New Mexico Geological Society Annual Spring Meeting — Abstracts

In mountainous terrain, hydrologic processes associated with severe fires are an effective mechanism for rapidly transporting large volumes of sediment. Where fire regimes include severe fire, these processes can have significant geomorphic impacts on millennial timescales. We use alluvial fan deposits from the Sacramento Mountains to investigate the importance of fire-related geomorphic processes in ponderosa pine forests, where fire-scar records suggest recent severe fires are an anomaly.

Consistent with fire-scar reconstructions for the last 400 years, the youngest fan deposits in the Sacramentos are generally not fire-related, suggesting severe fires were rare. In contrast, middle Holocene deposits are dominated by charcoal-rich debris-flow facies. These deposits, characteristic of episodic fire-related sedimentation, resulted in rapid fan aggradation. Increased severe fire activity during the middle Holocene was followed by a dramatic decrease in firerelated sedimentation during the early Neoglacial when deposits indicate gradual aggradation and cumulic soil development. During Medieval time and the transition to the Little Ice Age ca. 1500-500 calendar years BP fire-related sedimentation increased again, although it never returned to middle Holocene levels.

Our results indicate that alluvial fan aggradation rates in the Sacramento Mountains are strongly influenced by changes in severe fire occurrence and resulting fire-related sedimentation. This record also demonstrates that short tree-ring chronologies from the Southwest are unlikely to capture the full range of Holocene fire variability, and that severe fire may be an important driver of natural geomorphic change in some ponderosa pine forests.