USING DEBRIS FLOW ANALYSES TO QUANTIFY LONG-TERM FIRE REGIMES OF NORTHERN ARIZONA PONDEROSA PINE AND MIXED CONIFER FORESTS IN STEEP TERRAIN

Fire-related debris flow deposits provide a wealth of information about fire events, ranging from forest composition and burn severity to the date of the fire. Analyses of debris flows provide important ecological information for understanding long-term fire regimes in forested landscapes in steep terrain and the impacts of these fires on the physical environments. The setting for this study Kendrick Mountain, approximately 30 km NW of Flagstaff, AZ. The majority of the mountain burned in a mosaic of severities during the 2000 Pumpkin Fire, whose subsequent debris flows provided modern analogs for the sedimentary analyses. Preserved fire-related debris flow deposits from 15 exposures in four drainages were described and charcoal fragments dated with 14C AMS radiocarbon techniques. Modern forest composition in the study area ranges from purely ponderosa pine (Pinus ponderosa var. scopulorum) at the base to transitional mixed conifer forests and spruce-fir on the uppermost slopes.

High severity burning is necessary for development of fire-induced debris flows. The presence of fire-related deposition in the study area suggests that detailed surface fire regimes reconstructed by dendrochronologic methods for analogous forests in low gradient terrain do not fully describe fire behavior in complex terrain. Our results suggest that stand-replacing fires occurred on centennial and multi-centennial scales in both ponderosa pine and mixed-conifer forests in steep terrain. This temporal pattern more closely resembles the fire regime of higher elevation spruce-fir forests than the fire regime of neighboring low gradient ponderosa pine and mixed conifer forests. The deposits infer that stand-replacing fire was a natural phenomenon prior to the advent of fire suppression and forest composition and density changes linked to modern fires. It is likely that the surface fire regime transitions into a mixed-severity regime in steep terrain through increased fuels preheating by crown stacking. Identification of tree species from charcoal macrofossils in deposits will help clarify whether steep terrain or proximity to spruce-fir forests is responsible for cyclic stand-replacing fire events in the study area. Results of this latest development are pending.