Fire and Ecosystem Effects Interactions
How do fire frequency, rotation, and return interval relate to fuels?
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. They also found that as time between fires increased, severity also increased.
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. However, this effect decays as time since fire increases. Specifically, the drier forests of New Mexico, the ability of wildfire to act as a fuel break decayed after only 6 years. Weather conditions conducive to fire ignition and spread, however, can weaken the effect of a previous reburn to limit fire spread or severity.
The authors suggest that wildfire can act as a fuel treatment by reducing the severity of subsequent fire and increasing vegetation recovery, but in high-frequency/low-severity fire adapted ecosystems, increases in the frequency of high severity fire may lead to changes in vegetation of forested sites to alternative communities.
The authors found that spruce beetle outbreaks expanded coincident with increasing populations of spruce starting 94 years after the last stand-replacing fire and remained relatively small for another century. However, with the onset of fire exclusion, the increased density of spruce-fir has resulted in longer outbreak durations in the present century.
The authors 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. This mitigating effect lasted up to 22 years in some of the burned areas studied, but generally decayed as time since fire increased.
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.
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.
The authors found that the stand age distribution was multimodal with peaks of establishment due to interruptions to the historical frequent-fire regime. Due to fire exclusion, reduced understory competition, and favorable climate, abundant regeneration in the 20th century raised forest density from 60 trees/ha in 1876 to >3000 trees/ha in 1992.