Balsam Poplar and the Balm of Gilead

Issue: 
September-October 2004

HOPULUS BALSAMIFERA (also called P. candicans and P. tacamahaca) is one of Canada’s most wide ranging trees. It is characteristic of boreal forest species, growing almost to the tree line. Its southerly range includes the western mountains and the northern tier of the USA. It is the most northerly growing hardwood. Canada is also home to balsam fir (Abies balsamea) and balsam flowers (Impatiens spp. the touch-me-nots or jewel weeds (Balsaminaceae). ‘Balsamifera’ means balsam bearing, but what is balsam?

Balsam, shortened, gave rise to the world balm, and indeed, in biblical times and in Hebrew, similar words mean balm, sweet-smell and chief of oils. Various connections have been made to anointing oils, myrrh and the Balm of Gilead mentioned in the Bible – Gilead is a mountainous region of Jordan. Such plant products, from several species (none poplars), were of major ceremonial, economic and political importance. Thus the names of one of our most characteristic northern trees derive from the inter-combination of properties and traditions involving scent and medical uses, which in turn relate to its gummy resins. The latter should not to be confused with Canada balsam, a product of the balsam fir. Tacamahaca derives through Spanish from the Aztec ‘tecomahiyac’ or Nahualt ‘tecamaca’ and again relates to the scent and medicinal value of plant gum, but it is not from our poplar buds.

The medicinal properties of balsam poplar lie in the winter buds. These are black, upright and sticky. They are strongly aromatic and if chewed taste tarry and hot. It is not surprising that the buds contain and are covered with waxy resins, terpenes and phenolics with disinfectant properties. Humans and honeybees use the balsam for protection. Made into a tea, the balm relieves respiratory congestions and sore throat. Bees use it for propolis, a known antibiotic, for sealing their hives against winter and to keep out intruders.

Balsam poplar grows best in river flood plains, but colonizes many kinds of sites. It requires moist soils over most of its range, but occurs on sandy sites near the tree line, presumably where the permafrost keeps water close to the surface. Although it does not sucker and form extensive clones in the way aspen can, individual trees cling tenaciously to life. Disturbance and damage promote rooting of even quite large branches, suckering, stump sprouts and sprouting from roots. The main means of natural reproduction and site colonization is probably through seeds.

The tan-coloured seeds are small, each attached to a tuft of long, silky hairs by which they become readily airborne and may blow for many kilometres. Seeds may come down in rivers and lakes and be secondarily dispersed by water. They can germinate almost immediately and do not remain viable for more than a few weeks. Female trees can produce prodigious quantities of seeds from their mature catkins and the generic term ‘cotton wood’ describes them well.

Sexual reproduction takes place before the leaves emerge in spring. Male trees copiously produce air-borne pollen from quite conspicuous, short-lived, dangling catkins (up to 9 cm long) with many small flowers (about 3 mm long) and numerous reddish stamens. The female trees produce catkins that are about the same size, but they are retained on the trees as the seeds mature. The capsules are at first lustrous green but become dull when they split and shed seeds, early in the summer.

Yes, balsam poplar has a rapid sex life, starting at about age 8 years and relying on the wind to disperse first pollen and then seeds before the canopy is completely leafed out. Then, the seeds must germinate quickly; seedlings establish and grow rapidly, sometimes reaching 5 cm with several leaves by autumn.

Once established, balsam poplar does not hold back. It is among the fastest growing trees in Canada, up to a foot each year, especially when young. It can become over 25 m tall and attain a girth of over 3 to 4 m (DBH to about 1 to just under 2 m). The trees are short-lived, normally up to about 100 years.
The bark of balsam poplar thickens with age and becomes deeply fissured. It can be used for making fishing bobbers and for carving by artisans. It allows for some protection against fire and herbivory, but the trees are cut by beaver and browsed (even broken down) by moose. Young trees and saplings may be girdled by rodents and hares, especially in winter, but the stems usually sprout below the girdle. There are several serious insect pests that feed on the leaves and bore into the wood. Fungal infections are also common.

The ease of propagation and speed of growth make balsam poplar, and other poplars, valued timber trees. It is used for plywood, wafer board, veneer, pulp, and in construction. Hybrids of poplar are in use in plantations and for biomass conversion (removing CO2 from the air as part of atmospheric greenhouse gas reduction). It is not generally considered a horticultural tree, but its use in agroforestry and riparian rehabilitation is locally important.

— Peter Kevan, University of Guelph and Helen Murphy, University of Windsor. Photo from: Trees of Ontario, Lone Pine Publishing, 2001.

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