From "The Benefits of Being Burned," a chapter in Canada's Boreal Forest, by J. David Henry (Smithsonian Natural History Series).
Perhaps the jack pine's most impressive fire characteristic is cone serotiny--the ability of trees to retain cones filled with viable seeds for years. In experiments, more than half of the seeds from cones more than twenty years old were able to germinate. Only the heat of a blaze will open jack pine cones, allowing burnt-over areas to be dusted with a layer of jack pine seeds.
Jack pine cones even look unique. As hard as a piece of iron ore, with a surface reminiscent of a hand grenade, they consist of hard segments bonded tightly by a strong resinous glue. Between and behind these hard surface segments lie the compartments containing the seeds. The resinous glue seals off the compartments, protecting the seeds from rain, frost, bacterial and fungal infections, and even the gnawing teeth of most rodents. Red squirrels do persist and occasionally feed on jack pine cones, but given the choice, squirrels favor the soft, fleshy cones of black and white spruces.
Jack pine cones do not explode when exposed to a flame; rather they bloom like a flower filmed in time-lapse photography. The strong glue begins to melt when the temperature exceeds 122º°F (50º°C). But much more happens in the cone than just the melting of the bonding material. When exposed to heat, the central spine of the cone, to which all the hard surface segments are connected, curls back onto itself, and the openings of the seed compartments blossom into view.
However, the architecture of a jack pine cone has evolved even further than that; the cone also has an ability to protect the seeds from the damaging heat of fire. In the early 1960s, W R. Beaufant experimentally exposed jack pine cones to extremely intense heat and found that seeds survived when the cones were exposed to 1,650ºF (900º°C) for as long as thirty seconds. Cones held in a flame at 1,300º°F (700º°C) for as long as three minutes also showed no loss of seed viability. How is this protection accomplished? Jack pine cones' slightly corky interior provides good insulation, protecting the seeds, at least for a limited time, from damage by fire.
Although cone serotiny is an intriguing example of adaptation to fire, jack pin fis not locked into this reproductive strategy. Studies show that about 90 percent of the cones produced by jack pine in the taiga open only in response to the heat of fire. The rest open under the warmth of the Sun. Is this just inefficiency on the part of the jack pine? Or does it represent evolutionary wisdom? Many researchers believe that the 10 percent of jack pine cones that open when heated by the Sun represents an effort by jack pine to hedge its bets by using a mix of reproductive strategies. A small percentage of jack pine seeds are released every year, but the majority are banked until a major forest fire calls them forth. When there is a run on the seed bank, the results are impressive. After a typical fire in the taiga, as many as 2 million jack pine seeds per acre (5 million per ha) can be found on the surface of a burnt area.