For two weeks in August, the researchers flew a plane called the Carnegie Airborne Observatory over forests throughout the state and aimed a laser-guided sensor at the trees to measure the water content of their canopies.up to 58 million trees have suffered a water deficit classified as extremely threatening to long-term forest health. California’s years-long drought is not only having a distressing impact on humans but also on vegetation, as years of little or no rain is taking a big toll on the state’s forests, a new study revealed.