The mechanical influences of vegetation on slope stability are summarized in the table below.
Table 2. Mechanical__________Influence_____
_______wind dynamics _______adverse (can be beneficial)____
_______root binding_________ beneficial
Reinforcement, anchoring, and rootbinding are all fairly self explanatory as beneficial influences resulting from the action of the roots on the soil in which they are growing. Wind dynamics are generally considered to be an adverse influence because of the potential for windthrown trees to tear up great chunks of soil and possibly bring other trees down with them as they fall, or damage adjacent structures.. However a thick stand of mature trees tends to slow wind and be resistant to such damage except for during the most extreme weather conditions. This stabilizes conditions at a site, and so can be beneficial where such conditions exist.
Different types of root architecture (Wilde 1958). The soil binding abilities of roots are a function of mass, density, depth of penetration, and architecture.
The influence that roots have depends upon the type of roots that vegetation produces. Trees with plateroots such as the spruce in the picture above are more susceptible to being wind thrown than those with taproots. Meanwhile shrubs with a heartoot structure will produce greater rootbinding effects thereby doing more to resist soil erosion than a tree with a pronounced taproot. Similarly heartroots provide good upslope anchoring such as for the tree in the photo at the upper right, while one with a more defined taproot like the tree at lower right could be undermined by erosion and eventually fall.
Upslope tree roots anchor trees against gravity pulling them down slope. The mound around this tree also shows the effect of fine roots binding to the adjacent soil, while soil outside this area of influence has moved down slope.
Tree with strongly defined tap root. It is currently stable, however erosion is undermining it