FIRE SEVERITY VS. FIRE EFFECTS: WHAT ARE WE RECONSTRUCTING?

BROWN, P.M.

Rocky Mountain Tree-Ring Research, Ft. Collins, CO

Tree-ring evidence of fire timing and behavior includes fire scars and tree age structure. Fire scars provide relatively unequivocal evidence for non-lethal fires, although compositing of records from multiple trees is needed to minimize statistical errors. Even-aged tree cohorts are often evidence of coupled ecological/demographic processes: lethal fire followed by synchronous tree recruitment into canopy openings. Several recent studies have reconstructed mixed-severity fire regimes (i.e., including both lethal and non-lethal fire behavior) from combined fire scar and age data. However, these studies have done so without thorough consideration of interacting ecological and demographic processes involved in formation of static (i.e., contemporary) tree age structure. These processes include both climate forcing of variation in fire timing and tree demography and effects from multiple, repeated disturbances and their synergistic effects on tree recruitment.

In this talk, I will briefly review tree-ring methods for reconstructing fire regimes and present evidence for and against mixed-severity fires in ponderosa pine forests in the Black Hills and southwestern Colorado. Part of the review will be a synthesis of terminology and where more explicit definitions may be needed. Empirical data to be presented suggest that scaling up from individual stands to landscapes is critical to observe emergent patterns in stand initiation related to climate synchronization of both fire timing and optimal conditions for tree recruitment. These results also have important management implications. Rather than focus on restoring mixed-severity fire behavior in these areas as has been argued in recent papers, managers should instead focus on the mixed effects that interacting environmental processes (including, but certainly not limited to, mixed-severity fires) had on forest structure and composition over many decades to centuries.