Tree root throw and sediment transport: Field and modeling studies in a Canadian Rocky Mountain Forest

Background/Question/Methods :Tree toppling and root throw results in sediment disturbance, as soil associated with the root wad is upheaved in the form of a root plate, which eventually disintegrates. The disintegrating sediment may either be returned to the pit or may land on the ground forming a mound, which is then susceptible to transport by other processes. Although root throw is a widely recognized process, its role as an agent of sediment transport has not been widely considered. This study provides field data documenting the role of episodic root throw on sediment transport at a local scale. A detailed field program measured characteristics associated with the root throw process in burned and pre-fire scenarios (root plate dimensions, direction of tree fall, root wad disintegration).

Results/Conclusions: Larger regolith-based mass movements are relatively uncommon in this region, and transport due to root throw may play a notable role in sediment transfers. In the post-fire scenario, increases in root throw rates occurred as fire-killed trees toppled in the immediate post-fire years. Removal of the canopy increased root plate exposure and disintegration rates increased. We used our field data in conjunction with a forest population dynamics model to consider the temporal dynamics of tree-throw sediment transport at larger time scales (of order approximately 10³ years). Fire events, which drive the forest disturbance regime, are treated stochastically. Tree recruitment, growth, mortality, and toppling are simulated in our model. The model cycles through forest generations with life spans set by fire occurrence. At any time, the model keeps track of existing trees, new recruits and those that die by wildfire or other ecological causes (e.g., competition). Tree ages and age-related growth rates are determined by tree dbh; a minimum dbh is required for a tree to uproot significant amounts of sediment. Root plate dimensions and volume associated with tree topple are estimated from dbh, using relations based on field data. Disintegration rates of root plates in the model are calculated from field-derived relations. The temporal sequencing of sediment transport due to tree throw over millennial time scales is largely influenced by wildfire return intervals and associated tree population dynamics.