Previous burns and topography limit and reinforce fire severity in a large wildfire

Abstract

Abstract. In fire-prone forests, self-reinforcing fire behavior may generate a mosaic of vegetation types and structures. In forests long subject to fire exclusion, such feedbacks may result in forest loss when surface and canopy fuel accumulations lead to unusually severe fires. We examined drivers of fire severity in one large (>1000 km 2 ) wildfire in the western United States, the Rim Fire in the Sierra Nevada, California, and how it was influenced by severity of 21 previous fires to examine the influences on (1) the severity of the first fire since 1984 and (2) reburn severity. The random forest machine-learning statistical model was used to predict satellite-derived fire severity classes from geospatial datasets of fire history, topographic setting, weather, and vegetation type. Topography and inferred weather were the most important variables influencing the previous burn. Previous fire severity was the most important factor influencing reburn severity, and areas tended to reburn at the same severity class as the previous burn. However, areas reburned in <15 yr burned at lower severity than expected. Previous fire severity and Rim Fire severity were higher on ridges, at intermediate elevations (~750-1250 m), and on slopes <30°, indicating a consistent effect of topography on fire severity patterns in these forests. Areas burned with low severity prescribed fires burned at low severity again in the Rim Fire, and areas with long fire-free periods burned at higher severity. This fire history effect suggests that prescribed burning was an effective management tool, leading to lower fire severity in the previous burns and the subsequent reburn. Our results show that self-reinforcing fire behavior results mainly from effects of vegetation structure and fuels on fire severity and that this behavior is mediated by topographic setting and the time since last fire.

Mixed‐severity wildfire and habitat of an old‐forest obligate

“…Prescribed fire is generally considered to be the most effective way to reduce the likelihood of high-severity fire in combination with mechanical treatments (Stephens et al 2009). The 2013 Rim Fire in the Sierra Nevada, California, USA, burned with low severity in areas previously treated with prescribed fires, suggesting that prescribed burning was an effective management tool to reduce fire severity (Harris and Taylor 2017). Many fire-prone forests will require active management to restore ecosystem function, but no single prescription will be appropriate for all areas and, in some portions of the forests, minimal maintenance may be more sustainable in the long term .…”

Deconstructing the King megafire

“…() draw similar conclusions in the context of the Rim Fire, concluding strong prefrontal surface winds aligned with terrain drove some periods of extreme fire spread. Similarly, studies have used statistical analyses to identify environmental factors related to observed degrees of severity, such as the severity of previous burns and topographic slope (Harris and Taylor ) and, for the Rim Fire, the influence of forest structure, fire history, topographic, and weather conditions (Lyderson et al. ).…”

“…Brotak and Reifsnyder (1977) identify synoptic conditions associated with large fires, chiefly strong prefrontal surface winds preceding a trough, while Peterson et al (2015a) draw similar conclusions in the context of the Rim Fire, concluding strong prefrontal surface winds aligned with terrain drove some periods of extreme fire spread. Similarly, studies have used statistical analyses to identify environmental factors related to observed degrees of severity, such as the severity of previous burns and topographic slope (Harris and Taylor 2017) and, for the Rim Fire, the influence of forest structure, fire history, topographic, and weather conditions (Lyderson et al 2014). The latter study indicated that on days characterized by strong plume activity, severity was moderate to severe regardless of forest conditions.…”

Positive severity feedback between consecutive fires in dry eucalypt forests of southern Australia

“…, van Wagtendonk et al. , Harris and Taylor ), with most only having one response fire, and occurring mostly in conifer forests. These studies have consistently found that fires reburn at the same or higher severity as the previous fire, supporting the idea of a positive feedback.…”

“…Few studies have examined the effect of the severity of previous fires on the severity of subsequent fire. The handful of studies looking at this have been in North America (Thompson et al 2007, van Wagtendonk et al 2012, Harris and Taylor 2017, with most only having one response fire, and occurring mostly in conifer forests. These studies have consistently found that fires reburn at the same or higher severity as the previous fire, supporting the idea of a positive feedback.…”