ABSTRACT
In natural settings, trees commonly grow in groups or clusters, with mutual shading from tree to tree. Trees that have been weakened by environmental stresses can be attacked by pathogens that are normally not aggressive. For example, severely defoliated plants seem to be more susceptible to attack by root rots, cankers, and dieback pathogens. Considerable misunderstanding exists regarding the nature of tree root growth. A commonly held image is that trees develop an enormous carrot-like taproot, and that support roots extend a great distance below the soil surface. Space restrictions commonly affect street trees which are unable to develop their full crown potential. The average distance between a street tree planting pit and a building is only 6 feet. Limb breakage from trucks is one form of casual damage that regularly occurs to trees in urban sites. Several other forms of casual human interference involve the integrity of the tree trunk and particularly the bark.
