Abstract
High-severity fire creates patches of complex early seral forest (CESF) in mixed-severity fire complexes of the western USA. Some managers and researchers have expressed concerns that large high-severity patches are increasing and could adversely impact old forest extent or lead to type conversions. We used GIS databases for vegetation and fire severity to investigate trends in large (>400 ha) CESF patches in frequent-fire forests of the western USA, analyzing four equal time periods from 1984 to 2015. We detected a significant increase in the total area of large patches relative to the first time period only (1984–1991), but no significant upward trend since the early 1990s. There was no significant trend in the size of large CESF patches between 1984 and 2015. Fire rotation intervals for large CESF patches ranged from ~12 centuries to over 4000 years, depending on the region. Large CESF patches were highly heterogeneous, internally creating ample opportunities for fire-mediated biodiversity. Interior patch areas far removed from the nearest low/moderate-severity edges comprised a minor portion of high-severity patches but may be ecologically important in creating pockets of open forest. There was ample historical evidence of large CESF patches but no evidence of increases that might indicate a current risk of ecosystem-type shifts.
Highlights
High-severity fire patches represent the component in fires that kill all or most of the overstory trees within mixed-severity fire areas in conifer forests of the western USA [1,2], creating a unique forest habitat type known as the complex early seral forest (CESF) [3]
When the data were analyzed by time periods, there was only one significant difference in the annual area of CESF habitat created by high-severity fire relative to the earliest time period (1984–1991), but no significant differences were detected among time periods since the early 1990s (Table 1, Figure 2)
With regard to the size of individual large CESF patches, there were no significant differences detected among time periods (Table 2)
Summary
High-severity fire patches represent the component in fires that kill all or most of the overstory trees within mixed-severity fire areas in conifer forests of the western USA [1,2], creating a unique forest habitat type known as the complex early seral forest (CESF) [3]. CESFs are distributed as small (400 ha) in mixed-severity burns in the lower/middle-montane conifer forests of the Sierra Nevada [2] and within other frequent-fire forest types of the western USA [4,5,6]. Some have expressed concerns that large high-severity patches are increasing as a component of a recent increase in so-called megafires and that this may signal ecosystem-type shifts and the loss of old-growth
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