EPISODIC GEOMORPHIC IMPACT OF SEVERE FIRE IN PONDEROSA PINE AND MIXED-CONIFER FORESTS OF THE SACRAMENTO MOUNTAINS, NEW MEXICO

Where vegetation significantly modulates geomorphic processes, severe fires provide a mechanism for large episodic increases in sediment transport. However, in ponderosa pine forests, abundant fire-scar data from the last 500 years indicate severe fires were rare, implying minimal geomorphic impacts. Alluvial fan stratigraphic evidence was used to compile an 8000-yr fire-related sedimentation record for a ponderosa-dominated forest in southern NM and directly assess the geomorphic impact of fire on episodic erosion and fan aggradation.

Consistent with existing fire-scar records, deposits from the last 500 years provide little evidence of severe fire. In contrast, fire-related debris flows delivered large volumes of coarse charcoal-rich sediment to fans during the middle Holocene (5800-4200 cal BP). Fire-related deposition decreased sharply during the early Neoglacial when more gradual accumulation and cumulic soil development dominated fan processes. Fire-related deposition increased again, although not to middle Holocene levels, during Medieval time and the early Little Ice Age (1800-500 cal BP). Episodes of increased fire-related deposition correspond to generally warm dry periods when multidecadal climate variability in the western US may have been greater. This variability may be key in fuel-limited ponderosa forests where extended very wet conditions would encourage stand density increases, setting the stage for geomorphically significant fires in later droughts.

Although restricted to times when conditions were favorable, principally the middle Holocene and Medieval period, fire-related deposits account for a large percentage of fan thicknesses in the Sacramentos. Main valley aggradation was also most rapid during these periods, likely due to increased sediment supply and/or baselevel changes due to fan aggradation. Most recently, deep arroyos have formed along many of the main valleys. No previous episodes of arroyo cutting were identified in the stratigraphic record, suggesting that incision may have been the result of post-settlement land use changes that included intensive grazing and railroad logging. These results highlight the sensitivity of this environment to infrequent large events and demonstrate the need for records that can address millennial scale variability.