BIOGENIC DISTURBANCE AND TRANSPORT ON BURNED HILLSLOPES IN MOUNTAINOUS FORESTS OF INTERIOR BRITISH COLUMBIA

Tree topples are a natural occurrence in most forest types, but geomorphic effects of this process have largely been studied in deciduous forests. Frequency of uprooting and sediment volumes disturbed by upheaval may be less in coniferous forests but biogenic sediment disturbance and transport is nonetheless an active hillslope process. Geomorphic contributions of tree uprooting have been little studied in mountain forests, and effects of stand-replacing wildfire on this process have not previously been investigated.

The initial uprooting disturbs large volumes of sediment, much of which remains attached to the roots as a root-plate feature. The direction of tree fall and gradient of the hillslope are two factors influencing the distance of biogenic transport occurring during the uprooting event. As a root-plate disintegrates, these factors also influence the amount of sediment falling outside the originating pit, creating a mound of readily erodible sediment. This leads to creation of characteristic pit-mound topography on hillslopes.

After stand-replacing wildfire, tree topple frequency increases due to large numbers of fire-killed trees. In addition, the removal of vegetation exposes root-plates and mounds to erosional processes such as rainsplash and overland flow, facilitating an accelerated disintegration of both features. The post-fire environment thus creates an opportunity for amplified transport of sediment disturbed by uprooting.

Early field results from a post-fire study in Kootenay National Park, British Columbia indicate that tree uprooting is an important sediment disturbance and transport process operating in subalpine forests. Field data for the first year after fire indicate that little hillslope erosion occurred due overland flow, and increased topple rates of fire-killed trees was the dominant sediment transport mechanism. Vegetation removal has exposed large volumes of sediment on pre- and post-fire root-plates and mounds. However, rainfall amounts were not sufficient to generate significant hillslope sediment transport as observed in other post-fire studies (e.g. Martin and Moody 2001).

References: Martin and Moody 2001 Hydrol. Proc. 15:2893-2903