2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 4
Presentation Time: 9:05 AM


SANBORN, Paul, Forestry Program, Univ of Northern British Columbia, 3333 University Way, Prince George, BC V2N 4Z9, Canada, GEERTSEMA, Marten, Forest Sciences, Ministry of Forests, 1011 4th Ave, Prince George, BC V2L 3H9, Canada, JULL, A.J. Timothy, Dept of Geosciences, Univ of Arizona, P O Box 210081, Tucson, AZ 85721 and HAWKES, Brad, Pacific Forestry Centre, Canadian Forest Service, 506 West Burnside Road, Victoria, BC V8Z 1M5, Canada, sanborn@unbc.ca

The cool, wet Interior Cedar-Hemlock zone in the Rocky Mountains of east-central British Columbia, Canada, has a natural disturbance regime with infrequent but severe wildfires that can have significant geomorphic effects. AMS 14C dating of charcoal in colluvium and alluvial fans has been used to reconstruct the Holocene history of wildfire in the lower Morkill River valley, a headwaters tributary of the Fraser River.

Early Holocene dissection of thick (> 100 m) silty and sandy glaciolacustrine deposits in the Morkill River valley created deeply gullied terraces that are vulnerable to erosion and mass wasting when these highly erodible materials are exposed by severe wildfires. This geomorphic setting, combined with a wet climate, favours both creation and preservation of a rich record of datable charcoal. Excavations and natural exposures have yielded more than 160 samples of charcoal from 30 sites, with AMS 14C dates ranging from modern to 8500 BP. Paleosols containing apparent fire-reddened horizons, with or without associated charcoal, are also common at these sites.

Individual sections in toeslope colluvium can contain > 20 distinct but laterally discontinuous charcoal-rich horizons. Clustering of AMS 14C dates at such sites could indicate that a single wildfire triggered multiple depositional events. However, observations of modern post-fire erosion and slope failures suggest that such sequences of multiple buried charcoal layers can also form by rapid accumulation of approximately 0.5-m-thick blocks of surface soil, held together by the forest root mat. Such deposits at the base of steep glaciolacustrine terrace scarps often merge laterally downslope into alluvial fans that usually contain many fewer buried paleosols and charcoal layers. Both depositional settings have inherent, but differing, limitations as repositories of Holocene wildfire history.

Complementary studies of charcoal age and abundance in lake sediments are needed to help refine this historical record, but the radiocarbon dates suggest that major fire-related erosion and mass wasting events in this landscape occurred at approximately 500 to 1000-year intervals throughout the Holocene.