Altered vegetation structure from mechanical thinning treatments changed wildfire behaviour in the wildland-urban interface on the 2011 Wallow Fire, Arizona, USA

Fuel reduction treatments are designed to meet multiple management objectives, resulting in unique vegetation structures that do not conform to standard classifications and vary considerably over space and time. We evaluated how different post-treatment vegetation structures relate to patterns in wildfire severity. To reconstruct both untreated and treated pre-fire forest structure, we used post-fire stand data measured at three different fuel treatment units burned by the 2011 Wallow Fire (Arizona). We describe (1) how forest structure differs among the treatment units, both in the untreated forest and within the treated area; and (2) how those differences in forest structure explain variability in burn severity. We show that the retention of smaller trees (ladder fuels) for wildlife cover relates significantly to higher severity within one treatment unit. Further variability in within-treatment severity is explained by the severity of the wildfire in the untreated forest as the fire approached the treated area. The untreated forest structure and species composition constrain post-treatment structure and composition, which was related to within-treatment structure and post-fire composition and structure. The study design presented in this paper suggests that evaluations of fuel treatment effectiveness can move beyond simple classifications of treatment type and fire behaviour.