Dendrocalamus strictus (Roxb.) Nees
Linnaea 9: 476 (1834).
Gramineae
2n = 72 (hexaploid), 70
Origin and geographic distribution
D. strictus is widespread and native in India, Nepal, Bangladesh, Burma (Myanmar) and Thailand. For map click: Map511.TIF. It is most common in India, especially between the Ganges and Ramganga rivers. Outside its native area it is sometimes also cultivated, often only in botanical or experimental gardens (e.g. in Sri Lanka, Indo-China, Indonesia, Malaysia, the Philippines, Puerto Rico, Cuba, United States).
Uses
The culms of D. strictus are used for many purposes, such as for building material, furniture, mats, baskets, sticks, agricultural implements, rafts and woven wares. In India it is the principal source of paperpulp. In Thailand it is an important species in the bamboo board industry. Young shoots and seeds are edible. Leaves are used as forage.
Production and international trade
D. strictus is the most common, most widespread and most universally used bamboo in India, but no accurate statistics are available on its production and trade. The total area under bamboos in India (all species) is estimated at 10 million ha, with a potential annual production of 4.5 million t. About half of the area and production is ascribed to D. strictus. Production of paperpulp (for which D. strictus is the major source) is estimated at 3-3.5 million t per year.
Properties
Fibre dimensions of the culm are on average: length 2.45 mm, diameter 14.51 µm, lumen diameter 2.88 µm, wall thickness 5.20 µm. The fibre saturation point is 20%. The moisture content of 3-4-year-old green culms of D. strictus ranges from 46-77% (top) to 78-108% (bottom). At drying from green to oven dry, tangential shrinkage is 6-13%. At 12% moisture content and for green culms the density is approximately 600-700 kg/m3 and 540-780 kg/m3 respectively, the modulus of elasticity 13,730-17,650 N/mm2 and 6000-11,790 N/mm2, the modulus of rupture 84-98 N/mm2 and 50-153 N/mm2, the compression strength parallel to grain 56-66 N/mm2 and 42-57 N/mm2, the shear strength 13-15 N/mm2. Climatic and soil conditions affect some of the physical and strength properties of D. strictus. Culms grown in dry conditions have a higher modulus of elasticity and modulus of rupture than culms grown in humid areas. The chemical composition of the culms is approximately: holocellulose 60-62%, pentosans 15-18%, lignin 24-29%, ash 1-2%; the solubility in cold water is 3-7%, in hot water 4-8%, in alcohol-benzene 1.5-2%, in 1% NaOH 17-23%, in ether 1-2%.
Per 100 g edible portion the chemical composition of young shoots is approximately: water 90 g, protein 2.9 g, fat 0.3 g, carbohydrates 2 g, fibre 1 g, ash 1 g, Ca 15 mg, Fe 0.6 mg, P 46 mg, vitamin A 133 IU, vitamin C 0.2 mg; the energy value is about 90 kJ/100 g. Seed meal contains approximately 15.3% crude protein, 1.3% ether extract, 4.3% fibre, 77.4% N-free extract, ash 1.7%, Ca 0.35%, P 0.21%. The energy value per g dry matter is about 20 kJ.
Description
Densely tufted, sympodial bamboo. Culm much curved, (6-)8-16(-20) m tall, 2.5-8(-12.5) cm in diameter, thick-walled or solid, glaucous when young, turning dull green or yellowish, glabrous; internodes 30-45 cm long; nodes somewhat swollen, basal ones often with aerial roots. Branches arising from nearly all nodes (including the lower ones), usually several at each node, unequal, the central one dominant. Culm sheath variable, lower ones shortest, 8-30 cm long, striate, glabrous or with golden-brown hairs on the back; blade erect, triangular, up to half the length of the sheath, with stiff narrow tip and hairy at both sides; ligule 2-3 mm long, toothed, not bristly; auricles small and covered with short hairs, not bristles. Young shoot brownish-green with very thick dark brown hairs and short apex. Leaf blade linear-lanceolate, up to 25 cm x 3 cm, largest in moist areas, rough and often hairy adaxially, with soft hairs abaxially; ligule short; auricles sometimes with a few slender bristles. Inflorescence a large leafless branch with dense globular heads 4-5 cm apart and 2.5 cm in diameter; spikelets 7.5-12 mm x 2.5-5 mm, usually hairy, the fertile ones intermixed with many sterile smaller ones; fertile spikelets with 2-3 perfect florets and 2 or more empty glumes. Caryopsis ovoid to subglobose, about 7.5 mm long, brown, shiny.
