Tree information

Local name: 

Sal (साल)

Genus: 

Shorea

Species: 

robusta

Author: 

Gaertn.

Family: 

Dipterocarpaceae

Large tree. Leaves glabrous, ovate-oblong, 10-20 cm by 6-12 cm; apex a blunt point, base cordate, 12-14 pans of lateral veins; stalks 2- 2.5 cm long. Flowers yellow; about 2 cm across, in large showy branched clusters. Fruit ovoid, about 8 mm in diameter, with five wings, three long and two short the longer up to 7.5 cm long.

Characteristics

A large tree, exceptionally reaching a height of 45 m. It is deciduous but only for a short time in the year, except in very dry localities. It is a light-demander. It grows on a wide range of soil types, except in the very sandy, gravelly soils immediately adjoining rivers in the Bhabar Terai zone, where it is replaced by Dalbergia sissoo and Acacia catechu, and in waterlogged areas. It grows badly on stiff clays. It has been reported to avoid limestone areas, but this appears to be due to the dry soil often formed over limestone, as it grows quite well on soil containing limestone debris. The seedlings will tolerate some frost, but not annual heavy frosts such as occur in frost hollows. However in most of the S. robusta areas of Nepal frost is rare.

Seedlings exposed to unfavourable conditions, such as frost, drought and fire, frequently dieback. In nature many die completely, but in others the root remains alive and continues to send up new shoots each year, until eventually a very strong rootstock develops which produces a shoot which continues to grow and eventually forms a tree. This process may take 3-10 years. However this annual dieback is by no means universal, and under good conditions the seedlings will produce a shoot which will continue to grow without dying back.

This capability of the seedlings and young plants to shoot after having been cut back contributes to the remarkable ability of cut-over S. robusta forest to regenerate. On land which was previously S. robusta forest and cleared for cultivation, if the cultivation is abandoned after a few years, and the site is then protected against grazing, there will often be found, within a year, very numerous young S. robusta shoots of uniform height, arising from roots which have survived in the ground. This will not happen if the land has been cultivated too long, or heavy grazing takes place. This behaviour of the seedlings causes them to be considerably resistant to fire, as they may be burnt back annually for many years and shoot from the base annually, until eventually a stem is established. Older trees are also very resistant to fire. Young seedlings are very liable to damage by browsing, and in heavily grazed areas may be completely eliminated. Older S. robusta trees coppice well in most localities, but not if the stems are more than 20-30 cm in diameter. Coppicing should be completed before the onset of the hot season.

Shorea robusta is not usually severely affected by disease, but M. Karki (1992) records complete destruction of an area of forest at Hetauda by the root rot fungus, Polyporus shoreae.

Occurrence

Shorea robusta forest is dominant in the Bhabar Terai, except in areas of very high rainfall, where it is replaced by mixed forest, and along streams. It also covers most of the Siwalik Hills, and the duns between them. Along river valleys it penetrates deep into the midlands, along the lower slopes of the hills, sometimes up to 80 km from the plains. Its maximum altitude is about 1500 m, but it is not common above 1000 m. A number of different types of Shorea robusta forest have been distinguished; Dobremez (1976) lists nine, and Champion and Seth (1968) more, many of which can be expected to be found in Nepal. However for the present purposes it is sufficient, following Stainton (1972), to distinguish the Bhabar Terai and Terai S. robusta forests from the hill S. robusta forest which includes the forest along river valleys in the midlands. In the former the trees are much larger and the species composition richer, while in the latter the trees rarely exceed 15 m in height. At its upper limits it is replaced by Pinus roxburghii or Schima-Castanopsis forests.

Natural regeneration

The fruit ripens at the beginning of the rains, usually in June, and the propellor-like action of the wings may carry the seed up to 100 m from the mother tree, more in strong winds. Provided that there is rain soon after the seed falls germination will begin almost immediately; if there is no rain the seeds will die. About one year in three is a good seed year, and one in four moderate; the rest are poor. If the seed falls on a layer of dead leaves it may germinate, but often the root will fail to penetrate through the leaves and the seedling will die. On bare soil, such as is produced by fire, germination and establishment is more satisfactory. Best development is where there is full overhead light but light side shade. Regeneration is plentiful in most S. robusta areas, provided they are protected against grazing, but much comes from coppice and growth of seedlings which have died back in previous years, as described previously.

Artificial regeneration

In India for direct sowing under taungya, strips are hoed about 30 cm wide, separated from the taungya crop by uncultivated strips also 30 cm wide. Regular weeding of the seedlings in the lines is necessary during the first and second years, and in the second and third year the seedlings need to be thinned out.

