According to PBS News Hour, California’s four-year drought has killed millions of trees in the Sierra Nevada mountain range and has scientists worrying about the region’s giant sequoias. The famous thousand-year-old trees require more water than any other species on Earth, but warm conditions during the past two winters have reduced the snowpack that provides their main source of water. Sequoias pull water from the soil all the way up to the tops of the trees, according to University of California, Berkeley researcher Wendy Baxter. Examining how much tension the water is under as it travels upwards and into each leaf tells researchers the level of stress each tree is experiencing. The greater the pressure required to push the water out, the more stressed the tree becomes.