It is prohibited as a normal practice for any person or City department to top any street tree or other tree on public property. Topping is defined as the severe cutting back of limbs to stubs larger than three inches in diameter within the tree’s crown to such a degree so as to remove the normal canopy and disfigure the tree. Trees severely damaged by storms or other causes, or certain trees under utility wires or other obstructions where other pruning practices are impractical, may be exempted from this section. Tree trimming and tree topping conducted by employees of utility companies shall be exempt from this section.