Alnus (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Alnus Miller

Protologue: Gard. Dict. abr. ed. 4: 51 (1754).

Family: Betulaceae

Chromosome number: A. japonica: 2n= 42, A. nepalensis: 2n= 56.

Major species and synonyms

• Alnus japonica (Thunb.) Steud., Nomencl. bot. ed. 2, 1: 55 (1840), synonyms: A. oblongata Willd. ex Regel (1861), A. formosana (Burkill) Makino (1912), A. maritima (Marsh.) Nuttall (1842).

• Alnus nepalensis D. Don, Prodr. fl. Nepal.: 58 (1825).

Vernacular names

• General name: alder (En).

• A. japonica : Japanese alder (En).

• A. nepalensis : Indian, Nepal or Nepalese alder (En). Burma (Myanmar): maibau.

Origin and geographic distribution

Alnus consists of about 20(-35) species and is distributed mainly in temperate and subtropical areas of the Old World. A. japonica is a native of Taiwan, Japan and North-East Asia (China, Korea, Siberia) and is grown in the Philippines. A. nepalensis occurs naturally throughout the Himalayas from Pakistan through Nepal, northern India, Bhutan and Upper Burma (Myanmar) to southwest China and Indo-China. It has been introduced into South-East Asia, with particular success in the Philippines. Trial plantings of both species have been made in Malaysia and West Java. Plantations of A. nepalensis exist in tropical Africa, Costa Rica and Hawaii.

Uses

As many other Alnus spp., A. japonica and A. nepalensis are important sources of firewood. They are useful trees in agroforestry, planted to improve the stability of slopes liable to erosion and land-slides and for mine reclamation. Being nitrogen-fixing trees they can improve degraded lands. In Burma (Myanmar) A. nepalensis has been used effectively to reforest abandoned fields. In north-eastern India, Sikkim and Nepal A. nepalensis is interplanted with annual crops and used as a shade tree for greater cardamom (Elettaria subulatum Roxb.) and for Cinchona officinalis L. A. japonica is planted for shade in coffee and as a nurse tree in Pinus kesiya Royle ex Gordon plantations in the Philippines, where it has also been planted for reforestation. It is also grown as living posts supporting a network of wires for chayote (Sechium edule (Jacq.) Swartz), a fruit vegetable. Leaves are used as animal bedding. In the Philippines A. japonica has been found to be suitable as bed logs for shiitake mushroom (Cortinellus shiitake) cultivation, and is also grown as an ornamental.

A. nepalensis is pollarded for poles and its wood is used for boxes, match splints and for general carpentry, furniture parts, turnery, as well as for newsprint pulp and the production of charcoal. It is suitable as core material for plywood. The wood of A. japonica is suitable for making furniture, tools and packaging, and for the production of charcoal for gunpowder.

The bark of A. nepalensis has been used occasionally for tanning and dyeing. Its foliage is of low to moderate value as fodder for sheep and goats, but not suited for cattle.

Properties

Considerable quantities of nutrients are recycled through the litter of Alnus spp. Leaf and twig litter of A. nepalensis, grown in the eastern Himalayas and producing annually 3-6 t/ha litter, contains per 100 g dry matter: N 3.4-3.7 g, P 0.08-0.10 g, K 0.6-0.7 g, Ca 0.2 g.

The wood of A. nepalensis is moderately soft and lightweight with a density of 320-590 kg/m³. Its energy value is low (18 230-20 480 kJ/kg), but, like that of other alders, the wood dries rapidly and burns easily. The wood is pale brown or superficially bronze-coloured, with low lustre. Grain is variable, texture medium to fine. Although not among the best construction timbers, A. nepalensis seasons without excessive warping or splitting, but shrinkage figures are high. It is easy to saw and finish by hand or machine, with only slight blunting effects on tools. Planing and boring give good results while mortising and turning results are only fair with some picking up of grain. The wood preserves fairly well, but is non-durable in exposed conditions and is susceptible to discolouration by oxidation and fungal sap staining. The wood of A. japonica is largely similar to that of A. nepalensis; it shows slight discolouration by sunlight.

