Model Category: Numerical
Image: In these figures we show on the left the simulation set-up, for a root cone of radius 10 m, depth 5 m (after adjusting for scale). Box dimensions are 30 m by 15 m. Soil is composed of disks with diameter 0.035 m. All roots begin with diameter 0.035 m, and grow to diameters of 0.1 m, displacing surface of soil into a mound around the tree trunk (pink) above the initial soil surface (green). And on the right, root mound profiles for the two simulations for a flat surface. Simulations differ only in that the cone-shaped pattern within which roots are placed differs in its cone angle. From Figures 12 and 13 in Hoffman, B. S. S. and R. S. Anderson, 2014, Tree root mounds and their role in transporting soil on forested landscapes, Earth Surface Processes and Landforms, 39, 711-722, doi: 10.1002/esp.3470, published online 18 August 2013.
The growth and decay of tree roots can stir and transport soil. This is one process that contributes to the mass- movement of soil on hillslope. To explore the efficiency of this process, we document the mounding of soil beside Ponderosa and Lodgepole pine trees in the forests that dominate the mid-elevations of Colorado’s Boulder Creek watershed. Mounds are best expressed around Ponderosa pines, reaching vertical displacements above the far-field slopes of order 10–20 cm, fading into the slope by roughly 100 cm distance from the trunks with common diameters of 30 cm. Positive mounding occurs on all sides of trees on slopes, indicating that the mounding is not attributable to deflection of a creeping flow of soil around the tree, but rather to the insertion of root volume on all sides in the subsurface. Mounding is commonly asymmetric even on cross-slope profiles. Significant variation in the mound sizes results in no clear relationship between tree diameter and root volume displaced. These observations motivated the development of a discrete element model of tree root growth using the LIGGGHTS model, in which grains we specified to be ‘root cells’ were allowed to enlarge within the simulated granular matrix. Mounding could be reproduced, with the majority of the vertical displacement of the surface being attributable to reduction of the bulk density due to dilation of the granular matrix during root enlargement. Finally, we develop a previous analysis of the role of roots in transporting soil during growth and decay cycles. We find that even in shallow soils, the root-cycle can drive significant soil transport down forested montane slopes.
From Figures 12 and 13 in Hoffman, B. S. S. and R. S. Anderson, 2014, Tree root mounds and their role in transporting soil on forested landscapes, Earth Surface Processes and Landforms
Tree root mounds and their role in transporting soil on forested landscapes. Benjamin S. S. Hoffman and Robert S. Anderson (2013): EARTH SURFACE PROCESSES AND LANDFORMS Earth Surf. Process. Landforms