I recall when I was at school we were taught about erosion. That was a particular problem in the Gisborne and Hawkes Bay areas. Much of the problem was exacerbated by the removal of ground cover and of trees which lent a certain stability to the land.
Tree roots mechanically reinforce soil, increasing its shear strength (“root cohesion”) and helping soil layers act more like a single, stronger mass. Large roots can anchor through the soil into weathered rock, tying the shallow soil mantle to more stable material beneath and across potential slip planes.
