Geomorphic Impacts of Catastrophic Fire in Ponderosa Pine Ecosystems: Modern and Holocene Perspectives from Granitic Mountains of Central Idaho

University Of Oregon Eugene, Eugene OR

Investigators

Abstract

0000905 Meyer In recent decades, large canopy fires have swept through ponderosa pine forests of the semi-arid western United States. In central Idaho, ~300-year tree-ring records indicate a dramatic decrease in the frequency of low-intensity surface fires in the early 1900s, suggesting that fire suppression and fuel buildup due are largely responsible for recent catastrophic burns. The role of climatic variations in changing fire regimes, however, is much less understood. Intensified burning and ensuing slope erosion might also be a symptom of climatic warming over the last century, leading to unusually severe droughts and intense convective storms. Thus, are recent canopy fires, slope erosion, and debris flows in ponderosa pine forests truly exceptional in postglacial history? What has been the frequency and magnitude of these events in the past? And, has the frequency of severe fires and slope erosion changed in response to Holocene climatic variations? We will address these questions through the Holocene stratigraphic record in numerous small alluvial fans in the Payette River drainage of central Idaho. Recent debris-flow and flood events from burned basins allow development of facies models for distinguishing fire-related sediments in fan stratigraphic sections. Burned soil surfaces provide unambiguous stratigraphic markers and accurate fire dates, and fire-related debris-flow deposits are datable evidence of both intense canopy fires and geomorphic response. A large set of 14C dates will yield estimates of the probability and frequency of fire-related sedimentation events over time and space. Dating of colluvial slope deposits and mainstem fluvial activity will combine with alluvial fan records to allow comprehensive system-wide analysis of Holocene geomorphic response to fire, climatic change, and intrinsic controls. It will also provide a long-term context for understanding fire regimes in ponderosa pine ecosystems and natural hazards in mountain environments.