Historic fire data show that wildfires are not only getting larger, but are also becoming more intense. There are likely two primary reasons for these results. First, and best documented, is the fact that the density of trees and accumulated dead woody debris has increased due to historic grazing and fire suppression activities. This increase in woody debris and tree density has allowed recent fires to not only burn with greater intensity but also to exceed initial attack suppression response capabilities. The result has been more fires that cannot be suppressed at an early stage, more frequent evacuation of suppression crews from these areas due to the rapid build-up of dangerous wildfire conditions, and a more intensive fire that consumes a larger proportion of the stand canopy.
A second reason that fires throughout the west are getting larger and more intense is the fact that more and more fires are human caused. This is particularly true in and adjacent to wildland-urban interface areas due to increased exposure to human activity. Human caused fires also contribute to larger and more intense fires because these tend to occur during drier periods of the year when natural ignitions, generally lightning, did not occur. Naturally occurring fires are frequently associated with increased humidity and/or precipitation, generally reducing fire's rate of spread and intensity. These factors affect the ability of fire suppression crews to catch the fires early and reduce the size and intensity of fire starts.
WUI treatments have the goal of significantly reducing the possibility of future fires to exceed initial attack efforts and ensure that suppression forces can remain safely in place to suppress these fires. Reduction in the density of trees and dead woody debris adjacent to homes and powerlines is accomplished with mechanical treatments and controlled burning. These treatments aim to protect sensitive habitat, property, and, most importantly, lives in these adjoining areas.
Information from: http://www.fs.fed.us/r3/wui/