The mountain pine beetle is a major problem here in much of the Colorado mountains. It's a cycle which occurs every 125-150 years or so. The beetles lay their larva in the bark of the trees eventually killing them. Lodge pole pine are prime targets and just happen to be the primary type of pine trees in our area. The natural thing next to happen would be a major forest fire and then a rebirth with lots of young seedlings popping up everywhere.
Needless to say, a fire would be catastrophic, so one of the small things that can be done is to cut down the dead trees to lessen the fire's impact and to allow light in for new trees to grow. That's a simplified version of what's going on and probably not exact but it gives you an idea. One place near and dear to many of us is our local golf course which was once named the Jewel of the Rockies when it came to golf courses. As you can see by the sate light picture (click on it to enlarge), they've had to cut down most of the trees there. Hey Mac, I can see your house from space!
So far I've been lucky at home. I spray my trees every year but that is no guarantee. Also, I plant some new ones (lodgepole, spruce and aspen) and many that we planted years ago have grown to be 8 feet tall! It really breaks your heart to see the forest turn brown but it is Mother Nature's way. So go out and plant a tree or two. It's good to plant them in some one's memory or to honor somebody you know or to even give them a name like 'Bruce the Spruce' or 'Jennifer Juniper'. Maybe I'll name the next one 'Pickle' in memory of my liver.