The Jack Pine and Fire

The jack pine tree of Canada has several qualities that seem to equip it for a life with fire.As in many conifer trees,the branches nearer the ground tend to die off as the tree grows taller,not least because they find themselves without light.In the jack pine,however, these dead branches simply fall off.If the dead branches were allowed to persist,they would provide a “l(fā)adder”for the fire from the ground to the top.The physics of fire is in many ways counterintuitive. Crucially,a hot fire that burns itself out quickly can be less damaging than one that’s somewhat cooler but lasts longer.Jack pine needles are high in resin (a flammable organic substance secreted by trees) and often low in water,especially in the droughts of spring and summer when fires are likely,and so they burn hot but quick.On much the same principle,the jack pine’s bark is flaky.It picks up surface fires but then burns swiftly and does little harm.The stringy bark of eucalyptus trees in Australia,hanging loosely from the smooth trunk beneath,is protective in much the same way.In both cases the discarded bark prevents the fire from seriously harming the tree.

On the other hand,when jack pine bark accumulates on the ground (as it does if there is a long interval between fires),surface fires—particularly in spring and summer–can be very fierce.Then most trees of all kinds are killed.But the jack pine is typically the first to spring back.For a very hot fire in the summer burns both the leaf litter on the surface and the organic material in the soil itself,leaving a bare,mineral soil behind.Jack pines germinate well in such soil and, indeed,are inhibited by leaf litter.They like bright sunlight,too,and appreciate the open space.

By their fourth or fifth year many of the young jack pines are producing their first cones-which by tree standards is markedly young.Why are they so precocious:why not focus their precious energy on more growth rather than on reproduction?Forest fires often leave a lot of fuel behind,and sometimes a second fire quickly follows the first.It seems a good idea to scatter a few seeds before the possible subsequent fire.

But it is the cones and seeds of the jack pine that are adapted most impressively and specifically to fire.The cones are hard as iron, their scales tightly bound together with what could be called a “resinous glue.”Many creatures attack cones;but only the American red squirrel will take on the jack pine cone,and even the red squirrel much prefers the easier,fleshier meat of spruce cones.The cones may persist on the trees for many years,and the seeds within them remain viable.In one study more than half the seeds from cones that were more than twenty years old were able to germinate.The cones do not open until there is a fire:it takes heat of 50C to melt the resin that locks the scales together.Then,they open like flowers.Thus the seeds are not released until fire has cleared the ground of organic matter and of rivals and has created exactly the conditions they need. The output is prodigious.After a fire in the taiga (the northernmost forest,which then gives way to tundra),the burned ground may be scattered with twelve million jack pine trees per acre.

Although the cone responds to fire,and only to fire,it is remarkably fire resistant.It has been found that the seeds inside would survive for thirty seconds even when the cone was exposed to as much as 900C.It has also been shown that the cone does not respond simply to the presence of fire,like some crude unmonitored mechanical device:rather,as it is heated,it releases resin from within.which oozes to the surface and creates a gentle,lamp-like flame around the cone which lasts for about a minute and a half.All in all,it seems that, once ignited,the cone is programmed to provide a flame for the right amount of time to open the cone.?

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?The jack pine tree of Canada has several qualities that seem to equip it for a life with fire.As in many conifer trees,the branches nearer the ground tend to die off as the tree grows taller,not least because they find themselves without light.In the jack pine,however, these dead branches simply fall off.If the dead branches were allowed to persist,they would provide a “l(fā)adder”for the fire from the ground to the top.The physics of fire is in many ways counterintuitive. Crucially,a hot fire that burns itself out quickly can be less damaging than one that’s somewhat cooler but lasts longer.Jack pine needles are high in resin (a flammable organic substance secreted by trees) and often low in water,especially in the droughts of spring and summer when fires are likely,and so they burn hot but quick.On much the same principle,the jack pine’s bark is flaky.It picks up surface fires but then burns swiftly and does little harm.The stringy bark of eucalyptus trees in Australia,hanging loosely from the smooth trunk beneath,is protective in much the same way.In both cases the discarded bark prevents the fire from seriously harming the tree.

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