Mountain Pine Beetle.
The photo does not even begin to truly represent the destruction. These pesky little critters have eaten their way through 1000's and 1000's of trees. After they have dined their way through the forests the trees turn yellow and then bright red. They present a huge fire hazard, a tremendous expense...about $800 per tree for mandatory removal...and drastically alter the look of the mountains. Everyone has an opinion about these little guys...natural cycle of the forests or unacceptable plight that must be tended to no matter the cost. Regardless of your position on the matter you must concede it sure looks different up there. The town of Breckenridge estimates that 80-90% of the mature trees that the beetles have a taste for will be destroyed by their wild eating binge.