Bark beetles. They are not a new phenomenon for California's pine forests, however our ongoing drought, now in its fourth or fifth year depending on who you ask, has stressed the trees to the point that they're no longer producing much sap. This means that the trees' natural defense against the beetles the sap which drips out and envelops them after they've eaten some bark isn't as plentiful. This means that the beetles are succeeding in killing more trees, and as a result they're also reproducing in greater numbers and spreading across forests.
The upsetting but beautiful drone footage above, via Dane Christensen, shows how many trees in this one forest have been affected you can see by the brownness of their needles.
Says Christensen, "While outside Yosemite in Oakhurst, I saw the brown swaths of forest while driving. I threw up my drone and the aerial perspective revealed a haunting view of dying forest as far as the eye (or drone) could see. Tony, who lives in the forest, told me about the bark beetle epidemic and is the narrator in what I call a 'drone-umentary.'"