For raising nursery plants the seed should be sown directly into containers, two seeds per container, in June. No shade should normally be needed, but frequent root pruning is necessary as the species develops a strong taproot. Such seedlings will be ready for planting by the next monsoon. The use of stumps is still at a trial stage. Bare-root plants have no chance of survival.

Under very favourable conditions initial growth can be quite fast, with a height of 6 m after five years from seed. This, however is exceptional. In a planted plot at Adabhar at the age of 3.5 years mean height was 1.95 m, but there were only 46 per cent survival. In natural regeneration at Sagarnath, age 33 months, mean height was 3.5 m and dbh 1.2 cm. Selected trees such as might eventually have grown to maturity averaged 4.6 m in height by 2.1.cm in diameter. There were 4575 S. robusta trees and 1750 trees of other species ha-1 (KJ. White, 1988).

In the Tinau watershed area an area containing S. robusta was protected from grazing when the trees were knee high. After three years the mean height was 2.5 m, dbh 2.2 cm, and the average green weight per tree 2.9 kg. The total tree biomass was 53 t ha-1, of which S. robusta was 97 per cent (Fonzen, 1986). For timber production in India the rotation is from 100 to 120 years. Volume tables for S. robusta have been prepared by E.R. Sharma and Pukkala (1990).

The seed ripens in June. When ripe the fruits are pale green in colour, and the wings dry and brown; they can easily be detached from the branches by lightly shaking the trees. The most fertile seed is reported to be produced about the middle of the fruiting season. Germination is hypogeal; it is rapid, and often begins when the fruits are still on the tree. There are between 450 and 1000 fruits kg-1.

The seed loses its viability very rapidly and should be sown within a week after collection. Packing in lime is reported to have produced 45 per cent germination of seed 27 days old, as compared to an initial germination percentage of 90. Drying out of the seed causes rapid loss of viability, so keeping it moist may help to prolong viability a little. In general however the seed cannot be stored.

Importance in Nepal

The natural S. robusta forests in the Terai are the country's main source of building timber, and in the valleys of the hill region are a valuable source of timber, fuelwood, and fodder. Unfortunately in both these areas the S. robusta forests are wasting assets. In the plains their area is being continuously reduced by conversion to agriculture, legally or illegally, and little is being done to ensure regeneration. In the hills lopping and excessive cutting for fuelwood have reduced much of the S. robusta forest to degraded scrub.

Rather than establishing plantations of S. robusta, the most urgent task is to rehabilitate the existing forests. In the hills especially they respond dramatically to protection for a few years, and management of such forests by regular coppicing would not be difficult, and would produce a useful yield of fuelwood and poles. Some such forests have been seen which were heavily overstocked; a thinning in such areas would both produce a yield and also improve the growth of the remaining trees.

In India, artificial regeneration of S. robusta is used primarily to supplement natural regeneration of managed forests, which is sometimes difficult to obtain. The method used is mainly direct sowing under taungya, with nursery stock used to replace failures in the sown areas. In Nepal also any artificial regeneration should be confined to areas of S. robusta forest where natural regeneration is deficient. Up to the present S. robusta has scarcely been raised at all in Nepalese nurseries.

Shorea robusta timber is the main constructional timber used in Kathmandu. It is strong and elastic, and the heartwood is very durable. It has, however, the disadvantages of being difficult to season, and to plane, as it is cross-grained. It is used for construction, doors, window frames, planking, carts and carving. At one time there was considerable export of S. robusta logs to India for railway sleepers and other purposes. This was a very old practice; Hooker, in 1848, noted that S. robusta logs were floated to Calcutta from the eastern Terai (Hooker, 1891). The timber weighs between 800 and 960 kg m-3. It is an excellent fuelwood, with an energy content of about 22,700 kJ kg-1 for the heartwood, and 21,300 kJ kg-1 for the sapwood. It also makes a good charcoal. The leaves are lopped for fodder, though they are of only medium quality. They contain about 10 per cent crude protein and the total digestible nutrients are 43 per cent. The leaves are also widely used for temporary plates. The seeds produce an oil which sets hard and white in cold weather and nowadays is extracted commercially on a considerable scale. It is used as a substitute for cocoa butter, among other purposes. After extraction of the oil the cake can be used to supplement cattle feed. From 1984 to 1986 production averaged about 430,000 l of seed oil plus 3500 t of de-oiled cake.

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