3043

11: Auxiliary plants

Senna siamea (Lamk) Irwin & Barneby

Mem. New York Bot. Garden 35: 98 (1982).

LEGUMINOSAE - CAESALPINIOIDEAE

2n = 28

Cassia siamea Lamk (1785), Cassia florida Vahl (1794), Senna sumatrana (Roxb. ex Hornem.) Roxb. (1832).

Siamese senna, kassod tree, Thailand shower (En). Indonesia: johar (general), bujuk, dulang (Sumatra). Malaysia: johor, sebusok, guah hitam. Philippines: robles. Cambodia: 'ângkanh'. Laos: 'khi:z hlek. Thailand: khilek (general), khilek-luang (northern), khilek-yai (central). Vietnam: c[aa]y mu[oof]ng den, mu[oof]ng xi[ee]m, humbo (Thuân Hai).

Senna siamea is native to South and South-East Asia, from Thailand and Burma (Myanmar) to southern India and Sri Lanka. However, it has been cultivated for so long, that its exact origin is unknown. It is widely planted throughout the tropics and is locally naturalized.

Senna siamea is grown as a shade tree along roads and in coffee and tea plantations and is often planted as an ornamental. There is increasing interest in its use as a source of mulch, especially in alley-cropping systems. In drier regions such as northern India it is planted as a wind-break and shelter-belt, while in other tropical regions it is used extensively for rehabilitation of degraded lands. It is also used to revegetate aluminium mine tailings (red mud).
In Thailand and Indo-China, young fruits and leaves are eaten as a vegetable. During preparation, the cooking liquid is replaced three times to remove toxins. The flowers and young leaves are used in curries in Sri Lanka.
Senna siamea is not widely grown for fodder, but the trees are browsed and plantations should be protected from livestock and wildlife. In Bangladesh, browsing by deer caused a 70% reduction in growth in the first year in unfenced plots. The alkaloids and other secondary plant compounds in the leaves, flowers and pods of Senna siamea are highly toxic to non-ruminants, like pigs and poultry, and these animals should be excluded from plantations. Caution should be exercised when feeding the leaves to ruminants, since little is known about the long-term effects. Senna siamea is used as a host plant for the lac insect in China, while in India it is used as a host for sandalwood (Santalum sp.), a parasitic tree producing the well-known aromatic wood.
The oil from the seeds is a minor source of vernolic and cyclopropenoid fatty acids. The dark heartwood is used for joinery, inlaying, handles, sticks and other decorative uses. The wood has also been used for posts, poles, bridges, mine poles, beams and produces excellent firewood and charcoal. Large plantations of Senna siamea were established in Ghana, Nigeria and Sierra Leone in the 1920s mainly for this purpose.
All parts of the plant can be used for tanning. In traditional medicine, the root is used to charm away intestinal worms and to prevent convulsions in children. The heartwood is said to be laxative and in Cambodia a decoction is used against scabies.

Leaves of Senna siamea contain per 100 g dry matter approximately: N 1.7—3.0 g, P 0.1—0.2 g, K 0.5—1.2(—1.9) g, Ca 1.6—2.8 g, Mg 0.2—0.25 g, Na 0.02 g, S 0.2 g, polyphenols 1.5 g, cellulose 18 g, lignin 6.6 g, and hemicellulose 22 g. Annual leaf fall of Senna siamea as high as 6.1 t/ha has been reported, providing per ha: 113 kg N, 40 kg K, 91 kg Ca, and 13 kg Mg. Per 100 g dry matter, neutral detergent fibre and acid detergent fibre contents were 42 g and 40 g, respectively, the ash content was 5.2 g, lignin 23 g. In vitro dry matter digestibility has been measured at 62—65%, which is consistent with the relatively low fibre content. The leaves and other plant parts produce many chemicals: anthraquinones, anthrones, flavones, triterpenoids and alkaloids, including cassiadimine, a chromone alkaloid. Tannin is present in the bark (9%), leaves (17%) and fruits (7%). The dense, dark coloured wood of Senna siamea makes good fuel, although it does produce some smoke when burning. The energy value of the wood is 22 400 kJ/kg, the density is 600—800 kg/m³. The wood sometimes produces a yellow powder that may cause irritation to the skin.
The weight of 1000 seeds is 25—28 g.

