چکیده (انگلیسی):

Understanding how alternative vegetation types co-exist in a landscape is important in managing for biodiversity within an ecosystem. In California, mixed conifer forest is often interrupted by stands of shrubs known as montane chaparral. The development of chaparral stands following recent high severity or stand-replacing wildfires in mixed conifer forests has been well documented. Fire has been excluded from mixed conifer forests for over a century, and fuel loads are at historically high levels across much of this landscape. Despite contemporary post-fire research on mixed conifer forest, little is known about montane chaparral fire regimes or forest-chaparral dynamics in an ecosystem with a functioning fire regime. This study quantifies fire regimes in chaparral and adjacent forest and determines how chaparral responded to fire before fire exclusion in Lassen Volcanic National Park, California, a park that was never logged. Chaparral stems regenerated immediately after high severity fires in the 19th and early 20th century, and stem recruitment continued until the present. Fire return intervals in chaparral were longer than in adjacent forest (25 years vs. 11 years), and chaparral fires occurred during drier, potentially more extreme conditions. The apparent maintenance of stands of chaparral by less frequent, more severe fires suggests chaparral represents a self-reinforcing alternative stable state to forest. Following fire exclusion, chaparral stands gradually converted to forest as trees progressively invaded chaparral from the forest edge. Forest developing in chaparral is denser and more fir-enriched than the adjacent forest, similar to the understory that develops beneath a pine overstory following fire exclusion. Replacement of chaparral by forest reduces mixed conifer forest landscape diversity. However, the mixture of shrubs and trees in long unburned former chaparral is likely to burn with high severity effects in a subsequent fire. Since chaparral is also establishing in recent, very large high severity burn patches, chaparral extent may be expanding in the new configuration. If the decades needed for trees to invade from forest at the edge of severe burns exceed the fire return interval, chaparral may emerge as an alternative stable state to forest. Consequently, developing management strategies to increase resilience of mixed conifer forests to altered fire regimes is a pressing management challenge.