Jack Pine

Issue: 
September-October 2003

WHEN IT COMES TO WITHSTANDING COLD, jack pine is one of the hardiest of conifers, and the hardiest of the pines. It can be thought of as being a true Canadian, even though it does occur naturally at the very southern edge of its range in the northeastern and north central USA. It prefers to grow on sandy, well drained sites, including hilly terrain, with acidic soils. The waxy needles, in twisted pairs from 2 to 4 cm long, resist freezing winds. The long and slender reddish twigs tend to shed snow so that the branches do not break. The rough bark can also withstand being blasted by blowing ice. These same characteristics presumably counter drying conditions of the hot boreal summer. Summer drought, though, produces conditions ripe for forest fires, which, if not too hot, this tree can also withstand. The long, stringy roots, once used for stitching in making birch-bark canoes and tepees, provide strong anchorage against wind. This is one tough tree!

Relationship with Fire
Forest fires are natural in the continental boreal forest, or taiga. Jack pine is remarkably adapted to the ravages of fire. The rough, scaly, thick, dark bark provides some protection, but the remarkable feature is the serotinous cones. The resinous cones, 4 to 5 cm long, with their seeds enclosed, open when exposed to heat. Mature cones are often retained on the trees for many years. The seeds, winged and 4 to 5 mm long, germinate rapidly after release in warm weather. The seedlings thrive best in open habitats. Heat is not the only mechanism for release of seeds. Squirrels, chipmunks, and other boreal animals chew into the cones to feed on the seeds. Not all are eaten. Some are dropped, some are cached and forgotten.

When it comes to withstanding cold, jack pine is one of the hardiest of conifers, and the hardiest of the pines.

Because forest fires are commonplace in the taiga, jack pine tends not to have a long life. If not killed by fire, insects or fungi (there are many pests and diseases), the trees can live for about 70 years before showing signs of senescence. Jack pine is the smallest of the Canadian pines. Overall, the trees have a rough appearance, generally do not grow more that 20 m tall. On poor sites, they are reduced to scrubby bushes. The older and lower branches do not self-prune tidily. In forests protected from fire, insect outbreaks and flooding, jack pines may live over a century. They are not a tree of "old growth" forests, often co-occurring with aspen and birch. Once jack pines die out from old age, they tend to be replaced by such trees as red or white pine (associated with classical "old growth" forests in Ontario and Eastern Canada), balsam fir and black spruce with increasing northing.

Reproduction
Sexual reproduction in pines is a prolonged process. The polleniferous cones, or male strobili, are cylindrical, about 7 mm long, yellow and produced in clusters in early to mid summer. The ovuliferous cones are somewhat smaller, ovoid, and produce only singly or in pairs. Pollen liberation can be predicted accurately by heat units and forecast of sunny, dry weather. Just a slight tap on a twig bearing polleniferous cones at the right time produces a visible cloud of pollen that floats in the air. The pollen grains actually have bladder-like wings that contribute to their buoyancy in air. Huge clouds of conifer pollen are characteristic of the taiga and may form extensive yellow scum on lakes, ponds and pools. The pollen may be carried many miles on winds, but its effectiveness in fertilization is probably highly local.

The ovuliferous strobili are aerodynamically effective at directing air-currents and air-borne pollen around the minute scales so that pollen settles at the micropylar entrance to the ovule. Remarkably, that entrance faces to the axis of the strobilus. The tortuous path the fertilizing pollen takes to the micropyle is curious enough, but the ovuliferous strobili are excellent at pollen capture with equal efficiency at all normal wind-speeds.

After the pollen has reached its destination, it takes 13 months before the ovule is fertilized. Why that long duration is not understood, after all in most plants the time taken from pollination to fertilization is measured in hours.

After fertilization, the ovuliferous strobilus grows as the seeds within develop so that seeds are mature after another year. Jack pine seeds are the smallest in the pines and disperse not much more than 30 m from the parent depending on its stature (dispersal distance = 2 tree heights).

Name Origin
The scientific name "banksiana" is in honour of the great botanical explorer, Sir Joseph Banks, but the common name "jack" seems a mystery. The tree has various other common names including grey pine, black pine and scrub pine. The word "jack" has a ring of general utility about it, from machines, tools and people’s trades. Certainly, it is a very useful timber tree, used for construction and pulping. It has also been used as a source of varnish and pitch. It the days of the early settlers, it had a reputation for malevolence, perhaps because once the forest was cleared of the trees, very often the thin, sandy, acidic, dry soils were agriculturally non-productive. Thus, clearing jack pine stands for agriculture was destined to be unsuccessful.

For birders, jack pine stands are valued as habitat for the rare and endangered Kirtland’s warbler, endemic to jack pine barrens.

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

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