Changes in Tree Root Distribution, Tensile Force at Failure and Lateral Root Cohesion over a 10-year Period Following a Crown Wildfire, Kootenay National Park, Canadian Rockies

Wildfire disturbance influences tree population dynamics, ecosystem functioning, hydrology and geomorphic activity in subalpine forests of the Canadian Rockies. Tree roots within forest soils decay after crown wildfires, with changes to the tree root characteristics, distribution and strength expected over time. In addition to affecting forest soils and hydrological functioning, post-wildfire tree root decay may result in increased debris slide and debris flow activity prior to new tree establishment and growth. While many aspects of tree root characteristics and tree root decay after logging and/or wildfire have been studied previously, the present study provides results of an extensive field measurement program of tree root characteristics that was undertaken annually for a period of one decade following a crown wildfire. A large crown wildfire was ignited by lightning in Kootenay National Park, Canadian Rockies in 2003 and burned a total of 17 000 hectares. The field measurement program was completed in Hawk Creek, a small mountain drainage basin located in Kootenay National Park and that was part of the area subjected to this crown wildfire. Annual post-wildfire measurements of tree root characteristics and tensile strength at failure were made in a total of 15 soil pits, covering 5 slope gradient classes. It is anticipated that tree roots of small diameter may fully disintegrate before larger roots, thereby changing the total number of roots and spatial distribution of roots within forest soils. Post-wildfire tree root decay for all tree root diameter classes is expected, with an overall decrease in lateral root cohesion also expected over time. Detailed results of tree root measurements completed for each year include the frequency of tree roots in different diameter classes and the frequency of tree roots at different depths below the ground surface. Results show that tree root decay occurs preferentially for: (i) tree roots of smaller diameter; and (ii) tree roots located closer to the ground surface. Graphs of tree root diameter vs. tensile force at failure for any given year show a positive relation, with a rapid decrease in the strength of tree roots of a given diameter occurring in the several years after the wildfire. Lateral tree root cohesion decreases rapidly in the first two years following wildfire and then stabilizes at lower values (although some root strength still exists one decade after wildfire occurrence).