Vegetation has both hydrological, and mechanical influences on a site.
Mechanisms and Influences
The presence (or lack) of vegetation has both positive and negative effects on site stability as a result of altering the hydrologic and mechanical characteristics of a site. These are summarized in the tables that follow.
Table 1. Hydrological__________Influence_____(Greenway 1987)
_______infiltration_____________adverse (beneficial as well)
Interception by plants is beneficial as it breaks the impact of falling rain. This prevents or at least reduces the detachment of soil particles from the ground surface, which decreases erosion.
Infiltration can be adverse because water moving into the soil adds to the weight of the soil mass, while decreasing the friction that holds it together, potentially leading to a land slide. Plants increase the rate of infiltration because as their roots penetrate the soil they open up pathways for water as well. Increased infiltration also has beneficial effects, as it reduces runoff, which in turn decreases the transport of detached soil particles, thereby slowing erosion. So whether increased infiltration is beneficial or not depends upon the circumstances and site being considered.
Extraction refers to the pumping of water out of soils and into the atmosphere by plants as they transpire. This reduces the weight of the soil mass, and as the soil dries it increases the effect of friction holding a slope together.