Alnus japonica

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Local names:
English (Japanese alder), Japanese (hannoki)

Alnus japonica is a deciduous or evergreen shrub or small tree, 3-10(-20) m tall; crown dense; bark gray, smooth; twigs with a 3-angled pith and stalked, twig ends rather sharply triangular, perular glands, glabrous or subglabrous.

Leaves simple, alternate, in 3 rows, mostly with domatia in the vein axils and often glandular-lepidote below; leaf blade ovate-oblong to elliptical oblong, 6-9.5(-13) cm x 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; stipules early caducous.

Flowers in unisexual catkins. Male inflorescence a many-flowered pendulous catkin, 3-5 cm x 3-5 mm; 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 arranged in a terminal raceme on short shoots, catkin 1.5-2.5 cm x 1 cm, penducle 0.5 cm long, 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; nut obovate-orbicular, not emarginate, about 3 mm in diameter including the wings.

Ecology

The japanese alder is a native of Taiwan, Japan and North-east Asia (China, Korea, Siberia). As a pioneer, the tree grows well in full sunlight although shade is tolerated. It occurs in wet soils along streams and in swamps and also on exposed soils. In Japan, it occurs in marsh or swamp forest with a high water table and soil conditions tending to be anaerobic with high clay and organic matter contents. It does not require fertile soil, but prefers permeable soils and should not be planted in compact soils.

Native range
China, Japan, Korea, Republic of, Russian Federation, Taiwan, Province of China

Tree management

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, it retains leaves and can almost maintain its growth rate by forming adventitious roots. In the Philippines, the tree is planted at 15 m x 15 m to provide shade for coffee planted at 2 m x 2 m. The trees are pruned to a height of 3-5 m. The tree coppices easily and is sensitive to fire. On fertile sites, poles and firewood can be harvested after 5 years, small-diameter timber can be harvested in less than 10 years.

There are about 125 000 seeds/kg. Seeds retain viability for 3-6 months. Fruits are collected from the trees and seeds are released when fruits are left to dry in the sun. Treatment of seed with high temperature (90 deg C) increased germination of Alnus japonica by 88% in China. Seed storage behaviour orthodox. Hermetic air-dry storage at –20 deg C recommended.

The japanese alder is a native of Taiwan, Japan and North-east Asia (China, Korea, Siberia). As a pioneer, the tree grows well in full sunlight although shade is tolerated. It occurs in wet soils along streams and in swamps and also on exposed soils. In Japan, it occurs in marsh or swamp forest with a high water table and soil conditions tending to be anaerobic with high clay and organic matter contents. It does not require fertile soil, but prefers permeable soils and should not be planted in compact soils.

The tree is propagated by seed. 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 1 200 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. Clonal micropropagation is feasible on a commercial basis for this tree. In the Philippines, bare-rooted seedlings are generally used, planting out stock of 30-40 cm tall is recommended in areas with altitude over 600 m and rainfall less than 50 mm/month during 4-6 months.

It has been planted in reforestation for mine reclamation.

Erosion control:  The tree is planted to improve the stability on erodable slopes and landslides due to its extensive lateral root system.

The tree is an important source of firewood.

Timber:  In the Philippines, it has been found suitable as bed logs for shiitake mushroom (Cortinellus shiitake) cultivation. The wood is suitable for making furniture, tools, packaging, and production of charcoal for gunpowder.

Shade or shelter:  Alnus japonica is planted for shade in coffee plantations and as a nurse tree in Pinus kesiya plantations in the Philippines.

Nitrogen fixing:  This is a nitrogen fixing tree and has been planted to improve degraded land. It forms a symbiosis with N-fixing actinomycetes of the genus Frankia.

Ornamental:  The hannoki is planted as an ornamental.

It is planted as living posts supporting wires for chayote (Sechium edule), a fruit vegetable in the Philippines.