Rare is the day when all the resources that a tree needs are available in abundance. And for jack pine, a far more frugal and less aggressive species than, say, sugar maple, survival depends on gleaning something from nothing – every day. Its specialty is what might count as the tree equivalent of squatting: it benefits from forest fires so hellacious that they take out all the competition, leaving an available albeit depauperate space.
In most of the northeastern U.S., the living is so easy that jack pine can’t compete with faster-growing trees. It is, instead, a tree mainly of Canada’s boreal forest, which, despite the name, has lesser outposts in north-central Maine and the northern parts of the midwestern states. The most northern of all the pines, jack pine is an important presence throughout this great sweep of forest, also called the taiga, which stretches from Newfoundland to western Alaska and covers 1.5 million square miles – about a third of the land area of Canada – and is edged by tundra to the north.
It is odd to think of fire as a major player in the boreal forest, a place usually associated with snow cover and bitter cold. But fire does indeed figure prominently, and, although years may go by without widespread fires, few boreal stands are more than 200 years old and most are half or a quarter that age – all because of stand-destroying fires. Except for its cones, jack pine burns with alacrity. This species is not fire resistant; it is fire adapted.
In the boreal forest, there are two fire seasons: before green-up in spring, and summer. Spring fires typically don’t kill plant tissues below the surface of the frozen soil, and as soon as the ground has cooled, aspens sprout from their roots and birches from their root collars. Jack pine propagation depends on seeds, which are no match for these vigorous sprouts, so it’s the more devastating summer fires that help jack pine.
That’s because of its ability to retain cones full of viable seeds for years, which is called cone serotiny. Its cones stay on the tree, year after year, and half of the seeds from 20-year-old cones will germinate. In the fire-prone north, the hard scales of jack pine’s cones are glued so tightly together by resin that they look more like hand grenades than pine cones. The resin melts only when heated to temperatures above 140°F., and the seeds within remain viable unless the cone itself ignites. But jack pine cones resist ignition, even when exposed to 800°F for five whole minutes. At 900°F, the cones last for one minute without being consumed. These time-temperature limits safely exceed what is normally experienced in a raging crown fire. That’s when jack pine employs the other trick up its sleeve: the scales of the cones open in the heat, after which the cones then look like normal pine cones, but only a few seeds fall right away. The rest, held to the cone scales by minute hairs, wait for a decent interval, long enough for the ground to cool, and only then are they released, safe from a ground-level conflagration that might destroy them.
Summer fires, following periodic August droughts, reach well into the soil, frying the duff along with the aspen roots and birch stumps, and leaving bare, mineral soil. The 20-years’-worth of cones banked on each mature jack pine can amount to two million seeds per acre, and they have no competitors. The seedlings do best on mineral soil, require full sunlight, can survive dramatic temperature changes, and grow up to 14 inches a year. By the time they are four or five years old, they will begin producing cones, which optimizes their ability to capitalize on the next forest fire.
In the southern part of jack pine’s range, where fire is not common, the heat of the sun is enough to release the seeds. Interestingly, even in the fire-prone north, a small fraction of the cones will open in the heat of the sun.
Jack pine has to contend with temperatures at the other end of the scale as well. Hardy trees use supercooling to get through the winter, but that is only effective to -40°F. Very hardy trees – jack pine, spruce, tamarack, balsam fir, paper birch, and trembling aspen – export liquids from their cells into intercellular spaces, where they freeze solid, leaving the cell nucleus and other vital components unscathed
A tree with such elegant strategies has to give up something in return and, alas, jack pine is scraggly and small in stature and outclassed by its fellow conifers: red pine, black spruce, and white spruce. Used mostly for pulp, it nonetheless is an ecological marvel.