In the Philippines, kraft pulping of wood of an Alnus sp. showed a pulp yield of 47.6%. Bleaching improved the brightness to 76%. The pulp was suitable for the manufacture of good quality paper.

Seeds are very small: 1000 seeds of A. nepalensis weigh 0.28-0.43 g, seed of A. japonica is somewhat larger. Seed weight is sometimes given for seed including chaff, 1000 seeds weighing 8 g for A. japonica.

Description

• Monoecious shrubs and trees with a dense crown; bark generally grey and smooth; twigs with a 3-angled pith and stalked, perular buds.
• Leaves simple, alternate, in 3 rows, mostly with domatia in the vein axils and often glandular-lepidote below; stipules early caducous.
• Flowers in unisexual catkins.
• Male inflorescence a many-flowered pendulous catkin; flowers arranged in groups of 3 (triads) in the axil of a bract; flower with 4 perianth segments mostly connate at base; stamens 4, epipetalous, with short filaments.
• Female inflorescence a short, upright catkin; flowers in groups of 2 (diads) sustained by a bract concrescent with 4 bracteoles, without a perianth; styles 2 with stigmatose tip.
• Fruiting catkin cone-like, woody, with 5-lobed scales and minute 2-winged nutlets.
• Fruit a small nut, compressed, 1-seeded, crowned by the styles.
• Seed without endosperm.

A. japonica

• A deciduous or evergreen shrub or small tree, 3-10(-20) m tall; twig ends rather sharply triangular, glabrous or subglabrous.
• Leaf blade ovate-oblong to elliptical-oblong, 6-9.5(-13) cm × 2.7-5 cm, dentate, distinctly acuminate, base broadly or obtusely cuneate or subrotundate, with 6-7 pairs of lateral veins; petiole slender, 1-3 cm long.
• Male catkin 3-5 cm × 3-5 mm.
• Female catkins arranged in a terminal raceme on short shoots; catkin 1.5-2.5 cm × 1 cm; peduncle 0.5 cm long.
• Nut obovate-orbicular, not emarginate, about 3 mm in diameter including the wings.

A. nepalensis

• A deciduous or semideciduous tree, 8-15(-33) m tall, trunk straight, up to 80(-200) cm in diameter; twigs ribbed, glabrescent; bark thick, dark green or grey to silver-grey, often with yellowish patches and short, raised lenticels.
• Leaf blade ovate to oblong, 6-21 cm × 4-10 cm, shallowly crenate to subentire, acute to shortly acuminate, rounded or cuneate at the base, with 12-16 pairs of lateral veins; petiole strong, 1.5-2 cm long.
• Male catkins grouped in a terminal panicle up to 16 cm long; catkin 10-16(-25) cm long, yellow.
• Female inflorescences grouped in a short, axillary raceme of 3-8 catkins; catkin 1.0-1.7 cm × 0.6-0.7 cm; peduncle 3-6 mm long.
• Nut obtrapezoid, emarginate, 2 mm in diameter including the wings.

Growth and development

Both Alnus species develop an extensive lateral root system and are fast growing. Diameter growth of A. japonica is faster in open areas than in shade, while height growth is faster in shade. For A. nepalensis a mean annual diameter increment of 2 cm is not rare and an annual increment of 2.7 m in height and 2.9 cm in diameter have been recorded in Nepal. Exceptionally high annual increment figures of 4 m in height and 5 cm in diameter have been reported for A. japonica in the Philippines. Growth rates vary considerably, particularly in response to soil moisture and altitude.

A. japonica is shade tolerant and tends to retain its lower branches. While it is deciduous in Japan, it seems evergreen in the Philippines. Under flooded conditions A. japonica retains its leaves and can almost maintain its growth rate by forming adventitious roots with abundant aerenchyma.

