California's iconic coastal redwood trees are reliant on moisture from fog, especially with California's Mediterranean climate, which means they often receive no rainfall each summer. The number of foggy days in the past 50 years in their range has declined 30 percent. In the hotter and drier parts of their range, scientists have noticed redwoods dropping needles and getting thinner due to less fog.
The oldest trees in the world, the bristlecone pine, are being attacked by bark beetles and blister rust where mountain tops have been warming.
Scientists have noticed that the world's big trees have been dying at high rates, so this is not only a problem in California. There may be some things they can do to counteract some of their threats, such as find bristlecone pines with higher resistance to blister rust, or planting sequoias in more areas, but they are still very worried. I was lucky enough to grow up with redwoods in my "backyard" and walked through redwood forest to get to class every day at the University of California, Santa Cruz, so this is certainly an issue that I am worried about, too.