In a word, they’re dead.
Why? Blame the bark beetle.
Bark beetles are tiny. Often less than 4 mm in length, they’re almost impossible to see.
They reproduce in the inner bark of trees. Many species attack and kill live trees. Most often, bark beetles attack trees that are already weakened by disease, drought, smog, overcrowding, conspecific beetles, or physical damage.
Under outbreak conditions, like in Mariposa, the sheer number of bark beetles can overwhelm the tree’s defenses (sticky resin) and kill whole forests.