Growth and development
New rhizomes of seedlings first bend downwards before curving upwards to form aerial shoots, so successive shoots are not only larger than the preceding ones but arise from rhizomes deeper in the soil. Early shoots are thin, wiry and grass-like and die off sooner or later. After a stage with whippy culms, thin woody culms develop. A clump is considered mature when it starts producing full-sized culms. Under natural (forest) conditions, D. strictus seedlings require 11-13 years to form a mature clump; under artificial conditions, 6 years are required.
A mature clump continually grows in all directions, provided there are no obstructions (e.g. rocks or hard soil layers) until a balance is reached between the formation of new culms and the dying off of old culms. The ratio of new to old culms in a clump is usually 1:5. Climatic and soil conditions play a major role in culm and internode lengths. When the soil is poor, dry or hard, or when clumps suffer injury, clumps may become congested. In that case culms are packed tightly together and are often bent and twisted. This congestion can also be caused by unrestricted cutting at the periphery of a clump, by the browsing of young shoots by cattle, by harvesting young shoots for use as a vegetable, or by digging up young rhizomes for propagation.
Young shoots usually arise in the rainy season. Given sufficient rainfall, as many as 20 new culms may be produced annually from fair-sized clumps. A mature clump contains on average 20-40 culms, but up to 200 are possible. Individual culms are considered mature when they are 3 years old but they may become 7-8(-15) years old before dying. The life cycle of a D. strictus clump is most variable, ranging between 20 and 65 years.
Flowering in D. strictus is variable. Sporadic flowering, involving only a few culms in a clump, happens irregularly. The flowering culms die after flowering, but the clump does not. A possible explanation for this phenomenon might be that the clump has developed from several seedlings. Normally, a clump flowers gregariously at the end of its life. Usually, in a certain region, all or almost all culms in all clumps flower simultaneously within a period of 2-4 years and die thereafter. Fruiting is usually abundant after gregarious flowering. The phenomenon of gregarious flowering is not well understood; some authors believe that unfavourable circumstances (e.g. congestion, grazing, fire, poor soils) increase the intensity and frequency of gregarious flowering.
Other botanical information
In dry areas D. strictus is deciduous, but in humid climates or in moist environments it remains evergreen.
D. strictus is a polymorphic species, varying in habit of culms and clumps, thickness of culm wall, texture and pubescence of culm sheaths, branching habit, and size of leaves. Based on the hairiness of the lemmas, 2 varieties have been distinguished: var. strictus (var. prainiana Gamble) with lemmas almost glabrous, and var. sericeus Gamble with silky pubescent lemmas. A third named variety, var. argentea A. and C. Rivière has silvery white lines on the leaf blades and dark green and yellowish stripes on the culm sheaths.
In India, 3 habits are distinguished:
-the common type: including culms with rather thick walls (the ordinary form, medium sized), culm with thin walls (growing under favourable conditions, large), culms solid or almost so (growing under hot, dry conditions, small);
-the large type: culms very large with long, straight, smooth internodes and no side branches to a great height, clumps not congested (growing under optimum conditions);
-the dwarf type: culms very small, only exceptionally forming clumps, it is the poorest form (growing under unfavourable conditions).
Ecology
D. strictus occurs naturally in tropical and subtropical regions of South Asia. The optimum mean annual temperature is between 20-30ºC, but it can withstand extremes as low as -5ºC and as high as 45ºC. Mature plants are frost hardy but frost will kill young plants. Optimum annual rainfall is between 1000-3000 mm with 300 mm per month during the growing season. D. strictus is, however, a very drought resistant tropical bamboo, still growing rather well with 750-1000 mm rainfall per year. It prefers low relative humidity and is found from sea-level up to about 1200 m altitude, particularly on hilly ground with cooler and drier conditions. It grows on all soils with good drainage, preferring sandy loams on a stony subsoil with pH 5.5-7.5. D. strictus thrives in relatively open types of mixed deciduous forest.
