From the days of John Muir’s treks through the valleys and peaks of the Sierra Nevada, California’s majestic forests have attracted nature lovers from around the world. Since Muir’s death in 1914, though, dramatic changes have taken hold. Across the state’s forests, small trees are more common while large trees have declined in number, and oaks seem to be taking over where pines once ruled. A new study cites drought due to warming as a major driving force behind these changes.