Tree, 6—12(—30) m tall, with spreading branches forming a dense rounded crown. Bark almost smooth, grey, young shoots ribbed. Leaves simply paripinnate, oblong-elliptical in outline, 10—35 cm long; stipules subulate, 1 mm long, very early caducous; petiole terete but with a shallow ventral groove, 1.5—3.5 cm long, glandless; rachis 4.5—25 cm long, glandless; petiolule 2—4 mm long; leaflets in 4—16 pairs, subcoriaceous, oblong to ovate-oblong, 3—8 cm x 1—2.5 cm, 2—4 times as long as wide, base unequal-sided rounded to cuneate, apex rounded to retuse or blunt, often mucronate, glossy and glabrous above, dull and rough to delicately puberulous below. Inflorescence an erect, terminal, 10—60-flowered, leafy panicle, 15—60 cm long, composed of numerous dense corymbs up to 10 cm x 5—6 cm; peduncle robust, 5—7 cm long; bracts obovate in lower half, suddenly narrowing into a linear acute top 3—6 mm long, puberulous, early caducous; bracteoles absent; pedicel 2—3.5 cm long; sepals 5, unequal, rounded-ovate, 4—9 mm long, thick, puberulous, repanding-reflexed, long persistent; petals 5, orbicular-obovate, 1—2 cm long, yellow, glabrous, standard with 1—2 mm long claw; stamens 10, 3 lower ones with 6 mm long filaments and 5 mm long anthers, 3 upper ones staminodial, 4 meridian ones with 3—4 mm long filaments and 5 mm long anthers; ovary shortly tomentellous, style 4—5 mm long, stigma punctiform. Pod flattened, 20—30-seeded, 15—30 cm x 12—16 mm, alternately bulging and depressed in the centre, rim thick, glabrescent, dull, finally dehiscent. Seed very flat ovoid, 6.5—8 mm x 6 mm, light brown, glossy; areole oblong-elliptical, 3—4.5 mm x 1 mm.

Mature seed germinates readily after scarification. The first leaves are 1-jugate. Early seedling growth can be quite slow in comparison to Gliricidia sepium (Jacq.) Kunth ex Walp. and Leucaena leucocephala (Lamk) de Wit, growing to only 29 cm in 8 weeks after planting. After this early phase, growth may be quite rapid. Trees show Scarrone's architectural model, with an indeterminate trunk bearing tiers of orthotropic branches, which branch sympodially as a result of terminal flowering. Once established, flowering is precocious and abundant throughout the year. It starts flowering and fruiting at the age of 2—3 years. Fruits remain long on the tree. Unless carefully pruned, it ages ungracefully, the crown becoming straggling and misshapen with upright and drooping branches. In many species comparison trials, Senna siamea has ranked in the top 2 or 3 with respect to biomass production in the first 2—3 years. In a test of 17 multipurpose tree species in southern Sumatra, Senna siamea gave the highest leaf yield of up to 1200 g per plant per year. It also performed better than 16 other species tested in experiments in the highlands of Uganda, reaching a height of 8 m and a root collar diameter of 20 cm in 40 months.
The root system consists of a few thick roots, growing to considerable depth and a dense mat of rootlets in the top 10—20 cm of the soil, which may reach a distance of 7 m from the stem in 1 year and eventually a distance of up to 15 m.
As with many species of the subfamily Caesalpinioideae, Senna siamea does not nodulate and does not fix nitrogen through Rhizobium symbiosis, although there is some evidence that nitrogen-fixing activity may occur in the warty, lenticellate bark. Senna siamea forms ecto-mycorrhizae.

