There's a theory that decades of wildfire suppression have led to a buildup of material, so that when fires occur now they are much more severe than they would otherwise have been. These hotter fires kill a lot more, and are harder to recover from. [0, 1]

> "Historically, fires operated in a way that tended to be less severe in valley bottoms and lower slopes," Taylor added. "Now, because of fire suppression, they tend to burn at a higher severity and you lose that habitat." The practice of not allowing any fires means that more leaf litter and branches—the components of forest fuel—can accumulate on the floor, while small trees that would have burned in the past can grow large enough to carry fires into the crowns of the taller trees that had escaped the impact of less severe fires in the past, putting the landscape in jeopardy. [0]

[0] http://news.psu.edu/story/270206/2013/03/26/research/suppres... [1] http://snamp.cnr.berkeley.edu/static/documents/2012/06/13/St...