A. nepalensis and A. japonica form a symbiosis with N-fixing actinomycetes of the genus Frankia.

In its natural habitat A. japonica flowers and fruits from April to November, A. nepalensis from November to March, depending on geographical locality.

Other botanical information

A. japonica is sometimes considered to be different from the American A. maritima (Marsh.) Nuttall. A. maritima would have leaves that are more elliptical to obovate. A. japonica specimens from Taiwan have been accommodated in a separate variety ( A. japonica (Thunb.) Steud. var. formosana (Burkill) Callier or A. maritima (Marsh.) Nuttall var. formosana Burkill) or even a distinct species (A. formosana (Burkill) Makino), but at present they are not considered different from A. japonica. The tropical American A. acuminata O. Kuntze (synonym A. jorullensis Kunth) is occasionally tested as an alternative to A. japonica and A. nepalensis. It grows under comparable conditions. Pollination and seed dispersal of Alnus spp. are by wind.

Ecology

As pioneers, A. japonica and A. nepalensis grow well in full sunlight although shade is tolerated. A. nepalensis is found naturally in moist, cool, subtropical mountain monsoon climates, with a mean annual rainfall of 800-2500 mm and a dry season of 4-8 months. It occurs naturally at altitudes of 1000-1800(-3000) m, but it has been planted down to 300 m. Mean annual temperatures range from 13-26 °C. Alnus spp., including the 2 species discussed, occur mainly in wet soils along streams and in swamps and also on exposed soils. A. nepalensis prefers moist and well-drained soils, varying from loam and loamy sand to gravel, sand and clay. It can withstand some imperfect drainage but does not tolerate prolonged periods of waterlogging. It grows poorly on dry, exposed ridge-tops. A. japonica occurs naturally in marsh or swamp forest in Japan with a generally high water table, and soil conditions tending to be anaerobic with high clay and organic matter contents. A. japonica does not require very fertile soil, but prefers permeable soils and should not be planted in compact soils.

Propagation and planting

A. nepalensis is readily grown from seed, but may also be propagated vegetatively by tissue culture. Seed will retain its viability for at least a year if properly dried and stored in sealed containers. Likewise, A. japonica seeds retain their viability for 3-6 months. Fruits are collected from the trees and seed is released when fruits are left to dry in the sun. No pretreatment is needed. The fine seeds are broadcast in beds. Germination starts 1-2 weeks after sowing and is completed 2 weeks later. Transplanting seedlings into containers can begin 4-5 weeks after germination. Below 1200 m elevation seedlings reach a planting size of 25-35 cm in 4-5 months, but at higher altitudes they may take as long as 11 months. Young seedlings are liable to damage by ants and defoliation as a result of frost. Their survival rate is often very low.

Most planting of A. nepalensis is done with containerized seedlings, although bare-rooted seedlings have proven successful provided lifting and handling is done properly and moisture availability is high at the planting site. In the Philippines bare-rooted seedlings of A. japonica are generally used. Wildlings of A. nepalensis have been used successfully in Nepal, especially on north-facing slopes. Direct sowing is an alternative, even on exposed mineral soils. Seed must be fresh, as then it has a high germination rate. Ample quantities should be used. Good results are obtained when seed is mixed with soil from under old trees to facilitate even broadcasting and to introduce Frankia inoculum. Clonal micropropagation is feasible on a commercial basis for A. japonica, but other vegetative propagation methods have not been successful. Planting out stock of A. japonica of 30-45 cm tall is recommended for the Philippines in areas with altitudes over 600 m and a rainfall of less than 50 mm/month during 4-6 months.

A spacing of 2.5 m × 2.5 m is commonly used for plantations of A. nepalensis in Nepal, although a closer spacing is desirable for fuelwood crops. In the Philippines, A. japonica is planted at 15 m × 15 m to provide shade for coffee planted at 2 m × 2 m.

