Clethropsis nepalensis Spach

Alnus nepalensis is a deciduous or semi-deciduous tree with a straight trunk, up to 30 m in height and 60 cm (rarely to 2 m) in diameter; twigs ribbed, glabrescent; bark dark grey, often with yellowish patches and slightly raised lenticels. Leaves alternate, elliptical, ovate to oblong, 6-21 cm long, 4-10 cm wide, entire, denticulate or sinuate, shallowly crenate to subentire, rounded or cuneate at the base, with 12-16 pairs of lateral veins, top surface dull or shiny dark green, under surface pale with dotlike, yellow-brown scales; petiole strong, 1.5-2 cm long. Narrowly cylindrical clusters of tiny flowers or catkins occur in autumn as males or females, separate on the same or different twigs; male catkins grouped in a terminal panicle up to 16 cm long; catkins yellow, 10-16 (25 max.) cm long, hanging in clusters at the end of twigs; female inflorescence grouped in a short, axillary raceme of 3-8 catkins; catkin 1-1.7 x 0.6-0.7 cm, erect, woody, occurring on branching side twigs; peduncle 3-6 mm long. Fruits, which resemble the cones of the pine family, are dark brown, upright on short stalks, elliptical, composed of many spreading, hardwood scales; seeds light brown, circular and flat with 2 broad, membranous wings, more than 2 mm across. ‘Alnus’ is the classical Latin name for alder.

The catkins are produced in September and October in India and China, and from September to November in Nepal. The new cones appear between December and March (Nepal) and the seeds ripen in December in Yunnan (China).

Prefers moist and well-drained soils, including loam and loamy sand gravel, sand and clay. It does not require high soil fertility but prefers permeable soils. Grows well on soils with a high water content but not on waterlogged soils. It does poorly on dry, exposed ridgetops.

A. nepalensis is a pioneer species; it grows well in full light but will also tolerate shade. At lower altitudes it occurs in moist sites, such as near rivers, but it will colonize rocky sites exposed by landslides or land abandoned after cultivation. It occurs naturally in both pure and mixed stands and is common in streambeds, near streams, in ravines and in drier forests. It is found naturally in moist, cool, subtropical monsoon climates with a dry season of 4-8 months; it also grows in humid, cool or subtropical mountain areas in tropical zones with high rainfall.

A. nepalensis occurs naturally throughout the Himalayas, from Pakistan through Nepal, northern India, Bhutan and upper Burma to southwest China. It has been introduced into Southeast Asia, with particular success in the Philippines. Used in afforestation programmes in upper Myanmar, it has been planted in Costa Rica, experimentally included in species trials in Burundi where it was tested for the effects on growth of inoculation with Frankia sp, in Uganda, in the highlands of Java and in agroforestry trials in the altiplano of Bolivia, where it did not survive the harsh conditions.

A. nepalensis is readily grown from seed; no seed pretreatment is required. 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. In higher altitudes, they may take as long as 11 months. Young seedlings are liable to attack by ants and may defoliate as a result of frost. Their survival rate is often very low. Most tree planting is done with containerized seedlings, although bare-rooted seedlings have proven successful provided they are lifted and handled properly and moisture availability is high at the planting site. Wildings have been used successfully in Nepal, especially on slopes facing north. Direct sowing is an alternative, even on exposed mineral soils. Seeds must be fresh, as then they have a high germination rate. Ample quantities should be used. Good results are obtained when seed is mixed with soil obtained under old trees to facilitate even broadcasting and to introduce Frankia innoculum. Regeneration by tissue culture is also possible.

A spacing of 2.5 x 2.5 m is commonly used for plantations, although closer spacing is desirable for a fuelwood crop. A. nepalensis is pollarded for posts. On good sites, poles and fuelwood can be harvested after 5 years. Small-diameter timber can be harvested in less than 10 years. Longer rotations are needed for ordinary saw timber. A. nepalensis coppices after cutting, but successful regrowth seems to depend on seasonal and locality variations, with wet season felling and moist localities being best. Trees develop an extensive lateral root system and are fast growing. A mean annual diameter increment of 2 cm is common, and annual increments of 2.7 m in height and 2.9 cm in diameter have been recorded in Nepal. Alnus species are very susceptible to wind damage.

A. nepalensis is highly vulnerable to attack by defoliators Anomala spp. and Oreina spp. The stem borers Batocera spp. and possibly Zeuzera spp. may also become pests. An aphid, Eutrichosiphum alnifoliae, is a pest of economic importance, and ants usually attack young plants.

A. nepalensis seed is collected between November and March, depending on locality, when the cone-like fruits turn yellowish-brown and begin to open, but before the seeds have been dispersed by the wind. The seeds display orthodox storage behaviour. Viability is maintained for 15 months in hermetic storage at 4-5 deg. C with 5-10% mc. There are (min. 1.6) 2.3-3.5 million seeds/kg.

Erosion control: A well-known species that gives some stability to slopes that tend to slip and erode. Its seeds have been broadcast to stabilize landslides area. Shade or shelter: A. nepalensis is interplanted with annual crops and used as a shade tree for greater Cinchona officinalis and Eletaria subulatum. Reclamation: Effectively used to reforest abandoned taungya areas because it grows as a pioneer in degraded habitats with low fertility soils. It is also planted to improve the stability of slopes liable to erosion and landslides, and for mine reclamation. Nitrogen fixing: A. nepalensis forms a symbiosis with nitrogen-fixing actinomycetes of the genus Frankia and is therefore able to improve degraded lands. Soil improver: Considerable quantities of nutrients are recycled through the litter of Alnus spp. Leaf and twig litter of A. nepalensis may produce 3-6 t/ha litter annually, containing N 3.4-3.7 g, P 0.08-0.1 g, K 0.6-0.7 g and Ca 0.2 g per 100 g dry matter. Intercropping: On terraced slopes, the species is commonly pollarded for poles and interplanted with crops like maize, barley, chili and pumpkin. The cultivation of large cardamom (Amomum subulatum) or Cinchona spp. trees in combination with A. nepalensis is a common practice in the central Himalayas.

Fodder: The foliage is of low to moderate value as fodder for sheep and goats; it is not suitable for cattle. Fuel: Wood has a low calorific value of 18230 kJ/kg. It dries easily, burns well and is an important source of firewood and charcoal. Fibre: In the Philippines, kraft pulping of wood of Alnus spp. gives a pulp yield of 47.6%, and bleaching improves the brightness to 76%. It is suitable for the manufacture of high-quality paper. Timber: Although not among the best construction timbers, A. nepalensis has an even grain, seasons fairly well, and is easy to saw and finish by hand or machine. The wood preserves fairly well but is perishable if subjected to alternately wet and dry conditions. It is also subject to discolouration by oxidation and fungal sap stain. It is suitable for boxes, splints and matches, poles, general carpentry, furniture parts, turnery and newsprint. Tannin or dyestuff: The bark of A. nepalensis has been used occasionally for tanning and dyeing.