Albizia lebbeck

Albizia lebbeck is a tree well known in the Indian subcontinent for its range of uses. Although geographically widespread, little is known about the species outside India. It appears to have potential for increasing pastoral production in extensive systems in the wet-dry tropics where the major problem is low feed quality of the basal diet, mature tropical grasses. Albizia lebbeck addresses this problem in three ways: as a feed, as a supplement and by improving grass quality.

Nomenclature

Albizia lebbeck (L.) Benth (Mimosaceae) has a variety of vernacular names including siris, koko, vagai (India), tekik (Javanese), kitoke, tarisi (Sundanese), khago, ka se (Thai), East Indian walnut and Indian siris (timber trade). A number of names are trivial (mother-in-law's tongue, rattle-pod (West Indies)) or misleading (acacia, raintree (northern Australia)). The Indian name siris is most commonly used (Anon. 1980). Use of 'albizia' as a common name should be avoided as it is often applied to Paraserianthes falcataria, a species of major importance in the wet tropics, and very different from siris. Albizia is a genus of about 100 species, very similar to Acacia but formally distinguished by the stamens being fused at the base rather than free. The genus is more restricted to the tropics than the acacias, and none of the species is phyllodinous.

Botanical Description

A medium to large tree, of multi-stemmed widely spreading habit (to 30 m diameter) when grown in the open, but capable of good log form in plantation. Height to 20 m. Bark rough, grey; inner bark reddish. Leaves bipinnate, rachis 70-90 mm, rachillae 1-5 pairs, 50-70 mm. Leaflets 3-11 pairs, oblong to elliptic-oblong, asymmetrical, 15-65 mm x 5-35 mm, glabrous, entire, initially bright green and folding at night, maturing to a duller glaucous green and fixed rachis. Fully but briefly deciduous in the dry season. Inflorescence an axillary cluster of 15-40 pedicellate flowers. Peduncle to 100 mm, pedicel 1.5-5 mm, corolla inconspicuous, free filaments numerous, 15-30 mm. Entire inflorescence, fluffy, 60 mm diameter, yellow-green with distinctive pleasant fragrance. Pod flat oblong 120-350 mm x 30-60 mm, stiff-papery when ripe, swollen over seeds, dehiscent. Seeds 3-12 per pod, brown, flattened, 7 x 1.5 mm (Figure 2.5.1).

Distribution and Ecology

Siris is indigenous to the Indian subcontinent, to those areas of southeast Asia with a marked dry season (e.g. northeast Thailand, eastern islands of Indonesia) and to the monsoon areas of northern Australia. In this latter region, it has been recorded in such formations as 'semi-deciduous mesophyll vine forest' (Kabay and Burbidge 1977). Herbarium notes often place it at the rainforest-eucalypt woodland ecotone. These indigenous populations are probably declining as seedlings cannot establish under continuous grazing by cattle. It has been distributed widely around the tropics, mainly as a shade tree, and has occasionally naturalised. It can grow well under a wide range of rainfall regimes (600-2,500 mm) yet can be seen in areas with only 400 mm. It may be established in areas of highly variable rainfall but in its natural habitat probably requires a reliable wet season. In the Himalayas it is found to 1,600 m altitude. It is found on a wide range of soil types including those that are alkaline and saline (Prinsen 1986) but not subject to waterlogging.
Siris seedlings will not tolerate frost. Reserves in the root system enable young plants to survive total defoliation from fire or grazing, but with obvious setback to growth. Growth is opportunistic when conditions are suitable but ceases for 2-3 months before leaf drop. Trees are leafless for only 4-6 weeks, with new leaf produced at the height of the dry season, followed in the tropics by a gregarious flowering. Flowers are insect-pollinated. Seed dispersal seems to occur mainly due to strong wind, when intact pods can be carried hundreds of metres. Seeds are retained in pods until they fall. Some seed passes through the intestinal tract of cattle but not of smaller ruminants.

Productivity

Comprehensive yield data have not been published. However, it is evident that the species is productive when actively growing or regenerating or as undisturbed mature trees. Under best conditions, plants can grow to 5 m in one year; however, growth in areas with under 800 mm annual rainfall is much slower.

Fuelwood plantations produce 5 m³ ha/year (Anon. 1980). Isolated mature trees produce edible dry matter at the rate of 100-120 kg/year (Lowry 1989). Leaf litter fall under plantation conditions was 5,000 kg/ha/year (Pradhan and Dayal 1981). Wayside trees in the dry tropics show a crown diameter expansion of 22.2 m/year until mature (Lowry and Lowry 1991). Stands of mature trees with triennial pollarding yielded 1,700 kg/ha/year of edible material. Hedgerow stands browsed by cattle twice a year yielded 2,500 kg/ha/year in a subtropical low rainfall area where leucaena yielded 1,500 kg/ha/year (J.H. Prinsen, unpublished data). In Puerto Rico, plantings of 2,500, 10,000, and 40,000 trees/ha had leaf dry matter yields in the first 24 months of 1,710, 2,560 and 3,670 kg/ha respectively (Parrotta 1988).

Silviculture

Seeds are freely produced and are relatively large (7,000-8,000 seeds/kg). The species is not particularly hard-seeded and a proportion of seeds germinate immediately without any treatment, but for best results a 10 s immersion in boiling water is desirable. Siris is not Rhizobium specific and naturalised. forms are nearly always capable of producing an abundance of nodules. Plants can be sown directly, container grown, or raised in a massed seedbed and planted out as bare-rooted stumps (Anon. 1970).
Establishment is of course dependent on initial provision of water and protection from grass or weed competition, but there are few published data on this (Lowry 1991). Observations at a 725 mm annual rainfall site in southeast Queensland indicate that at least 3 years are required from planting to initial utilisation by cattle (D.M. Burrows, unpublished data).

Diseases and Pests

Establishment can be affected by attack on young plants by mice or rabbits, marsupials and domestic ruminants. Leaves are largely unaffected by insects, but young leaves may be subject to heavy predation by larvae of the grass yellow butterfly (Eurema hecoba). This appears to be a very short-lived effect. The most serious pests are bark-feeding larvae of longicorn beetles. These do not affect small stems and have little effect on large stems, but complete girdling can cause dieback in stems in the diameter range 40-100 mm. There is considerable variation in susceptibility of individual trees. Trees may be more susceptible under prolonged water stress. Recently a psyllid, probably of the genus Heteropsylla, was reported as seriously affecting seedlings in India (Hegde and Relwani 1988). The infestation was controlled by two applications of Nuvacron (0.05%) but not by Malathion.