Fire and Ecosystem Effects Interactions
How does fire severity relate to fuels?
The presence of live fuel was the most influential factor in predicting low-severity fire followed by year-of-fire climate. They found that as live fuel decreased, the likelihood of low-severity fire increased, i.e. a reduction in fuel reduced fire severity.
The authors found that spruce beetle infestation (< 5 years) did not correlate to fire severity regardless of burning conditions. Instead topography, pre-outbreak basal area, climate and short-term weather conditions during the fires all had a stronger influence on fire severity.
The authors found that areas that burned initially as high- or moderate-severity fire were more likely to reburn at high severity due to the increase in snags and shrub vegetation as a result of the first fire entry.
Low severity fire reduced fuel load density in the dry mixed-conifer stands.
High-severity fire was common in mixed-conifer forests, historically, however, changes in species composition and density due to fire suppression over the last century has changed how fire burns on these landscapes. Maintaining heterogeneity may provide resiliency against future climate change.
Coop et al. (2015) found that previous wildfires generally reduced the severity of a subsequent fire. This effect, however, diminished with increasing time-since-fire.
The authors found a positive feedback between the initial severity of a fire and the severity of a later reburn, especially concerning high severity forest fire. Conversely, low- or moderate-severity fires may regulate future fires to similar severities, which follows historical patterns of frequent, low-severity fires in dry coniferous forest systems.
The authors found a positive feedback between the initial severity of a fire and the severity of a later reburn when the first fire burned at high severity. They suggest that high severity fires can leave behind dense stands of fire-killed trees, potentially creating heavy fuel loads and ladder fuels when a subsequent fire strikes. Alternatively, a shift to a shrub state after high severity fire can result in a subsequent reburn of high severity fire. However, they also found that stands that are within unaltered, short-interval fire regimes tend to self-regulate the burn severity of secondary fires, and burn at the same or lower severity, suggesting that the initial fire moderated the burn severity of the second fire.
Using the Spearman’s Rank Correlation analysis, they found that tree density strongly correlated with CBI scores, so that denser stands on ponderosa pine burned at higher severities.
The authors found that snag density rapidly declined as time since fire increased as snags fell rapidly in the first few years post-fire and transitioned to other snag condition classes soon after. Course woody debris loading increased with time since fire and peaked between 6-12 years post-fire, but was still within recommended management values, until it turned into rotten wood at approximately 16 years post-fire.
The authors found that low- or moderate-severity fires generally regulated subsequent fires to similar severities, similar to historical patterns of frequent, low-severity fires in dry coniferous forest systems. Smaller burns created openings in both the understory and overstory vegetation, which adds diversity to the landscape, provides mechanisms for plant succession, and promotes differential tree establishment. In contrast to small burns, they found that large burns may create a homogeneous setting of burned areas which may not enhance species diversity following fire. Finally, the authors suggest that although typically previous fires kept the spread of subsequent fires in check most of the time, climate and weather can override fuel limitations and spread into recently burned landscapes.
Prescribed fire reduced the severity of subsequent wildfire compared to untreated areas suggesting that prescribed fire is an effective treatment to reduce fuel loads and high severity fire in dry mixed conifer ecosystems. Pre-treatment vegetation volume, heat load, and prescribed fire burn severity were all significantly related to the severity of the subsequent wildfire; however, the interactions between these variables were complex. But, higher severities in the prescribed fire were more effective at reducing fire severity in the following wildfire.
The authors also found that landscape context of the burn strongly influenced wildfire severities. Fire severity was lower further inside contiguous patches of burned area and increased with increasing distance from the center of the disturbance. The authors suggest that fire size and burn continuity may be more important in reducing subsequent fire severity than the severity of the prescribed fire.
The authors found that in ponderosa pine forests, low- and moderate- severity resource benefit fires more effectively reduced basal area and crown fire potential more so than prescribed fires. Areas burned multiple times for resource benefit maintained stand structure and fuel loads more consistent with historical forest conditions. Low severity fire in pinyon-juniper woodlands did not have an effect on forest structure and fuels, however, moderate severity fire did have beneficial effects on these ecosystems.
This article found strong relationships between topographic variables and high severity fire occurrence. Severe fire was more likely to occur on north-facing slopes at high elevations due to the interaction of biomass production and fuel accumulation which in turn influences burn severity. Without the influence of humans on this landscape, climate, topography, and vegetation interact strongly with each other to control burn severity in a region dominated by semi-arid forest, where moisture limits vegetation production.
The authors found a positive feedback between the initial severity of a fire and the severity of the reburn fire. High severity reburns usually occurred after a high severity burn. This pattern was stronger in the mesic, high-elevation forests of the Gila National Forest where the frequency of fire is longer. Conversely, low severity fires were typically followed by subsequent low severity fires, thereby maintaining the low-severity fire regime characteristics typical of these ecosystems.
At the higher elevation sites, despite long fire-free periods, fire severity was consistent with historical fire regime patterns and resulted in fuel conditions closer to the historical range of variation. Fire effects on cool, mixed-conifer ecosystems may allow the composition and structure of these ecosystems to be more resilient to the effect of catastrophic wildfire due to climate change.
Periods of drought or wind events can override fuel conditions leading to higher fire severities and variation from year to year. Furthermore, under a more variable-severity fire regime, fuel loads tend to be more spatially heterogeneous which is consistent with fuel data from the US Rocky Mountains.
The prescribed fire treatments constituted the greatest treatment impact on future crown fire behavior potential by raising the canopy base height. The thin and burn treatment and prescribed fire only treatment had nearly indistinguishable effects on forest structure, suggesting that resources may be better spent on increasing prescribed fire than on minimal thinning activities.
Snag densities declined rapidly with increased time since high severity fire, and most snags had fallen either partially or completely after five years. Total course woody debris biomass in the surface fuel load was similar between 8–9-year-old fires to a 27-year-old fire, but the wood became rotten with increasing time. The authors suggest that the increase in rotten wood could increase in ignitability with increasing time since fire, but fine fuels in the study sites were relatively low.
The authors found that prescribed fire treatment effectiveness at reducing future fire severity diminished as time since fire increased. The size of the treatment was also related to its effectiveness at reducing future fire severity so that larger treatments reduced severity to a greater extent. Fire severity in untreated areas was significantly higher than in treated areas across all sites.
Stands affected by beetle outbreak burned at higher severities and more often than expected, although forest cover type and vegetation were stronger predictors of severity than previous disturbance or elevation. Furthermore, the authors point to other studies in nearby stands that did not see a relationship between fire and beetle outbreaks. They suggest that beetle outbreaks followed by extreme drought, as occurred prior to the fire in the study, or wind are required to increase the flammability of the large, dead fuels, resulting in increased fire severity.
The authors found that all of the treated stands had lower fire severity and reduced crown scorch than the untreated plots.