Effect of thinning and prescribed burning on crown fire severity in ponderosa pine forests

Abstract

Fire exclusion policies have affected stand structure and wildfire hazard in north American ponderosa pine forests. Wildfires are becoming more severe in stands where trees are densely stocked with shade-tolerant understory trees. Although forest managers have been employing fuel treatment techniques to reduce wildfire hazard for decades, little scientific evidence documents the success of treatments in reducing fire severity. Our research quantitatively examined fire effects in treated and untreated stands in western United States national forests. Four ponderosa pine sites in Montana, Washington, California and Arizona were selected for study. Fuel treatments studied include: prescribed fire only, whole-tree thinning, and thinning followed by prescribed fire. On-the-ground fire effects were measured in adjacent treated and untreated forests. We developed post facto fire severity and stand structure measurement techniques to complete field data collection. We found that crown fire severity was mitigated in stands that had some type of fuel treatment compared to stands without any treatment. At all four of the sites, the fire severity and crown scorch were significantly lower at the treated sites. Results from this research indicate that fuel treatments, which remove small diameter trees, may be beneficial for reducing crown fire hazard in ponderosa pine sites.

Fire behaviour and severity in a maritime pine stand under differing fuel conditions

“…[36]. This assumption is soundly supported by theory, common sense and informal observation, which might explain the surprising paucity of studies examining the subject in a scientific context with field data, by looking at fire behaviour and severity differences between adjacent treated and untreated forest stands [31,45].…”

The influences of forest stand management on biotic and abiotic risks of damage

“…Modification of any of these fuel strata by silvicultural operations will thus have implications for fire behaviour, severity and suppression effectiveness (Peterson et al, 2005). Mature even-aged stands are usually more resistant to fire than multi-layered or young stands (Agee et al, 2000;Fernandes and Rigolot, 2007;González et al, 2007b;Omi and Martinson, 2004;Pollet and Omi, 2002), and have lower probability of being affected by fire (González et al, 2006). Thus, regeneration methods that drive the development of stands to even-aged structures (clear cuttings followed by planting or very intense shelterwood methods) are expected to reduce susceptibility to fire.…”

“…This accumulation of dead fuel can produce a temporal increase in the risk of fire if no additional treatment is considered to reduce it (Carey and Schumman, 2003). Additionally, during early stages of development, even-aged stands are characterized by abundant understorey vegetation due to canopy opening, which can lead to the accumulation of very flammable surface fuels (Gonzalez and Pukkala, 2007;Gonzalez et al, 2007a;Moreira et al, 2001;Pollet and Omi, 2002).…”

“…Similarly, thinning and pruning may produce an increase of dead surface fuels (slash) that enhance the risk of forest fires (Carey and Shumann, 2003) and decrease the moisture content through increased light and surface wind speed, affecting surface fire behaviour (Fernandes and Rigolot, 2007;Pollet and Omi, 2002). For this reason, management operations that reduce the amount of thinning residues are recommended to make thinning more effective in reducing fire risk (Peterson et al, 2005;Pollet and Omi, 2002).…”

“…One of the principles of forest fuel reduction treatments is to decrease crown density to reduce crown fire potential. As forest thinning achieves this, it can successfully decrease the potential for fire transmission between adjacent tree crowns (Agee and Skinner, 2005), reducing the probability of tree mortality (González et al, 2007b;Kalabokidis and Omi, 1998;Pollet and Omi, 2002). Shaded fuelbreaks (spatial buffers) which are thinned to reduce crown fuels and treated to reduce surface fuels are common tools to reduce fire risk at a landscape level.…”