The pine beetle is native to Colorado. Its population is normally tempered by cold winters. With increasingly warmer winters brought on by global climate change, the pine beetle is left unchecked.
It seems like a similar process is playing out in California due to the native bark beetle.
Colorado seems to be doing better these days. I think the pine beetle mitigation efforts are showing some progress and I would assume that the trees left standing are maybe naturally more resistant.
I would think, although possibly wrongly, that if this was drought induced the die off would be more uniform. The pictures look similar to what I saw in Wyoming a decade ago due to pine beetles.
Both are likely related, reduced water makes the trees more susceptible to the ravages of the pests?
I spent half my childhood in South Lake Tahoe and have lived in the region most of my life. During the 80's and 90's, much of which was a period of drought for the region, beetles were a problem and being actively mitigated by the forestry service. You can go through the national forest all through the sierras and see trees that have been tagged for removal due to beetles. The problem is very severe right now and certainly the worst I can remember. I've got a couple of friends that work for the us forestry service and they say it's so bad they can't keep up with it. Other sources point out that beetle infestation is part of the problem.
Reduced water does indeed make pine trees more susceptible to beetles. Pines defend themselves by producing pitch to trap and expel the beetles, and that works better when the trees have enough water.
