Lodgepole-pine cones only open when the resin holding shut the scales starts to melt. These mountain forests burn every hundred years because of the cool but dry climate and frequent lightning strikes, combusting the whole stand and consuming the overstory. The scattered alders help replenish the nitrogen gassed out by the wildfire. They do this by supporting special symbiotic bacteria in their roots that convert the nitrogen gas back into forms that the plants and trees can use. In the absence of recurring fire, the light-loving pines would naturally die out in a hundred years, and
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