Root Throw and Sediment Transport in the Rocky Mountains of Western Canada: Field and Modelling Investigations
Abstract
Sediment transport associated with root throw was investigated in Kootenay National Park in southeastern British Columbia. Tree toppling and root throw may result in sediment transport, as soil associated with the root wad is upheaved in the form of a root plate, which eventually disintegrates and deposits on the ground forming a mound. Although root throw is a widely recognized process, its role as an agent of sediment transport has not been widely considered. This study provides critical field data documenting the role of episodic root throw on sediment transport at a local scale (order of magnitude approximately 1 m). We then use these data in conjunction with a forest population dynamics model to consider the contribution of root throw to larger-scale sediment routing and its temporal dynamics. A detailed field measurement program documented characteristics associated with root throw in burned and pre- fire scenarios (e.g., root plate dimensions, direction of tree fall, root wad disintegration). Sediment transport rates due to root throw are relatively low; approximately two orders-of-magnitude lower than typical creep rates defined in the literature. However, in a landscape where larger mass movements are relatively uncommon, this small transport plays a role in sediment transfers, and contributes to soil mixing and formation of microtopography. In the post-fire scenario, an increase in root throw occurred as fire-killed trees toppled in the first years following the fire. The removal of vegetation also exposed root plates to erosion, and disintegration rates increased. A tree population model is developed to simulate tree recruitment, growth, mortality, and toppling rates over time scales greater than 1000 years. In the model, fire events occur as a stochastic disturbance which kills all trees, and recruits new trees. Thus, the model cycles through forest generations with lifespan determined by fire events. The number of trees at any time interval is the net result of recruits minus the proportion that die within the time interval by either wildfire or other ecological causes e.g., competition. Tree dbh is based on tree age and age- related growth rate, with a minimum dbh required before a tree will uproot significant amounts of sediment. Root plate volume, width, height are based on dbh and calibrated using the field data. Some literature suggests that dead trees uproot significantly less sediment than live trees when they fall over, and this option can be incorporated in the model. Subsequent disintegration rates of root plates are calibrated using the field data to derive transport rates in the model. The temporal sequencing of sediment transport rates due to tree topple over millennial time scales is influenced by the tree population dynamics and the associated fire return intervals. Results of the simulations show that peaks in sediment transport occur at approximately one half and one forth of the average fire interval. The sediment transport becomes more continuous and less pulsed as the fire interval increases. Both of these effects are a result of the interaction of tree topple with fire and competition and old age.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFM.H53C1401G
- Keywords:
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- 1815 Erosion;
- 1826 Geomorphology: hillslope (1625);
- 1851 Plant ecology (0476);
- 1862 Sediment transport (4558);
- 1865 Soils (0486)