covered with dense clusters of creamy white flowers, a magnificent sight. The tree is considered sacred by Buddhists since it is associated with the birth of Buddha. In India it is called the sal tree. Maya Devi, the mother of Prince Siddhartha, on a journey to visit her parents, stopped to rest in a grove of these. As she stretched her hand upwards to pluck some flowers, Siddharta was born, and the sal tree showered flowers on the newborn child. It is also believed that the Lord Buddha died in a grove of them. In Chiang Mai, the sight of a number of lovely phayom trees was once a joy for everyone visiting the Phayom Market on Suthep Road. However, a few years back, they were sacrified to road widening.

Some species of the dipterocarps grow to an enormous size. Numerous villages in Thailand have their venerated yang to (‘Giant Yang Tree’), many of them in recent decades having been the topic of fierce disputes between villagers and entrepreneurs eager to fell them for their timber. Other villages still have a site with a number of large yang trees, and usually the shrine of the local tutelary spirit is located there.

In the past, many hamlets were surrounded by pa yang (yang forest) or pa tueng (tueng forest), which is reflected in many place names. Interestingly, historian Ron Renard suggests that in at least some of the pa yang names, ‘yang’ is actually an allusion to the Karen people (also called yang in Northern Thai), who may have lived there long before they emerged in the historical sources, and who paid their tribute to their Thai liege lords with produce from these forests.

A famous, giant yang tree towering in a village a dozen kilometres outside Uttaradit town, did not survive the torrential floods of 2006, but Thailand’s largest dipterocarp, a 700 year old giant krabak/กราบาก (Anisoptera costata), is still the pride of the Taksin Maharat national park in Tak province. (Elsewhere, the krabak is a rare sight in Thailand’s forests.) This colossus is fifty metres high, while at the base its trunk has a circumference of 16.10 metres, with its wrinkled buttress roots resembling the leg of a giant dinosaur. Nearer to Chiang Mai, in Lamphun’s little district town of Thung Hua Chang, a large and locally famous thakian thong soars skywards, but the tree has suffered much damage in its old age. However, the yang na in Chiang Mai’s Wat Chedi Luang will also impress visitors and give them an idea of the size these stately trees can attain in just 200 years.

©Sjon Hauser: text and pictures

To learn more about dipterocarps and to identify the dozen species occurring in the North (and some 700 other tree species), consult the excellent illustrated Field Guide to Forest Trees of Northern Thailand by Simon Gardner, Pindar Sidisunthorn and Vilaiwan Anusarnsunthorn.  It is for sale at Dokmai Garden in Hang Dong (www.dokmaigarden.co.th).