In older literature, this species is best known as Cassia siamea. Until the beginning of the 1980s, Cassia L. was considered to be a genus with over 500 species. The large genus Cassia L. emend. Gaertner has now been subdivided into 3 genera: Cassia (trees, some filaments curved, bracteoles present, no areoles on seed), Senna Miller (herbs, shrubs or trees, all filaments straight, bracteoles absent, areoles on seed) and Chamaecrista Moench (herbs or shrubs, all filaments straight, bracteoles present, no areoles on seed). Cassia now has about 30 species, Senna and Chamaecrista comprise about equal numbers of species. A chemotaxonomic study of 28 Indian species of Senna and Cassia revealed that the terpenoid lupeol (considered a primitive character), was present only in Senna siamea, suggesting that it may be the ancestor of the modern sennas and cassias.

Senna siamea will grow in a range of climatic conditions, but is particularly suited to the lowland tropics with a monsoon climate with a mean annual rainfall of 500—2800 mm with an optimum of about 1000 mm. Under semi-arid conditions (500—700 mm), Senna siamea will grow only when its roots have access to groundwater. It requires a mean minimum temperature of 20°C, ranging from 14—28°C, and a mean maximum temperature of 31°C, ranging from 24—36°C. The maximum length of the dry period should not exceed 4—8 months. It is susceptible to cold and frost and does not do well at altitudes above 1300 m. Light requirements are high.
Senna siamea performs best on deep, well-drained, fertile soils with pH 5.5—7.5, but will grow on degraded, lateritic soils provided drainage is not impeded. It grows poorly on infertile, poorly drained podzolic soils. It is not tolerant of salinity, but is reasonably tolerant of acid soil conditions.

Senna siamea is normally propagated by seed, and plantations are often established by direct seeding. Mature seed has a hard seed-coat and scarification is required. Immersion in concentrated sulphuric acid for 10—30 minutes has been shown to be very effective, although treatment with boiling water is normally sufficient. Using the first method seed germinates for about 90% within 60 days. Germination of untreated seed is about 75% in 4—29 days. Seedlings can be raised in polythene bags in nurseries using standard techniques. Seed should be sown in full light as the slightest shade reduces germination quite considerably. The seedlings are usually transplanted when 25—40 cm tall, about 8—12 weeks after germination. Plants taller than 40 cm should be trimmed before transplanting, to prevent excessive transpiration. Planting density varies according to use. In fuelwood plantations, spacings range from 1 m x 1(—3) m. In hedges used for alley cropping or as shelter-belt, spacing between plants in the row should be 25—50 cm. Senna siamea can be propagated using tissue culture techniques, but this is not a common practice.

The relatively slow-growing seedlings are susceptible to competition from weeds during early growth. Senna siamea needs weeding for the first 1 or 2 years. Cultivation before sowing and the use of selective herbicides (such as fuazifop) helps to reduce weed competition. Alternatively, a broad spectrum herbicide such as glyphosate can be applied in a strip before planting Senna siamea to kill grasses and weeds, which then tend to act as a mulch for the young seedlings planted in the centre of the strip.
Although not N-fixing, Senna siamea has been increasingly used in alley-cropping systems, largely because of its coppicing ability and high biomass production. However, a number of studies have shown possible competition between Senna siamea and associated crops. In Togo, maize yields in rows adjacent to Senna siamea hedges were significantly reduced in the area where Senna siamea roots dominated the top 0—30 cm of soil. The very extensive root system of Senna siamea may prove a disadvantage in alley-cropping systems, unless it can be controlled by careful pruning.
The decomposition rate of the leaves is neither fast nor slow. In experiments in Nigeria and in Hawaii, mulching with Senna siamea leaves caused initial N-immobilization during 4 weeks. Thereafter, a slight increase in soil mineral nitrogen was found, reaching levels comparable to Leucaena leucocephala and Sesbania sesban (L.) Merrill. Where a mulch is used to control erosion, Senna siamea performs better than Grevillea robusta A. Cunn. ex R. Br. because its finer leaves retain water better and also better than Gliricidia sepium, which decomposes more quickly.

