It is a common observation that much of the shallow landsliding in the Portland area occurs during the winter rainy seasons. In developed areas, local slope saturation may be a result of concentrated runoff from man-made structures. This higher than normal concentration of runoff from buildings and paved surfaces can raise the water table on slopes that might not normally have such a high water table, even during the rainy seasons. Trees can act as pumps to reduce the water table and consequent pore water pressure. Another human related cause of shear strength reduction is the removal of vegetation from a slope.
Trees act as water pumps, reducing the pore water in the soil.
The tree roots play the same role as cohesion in equation 2-4. This type of analysis displays the role of different portions of a landslide. It should now be easy to see that a change in either of these properties by human and natural agencies can affect the factor of safety of the slope or an existing landslide on that slope.