Tree Root Decay in Soils Following Tree Death after a Crown Wildfire, Canadian Rockies: A Field Investigation

Wildfire disturbances due to lighting strikes are a relatively common occurrence in subalpine forests of the Canadian Rockies. Tree roots found within forest soils are known to undergo decomposition after crown wildfires. As decay progress, this results in changes to tree root characteristics, including the number of remaining tree roots, distribution of tree root diameters and tensile force at failure of remaining roots. These changes, in turn, may impact soil hydrology and the likelihood of geomorphic process occurrence, including debris slides and debris flows. Herein, we present results of an intensive, annual field measurement program covering a period of about one decade that provides information about tree root decay following a crown wildfire in the Canadian Rockies. The crown wildfire burned a total of 17 000 hectares in Kootenay National Park and provided an opportunity to undertake this field measurement program. Hawk Creek drainage basin was the location in which field sampling of tree root data occurred. A total of 15 soil pits over a range of hillslope gradients were dug and key characteristics of all tree roots emerging from walls of soil pits were measured. Numerical analysis of tree root measurements in each year includes the frequency of tree roots in different diameter classes, frequency of tree roots at different depths below the ground surface and lateral root cohesion. One key finding is that tree roots having a smaller diameter fully disintegrate before larger tree roots. In addition, tree roots situated higher in the soil profile (i.e., closer to the surface) decay preferentially compared to tree roots located lower in the soil profile. Results also show that for a given tree root diameter class, the tensile force at failure decreased very rapidly in the first two years after wildfire occurrence. Similarly, the lateral root cohesion decreased rapidly in the first several years following the wildfire, although some root strength was documented even one decade after wildfire occurrence.