Excerpts from “Why have all the trees been dying?” in a Lake Tahoe area paper [emphasis added]. I think this mortality will spread north in to the Cascades in Oregon and Washington.
Jonathan Cook-Fisher, District Ranger for the Tahoe National Forest in Truckee said, “It is happening from the west shore to the north. The fir trees are the first to go. There are some beetles, but the primary driver appears to be overstocked forest stands with drought conditions.”
Overstocked forests didn’t just happen in the last few years. It has been a growing (sorry couldn’t resist) problem for the past 100 years. Sierra Nevada forests (and forests throughout the west) have adapted to regular fires that were spurred by lightning storms. Trees adapted by growing rapidly and close together in an attempt to “out-grow” the fires. The lightning caused fires stayed low to the ground, burned out the brush and those swiftly growing young firs before the stands could get too thick, leaving a forest of primarily mature, bigger trees spread out around the forest. Fewer, large trees were better able to fend off pests and drought.
In a scientific study in the Journal of Ecological Applications  on the impact of tree mortality on wildfire severity entitled “Recent bark beetle outbreaks influence wildfire severity in mixed-confer forests of the Sierra Nevada,” Rebecca Wayman and Hugh Safford found:
“Our analyses identified prefire tree mortality as influential on all measures of wildfire severity… All measures of fire severity increased as prefire mortality increased…Managers of historically frequent-fire forests will benefit from utilizing this information when prioritizing fuels reduction treatments in areas of recent tree mortality, as it is the first empirical study to document a relationship between prefire mortality and subsequent wildfire severity in these systems.”