Propagation and planting
D. strictus can be propagated by seed, rhizome and culm cuttings, or by tissue culture.
Propagation by seed is the commonest method for the large-scale production of propagules. Seed weight is very variable, ranging from 0.1-1.4 g. Seeds lose their viability within 2-3 months. With moisture content reduced to 8%, stored at a temperature of 3-5ºC, seeds keep their viability for at least 3 years. Germination percentage is normally 60-70%. Direct sowing in the field is possible, requiring about 1 kg seed per ha, after which thinning and weeding are necessary. Sowing in a nursery on beds is more common. A bed of 10 m x 1.5 m may produce 4000 plants when 0.2-0.4 kg seed is sown in drills about 20 cm apart and lightly covered. Seedlings are usually transplanted to the field when they are one year old. Sowing in polythene bags (23 cm x 23 cm) also gives excellent results and it economizes seed when seed is scarce. A vegetative propagation system called 'macro-proliferation of seedlings' has been successfully developed in India for large-scale propagule production. Seed is sown in polythene bags (24 cm x 18 cm) filled with fertilized soil (NPK). In 8 months 3-8 young culms develop per seedling (per bag) which are then separated into propagules, each with some rhizome and roots. On average, 6 propagules are available per bag, 5 of which are planted out in the field, leaving one for the next multiplication round following the same system. This method ensures a continuous supply of propagules.
Rhizome cuttings (or single-culm clump division) are the traditionally preferred method of propagation, but this method is not practised for large-scale propagation. Preferably, one-year-old culms with rhizome should be used.
Culm cuttings are not very successful in D. strictus when parts of a culm are used. In an experiment, whole culms (1-2 years old) were used and gave good results: 1-1.5 new plants per 3 m culm (most from the middle and top parts).
Tissue culture applied to propagate D. strictus is still in its experimental stage but results are promising.
During the early stages of development of D. strictus some shading might be beneficial or even essential, since in areas exposed to full sun natural regeneration is conspicuously absent.
Sowing and transplanting should be done at the beginning of the rainy season. Transplanting seedlings when about 30 cm tall from natural forest is very successful and cheap. Recommended planting distance in the field is 3-5 m x 3-5 m, resulting in 400-1000 clumps per ha. A well-stocked natural D. strictus stand contains on average 150-225 clumps/ha.
Husbandry
To assure a good regeneration of a natural stand or forest of D. strictus it should be protected from grazing and fire for at least 6-7 years after the gregarious flowering period.
In a plantation, regular weeding and supply of sufficient water (by rainfall or artificially) are necessary until the plants are well established. NPK application is recommended (e.g. 200 kg/ha of a 15-15-15 mixture). In India, growth of new shoots is promoted by applying green manure (leaves of bushes) before the onset of the rainy season. To obtain a higher production of culms, M-shaped thinning of the clump is recommended.
Diseases and pests
Major nursery diseases of D. strictus are damping-off (caused by Rhizoctonia solani and Fusarium spp.) and leaf blight (many causal fungi, e.g. Alternaria spp., Colletotrichum gloeosporioides, Cercospora sp., Dactylaria sp.). Major diseases of adult plants are: rhizome rot (Ganoderma lucidum), culm rot (Fusarium sp.), culm sheath rot (Glomerella cingulata) and leaf rust (Dasturella divina). Witches' broom disease is rather common in D. strictus (infected plants show excessive branches at the nodes), but does not visually harm the culms.
Major pests are defoliators (e.g. the greater bamboo leaf roller Pyrausta coclesalis), shoot and culm borers (e.g. bamboo weevils (Cyrtotrachelus spp.) and the bamboo hispine beetle (Estigmena chinensis), and sap-suckers of shoots (e.g. the aphid Oregma bambusae), stems, leaves and seeds (e.g. the pentatomid bug Ochrophara montana). Integrated pest management practices with the emphasis on cultural, biological and genetic control still have to be evolved.
