MILLIONS OF PEOPLE travel to California’s redwood forests every year to marvel at the few remaining stands of old-growth trees. Sequoia sempervirens, or the coast redwood, thrives in the damp climate along the Pacific Ocean. Some are more than 2,000 years old, and the tallest can reach 380 feet. The state is also home to a second type of giant redwood, Sequoiadendron giganteum, or giant sequoia. The world’s most massive tree, it grows in the inland Sierra Nevada mountains, sustained by melting snowpack. The oldest sequoias are 3,000 years old, and although not as tall as coast redwoods, they are wider, sometimes reaching an impressive 30 feet in diameter.