Holocene Fire Regimes and Geomorphic Response in Central Idaho

Supported by National Science Foundation, Geology and Paleontology Program, EAR-0000905

Grant Meyer, PI

Jennifer Pierce [PhD UNM 2004] and Spencer Wood, Boise State University

Tim Jull, NSF-Arizona Accelerator Facility, collaborators

PROJECT SUMMARY

One of the possible symptoms of a warming global climate is heightened forest fire activity, involving increased magnitude, intensity, and/or frequency of fires. In mountain environments, major canopy fires are often followed by accelerated slope erosion and catastrophic sediment transport in debris flows and floods. Over the last two decades, numerous large canopy fires have swept through conifer forests of the western U.S., where ponderosa pine forests have experienced particularly severe burns.  In central Idaho, ~300-year tree-ring records indicate that the frequency of fire decreased dramatically in the early 1900s; however, the intensity and magnitude of recent fires are much greater than before 1900. These changes suggest that fuel buildup due to fire suppression is largely responsible for recent catastrophic fires. The role of climatic variations in changing fire regimes, however, is much less understood. Enhanced fire activity and slope erosion might also be a symptom of climatic warming over the last century, leading to unusually severe droughts and intensified storms. Are recent canopy fires, slope erosion, and debris flows in ponderosa pine forests truly exceptional in the context of postglacial history? What has been the frequency and magnitude of such events in the past? Has the frequency of severe fires and slope erosion changed in response to Holocene climatic variations? The fundamental problem in addressing these questions is that very few data exist on fire regimes prior to AD 1500 or on Holocene-scale geomorphic processes in the northern range of ponderosa pine.

We are addressing this problem through the stratigraphy of numerous small alluvial fans in the South Fork Payette River drainage of central Idaho. These fans contain clear records of Holocene fire-related and storm-generated sedimentation events. Recent debris-flow and flood events from burned basins in the Payette and adjacent drainages allow actualistic study and development of facies models for distinguishing fire-related sediments in fan stratigraphic sections. Burned soil surfaces within fan sequences provide unambiguous stratigraphic markers and accurate ¹⁴C dates for fires, and fire-related debris-flow deposits are datable evidence of both intense canopy fires and geomorphic response. A large set of dates permits accurate estimates of the probability and frequency of fire-related sedimentation events over time, and allows analysis of fire-related events over a range of spatial scales. Dating of changes in the Payette River fluvial system will help in understanding concurrent hydroclimatic and runoff variations. In addition, datable colluvial stratigraphy will combine with alluvial fan and fluvial records to allow comprehensive analysis of hillslope-alluvial system dynamics in response to fire, climatic change, and intrinsic controls. Geomorphic process-response models will provide valuable comparisons to Yellowstone alluvial systems, which are influenced by catastrophic forest fires in a contrasting high-elevation, glaciated landscape (Meyer et al. 1992, 1995).

Publications:

· Pierce, J.L., Meyer, G.A., and Jull, A.J.T., 2004, Fire-induced erosion and millennial-scale climate change in northern ponderosa pine forests: Nature, v. 432, p. 87-90.

· News and Views commentary by Cathy Whitlock on the above article

· Meyer, G.A., and Pierce, J.L., 2003, Climatic controls on fire-induced sediment pulses in Yellowstone National Park and Central Idaho: a long-term perspective: Forest Ecology and Management, v. 178, p. 89-104.

· Meyer, G.A., Pierce, J.L., Wood, S.H., and Jull, A.J.T., 2001, Fires, storms, and sediment yield in the Idaho batholith: Hydrological Processes, v. 15, p. 3025-3038.

Figure 3 from Pierce et al. 2004 showing variations in the relative thickness of fire-related alluvial fan deposits over the late Holocene. Note that a relatively few large fire-related debris-flows – probably from severe canopy fires – account for most of the sediment deposited over the last millennium. Most of these were emplaced during during the Medieval climatic anomaly, a time of widespread, severe multidecadal droughts in the western US (Cook et al., Science 306:1015, 2004), highlighting the importance of climate variations in controlling fire regimes and geomorphic response in forested mountain landscapes of the West.