Husbandry

After coppicing, regrowth is best when felling is done during the wet season and in moist localities. Alnus spp. are highly susceptible to wind damage. Trials in East Java with A. nepalensis were not successful because of a mortality rate of 95-100%. A. japonica shade trees in coffee plantations are pruned to a height of 3-5 m and branches are used as fuelwood. It is sufficiently tolerant of shade to be planted in Pinus kesiya stands transmitting as little as 30% light. A. japonica is reported to coppice easily and to be fire-sensitive.

On fertile sites poles and fuelwood can be harvested after 5 years. Small-diameter timber can be harvested in less than 10 years.

Diseases and pests

A. nepalensis is very susceptible to attacks by defoliators (Anomala spp., Oreina spp.). Stem borers (Batocera spp. and possibly Zeuzera spp.) may also become pests. An aphid (Eutrichosiphum alnifoliae) is a pest of economic importance. Where A. japonica is introduced it suffers only very mild attacks by the sawfly (Fenusa dohrnii) compared with other Alnus spp. In Japan A. japonica is a host for Eotetranychus tiliarium, while in China both the larvae and adults of Agelastica coerulea feed on its leaves.

Yield

In northern India the yield of A. nepalensis grown in plantations for timber and ranging in age from 7-56 years was estimated. When 7 years old and with a plant density of 715 trees/ha, bole biomass was 53 m³/ha, at 17 years with 545 trees/ha bole biomass had increased to 138 m³/ha, and at 56 years with 435 trees/ha it had reached 394 m³/ha.

Breeding

Research in Nepal on A. nepalensis has shown that local provenances perform best at any given site. None of the provenances, however, showed overall superiority. Interspecific crosses with the black alder, A. rubra Bong., have been made.

Prospects

As Alnus spp. are capable of fixing atmospheric nitrogen, they have the ability to enrich soils. They are generally well suited for reforestation purposes, particularly in moist areas, and for improving soil fertility. On unstable slopes their extensive lateral root system contributes to watershed protection and erosion control. Further research into the genus seems appropriate as, to date, the full potential of these multipurpose trees in forestry and agroforestry has still to be uncovered.

Literature

• Costales, E.F., Jr. & Costales, A.B., 1985. Effects of plant combination on the protection/stabilization of mined waste areas. Sylvatrop 10: 187-201.
• Jackson, J.K., 1987. Manual of afforestation for Nepal. XII. Nepal-UK Forestry Research project, Kathmandu, Nepal. pp. 190-193.
• Neil, P.E., 1990. Alnus nepalensis: A multipurpose tree for tropical highlands. NFT Highlights, NFTA 90-06, November 1990. Nitrogen Fixing Tree Association, Waimanalo, Hawaii, United States. 2 pp.
• Peñafiel, S.R., 1985 Growth of Japanese alder (Alnus japonica Nutt.) under two methods of inoculation. Sylvatrop 10: 69-75.
• Peñafiel, S.R., Noble, B.F. & Ngales, L.P., 1982. Growth of Benguet pine (Pinus kesiya Royle ex Gordon) seedlings planted under alnus (Alnus japonica Nutt.) stand. Sylvatrop 7: 45-48.
• Ramoran, E.B. & Panot, I.A., 1981. The potentials of Alnus species - Let's harness them. Canopy International 7(12): 8-9.
• Sharma, E., 1993. Nutrient dynamics in Himalayan alder plantations. Annals of Botany 72: 329-336.
• Sharma, E. & Ambasht, R.S., 1991. Biomass, productivity and energetics in Himalayan alder plantations. Annals of Botany 67: 285-293.
• van Steenis, C.G.G.J., 1955. Betulaceae. In: van Steenis, C.G.G.J. (Editor): Flora Malesiana. Series 1, Vol. 5. Noordhoff-Kolff, Djakarta, Indonesia. pp. 207-208; Vol. 6, p. 917.
• Yamamoto, K., Sakata, T. & Terazawa, K., 1995. Growth, morphology, stem anatomy, and ethylene production in flooded Alnus japonica seedlings. IAWA (International Association of Wood Anatomists) Journal 16(1): 47-59.

Authors

P.E. Neil