National Geographic : 2012 Dec
a tree can become huge and then keep growing. Giant sequoias are gigantic because they are very, very old. ey are so old because they have survived all the threats that could have killed them. ey're too strong to be knocked over by wind. eir heartwood and bark are infused with tannic ac- ids and other chemicals that protect against fun- gal rot. Wood-boring beetles hardly faze them. eir thick bark is ame resistant. Ground res, in fact, are good for sequoia populations, burn- ing away competitors, opening sequoia cones, allowing sequoia seedlings to get started amid the sunlight and nurturing ash. Lightning hurts the big adults but usually doesn't kill them. So they grow older and bigger across the millennia. Another factor that can end the lives of big trees, of course, is logging. Many giant sequoias fell to the ax during the late 19th and early 20th centuries. But the wood of the old giants was so brittle that trunks o en shattered when they hit the ground, and what remained had little value as lumber. It went into shingles, fence posts, grape stakes, and other scrappy products. Given the di culties of dealing with logs 20 feet thick, The giant sequoia is a snow tree, Sillett says, adapted for long winters in the Sierra. But it's a fire tree too. Thick bark protects it from burning in lightning-caused fires, which open cones and clear the understory, allowing saplings to find light and prosper. Download our iPad and Kindle Fire editions to watch a video about how these giants were photographed by Michael Nichols and his team.