Sediment transport due to tree root throw: integrating tree population dynamics, wildfire and geomorphic response

摘要

A field study was conducted to analyze root throw and associated sediment transport in Hawk Creek Watershed, Canadian Rockies. A large crown fire in 2003 allowed the opportunity to study pre-fire and post-fire root throw. Based on field data, a significant relation was found between gradient and root plate volume, as well as individual root plate dimensions. Given that tree diameters increase as trees age and that a relation in the field data was found between tree diameter and root plate volumes, sediment transport clue to root throw is expected to change in response to forest disturbance and stand age. Sediment disturbance, which is the amount of sediment upheaved during tree topple and does not take into account transport distance, shows higher values on steeper gradients. Sediment transport was notable for the steepest plots, with pre-fire values of 0.016 cm(3) cm(-1) a(-1) and post-fire values of 0.18 cm(3) cm(-1) a(-1). A tree population dynamics model is then integrated with a root throw transport model calibrated for the Canadian Rockies to examine the temporal dynamics of sediment transport. Fire is incorporated as a disturbance that initiates development of a new forest, with the model cycling through generations of forest. Trees fall according to an exponential rate that is based on time since death, resulting in a time lag between tree mortality and sediment transport. When values of time-since-previous-fire are short, trees are generally <13 cm, and minimal sediment is upheaved during toppling. If trees reach a critical diameter at breast height (dbh) at time of fire, a pulse of sediment occurs in the immediate post-fire years clue to falling of killed trees, with tree fall rates decreasing exponentially with time-since-fire. A second pulse of root throw begins at about 50 years after the previous fire, once new recruits reach a critical dbh and with initiation of competition-included mortality.