Quantifying the influence of past wildfires on the severity and size of subsequent wildfires

Wildfire is arguably one of the most important and widespread natural disturbance agents in western U.S. forests. It has a substantial impact on ecosystem structure and function by influencing soils, nutrients, carbon budgets, wildlife habitat, and vegetation. Wildfires also influence fuel amount, type, and structure, potentially influencing the severity and size of subsequent wildfires through site- and landscape-level feedback mechanisms. Until relatively recently, the ability to quantitatively evaluate how these feedback mechanisms operate has not been feasible because of data limitations (i.e. there has not been enough wildfire). However, due to increased fire activity over the last ~25 years, there are a number of examples of wildfires “interacting” with subsequent fires, where a wildfire either burns within the perimeter of a previously burned area (i.e. it reburns) or burns up to (but not in to) a previously burned area. This recent surge in fire activity, along with increased availability of remotely sensed data, now makes it possible to evaluate how wildfires influence subsequent fire severity and size over large landscapes. Some studies have suggested that extreme weather conditions may decrease the strength of the feedback mechanisms associated with interacting fires, and consequently, evaluating the influence of weather on such relationships is increasingly important, especially given that climate change is expected to result in more extreme weather events. This dissertation is composed of three chapters. The first chapter quantifies how previous wildfire influences the severity of subsequent fires. In my second chapter, I develop and evaluate several approaches to estimate day-of-burning for each point within a fire perimeter using coarse-resolution MODIS fire detection data. Knowing the day-ofburning is essential in order to evaluate the influence of observed weather (e.g., from a nearby weather station) on observed fire-related effects, such as smoke production or the previously mentioned feedback mechanisms of fire. My third chapter evaluates the ability of wildfire to act as a fuel break by limiting the extent (i.e. size) of subsequent fire. Using the methods from Chapter Two to estimate day-of-burning, I was also able to evaluate the influence of weather in weakening the strength of this feedback