No serious diseases or pests have been recorded for Senna siamea, but minor damage has occurred in a number of locations. The fungus Phaeolus manihotis occasionally causes damage to the root system. In Indonesia, Ganoderma lucidum is locally a serious disease, causing wood rot on young plants. In Vietnam, the butterfly Catopsylia crocale is a serious pest, its larvae feeding on the foliage. The castor slug caterpillar Parasa lepida has been observed feeding on the leaves of Senna siamea in India, while the caterpillar Enerma blanda has caused damage to the terminal buds in plantations in Sri Lanka.

Senna siamea is very tolerant of coppicing, lopping, or pollarding. Plantations can be harvested for fuelwood every 5—7 years, although shorter rotations are often practised in favourable environments. Where mulch or leaf production is the primary aim of a plantation, the first cut may be 12—18 months after sowing, followed by 3—4 cuts per year thereafter.

Very high biomass yields can be achieved under favourable conditions. Under irrigation in Karnataka (India) Senna siamea was the highest-yielding of 13 species with 55.6 t/ha per year. Unirrigated, its yield was 18.7 t/ha per year, second to Acacia auriculiformis A. Cunn. ex Benth. In lowland Nepal it was the most productive species tested, giving a mean annual increment for oven-dry fuelwood of 10 t/ha at 2.5 years of age. Total yields of wood for timber, poles and fuelwood may reach 10—15 m³/ha per year.

Senna siamea has long been cultivated for fuelwood in many tropical countries. It was the most widely used plantation species in Africa in the 1920s and is still highly regarded as a fuelwood because it grows rapidly, coppices well and produces small-size wood, easily handled by smallholder producers. It is worth trying Senna siamea as a timber plantation tree. Recently, it has been utilized in sustainable production systems such as alley cropping, and this use will expand. Further studies are required to evaluate its potential as a fodder crop for ruminants. Its relatively high in vitro digestibility, high nutrient content and low fibre content suggest potential in this regard, if the problem of anti-nutritive secondary plant compounds can be overcome.

Akkasaeng, R., Gutteridge, R.C. & Wanapat, M., 1989. Evaluation of trees and shrubs for forage and fuelwood in northeast Thailand. International Tree Crop Journal 5: 209-220.
Blair, G.J., Panjaitan, M., Ivory, D.A., Palmer, B. & Sudjadi, M., 1988. An evaluation of tree legumes on an acid ultisol in South Sumatra, Indonesia. Journal of Agricultural Science (Cambridge) 111: 435-441.
de Wit, H.C.D., 1955. A revision of the genus Cassia in Malaysia. Webbia 11: 263-265.
Gutteridge, R.C., 1990. Agronomic evaluation of tree and shrub species in southeast Queensland. Tropical Grasslands 24: 29-34.
Ruhigwa, B.A., Gichuru, M.P. & Spencer, D.S.C., 1994. Economic analysis of cut and carry and alley cropping systems of mulch production for plantains in south-eastern Nigeria. Agroforestry Systems 26: 131-138.
Schroth, G., 1989. Competition between the roots of trees and crop plants in the agroforestry system at Kazaboua, Central Togo. Mitteilungen der Deutschen Bodenkundlichen Gesellschaft 59: 797-802.
Tian, G., Kang, B.T. & Brussaard, L., 1992. Biological effects of plant residues with contrasting chemical compositions under humid tropical conditions - decomposition and nutrient release. Soil Biology and Biochemistry 24: 1051-1060.
Yamoah, C.F., Agboola, A.A. & Wilson, G.R., 1986. Nutrient contribution and maize performance in alley cropping systems. Agroforestry Systems 4: 247-254.
Yatazawa, M., Hambali, G.G. & Uchino, F., 1983. Nitrogen fixing activity in warty lenticellate tree barks. Soil Science and Plant Nutrition 29: 285-294.

R.C. Gutteridge

Gutteridge, R.C., 1997. Senna siamea (Lamk) Irwin & Barneby. In: Faridah Hanum, I & van der Maesen, L.J.G. (Editors): Plant Resources of South-East Asia No 11: Auxiliary plants. PROSEA Foundation, Bogor, Indonesia. Database record: prota4u.org/prosea