Bamboo seedlings have many natural enemies (e.g. rats, squirrels, pigs, porcupines, hares, deer, goats and cattle). The major pests of felled or dried culms are the powder-post beetles Dinoderus ocellaris, D. minutus and D. brevis, and termites, which may cause immense damage. Protection can be obtained by prophylactic and preservative treatments (e.g. soaking in a 5% aqueous solution of a copper-chrome-arsenic (CCA) mixture gives good results) but safer, environmentally friendly insecticides still have to be developed.
Harvesting
Harvesting may start 3-4 years after a clump has begun to produce culms of maximum size. Only culms older than 3 years are harvestable and harvesting should never be done during the growing season. It is recommended to cut the culms lower than 30 cm above the ground level, but not below the 2nd node. Debris and cut branches should always be removed completely.
In India various felling cycles are followed with various felling intensities, each giving good results. Much depends on the size of the clump, the number of old and new culms per clump and the average diameter of the culms when determining a sustainable harvesting method and cycle. Some general rules are widely accepted: clear-felling usually kills the clump; in every clump, in addition to the young culms (younger than 1 year) some older culms should also be left standing; loosening the soil and heaping it up around the base of the clump stimulates young shoot growth; culms left standing should be evenly distributed over the clump. A felling cycle of 3-4 years leaving all new culms and double that number of old culms per clump seems suitable. After gregarious flowering, clumps need to be harvested completely as soon as possible after seeds have developed and matured.
Yield
Yield figures for D. strictus culms in India vary considerably and depend on many factors. A plantation of 400 clumps/ha may produce 3.5 t culms per ha annually. A natural D. strictus forest with 200 clumps/ha may produce 2.8 t culms per ha annually, but the yield is usually lower. In a well-managed natural stand in India, with earthing up, M-shaped thinning and application of NPK fertilizer (0.5 kg) to the clumps, annual yields of about 17 t green culms per ha could be obtained.
Handling after harvest
Air drying of harvested culms is most practical and takes about 3 months in India. Drying in hot and dry winds should be done carefully (slowly and gradually) to avoid cracking. Mature culms (3-7 years old) dry satisfactorily, immature culms (younger than 3 years) may deform or show a certain amount of collapse. On average mature culms show a wall thickness shrinkage of 11% and a diameter shrinkage of 12% on air drying. For immature culms, those figures are 25% and 23%, respectively.
Culms of D. strictus are not durable. In the tropics in the open their life time is 2-3 years (they are destroyed by powder-post beetles, termites and fungi). Various preservative treatments (e.g. soaking in acidic copper-chrome solutions or in a mixture of 1:1 creosote and fuel oil) can extend the durability satisfactorily. Non-poisonous preservation methods still have to be developed. The traditional method to submerge culms for 10-20 days in running water reduces starch and sugar content and thus makes them less attractive for, but not resistant to, insect borers.
In Thailand, to obtain a shiny surface, culms are rubbed with ash, coconut husks or rice straw. To obtain a smooth surface (not shiny) culms can be rubbed with sandpaper. To make furniture, culms are roasted above fire at 110-130ºC for 15-20 minutes 2-3 times.
Genetic resources and breeding
Germplasm collections of D. strictus are being built up in India by the National Bureau of Plant Genetic Resources, New Delhi and its stations in Trichur, Shillong and Ranchi, and by the Indian Council for Agricultural Research in the Arunachal Pradesh Centre. Due to overexploitation, the natural D. strictus forests in India are suffering and protective conservation measures, in addition to germplasm collections, are badly needed. In India, several breeding programmes exist to produce superior cultivars of D. strictus.
Prospects
The prospects for D. strictus are promising. It is one of the best studied bamboos and its introduction on a larger scale in the drier parts of South-East Asia is worthwhile. In its natural area, conservation programmes and extended germplasm collections are absolutely necessary to retain its rich variability.
P.C.M. Jansen and S. Duriyaprapan
For additional information about author(s) see Contributors or Editors.