It's not that hotter and drier summers kill trees by themselves. But when winters are mild, creatures that kill the trees don't die, and continue to feast upon them.
That appears to be at least part of what is happening to Douglas firs, particularly in Oregon. Flatheaded fir borers have killed trees by the tens or thousands, by BLM counts.
Bill Schaupp is an entomologist at the Forest Health Protection office of the U.S. Forest Service, and well-versed in tree-killing bugs.
He gives us the big picture on what's happening to the Douglas firs.
Here's a link to info about the insect.