It shall be unlawful practice for any person, firm or city department to top any street tree, park tree or other tree on public property without authorization from the City Forester or Engineer. TOPPING is defined as the severe cutting back of limbs to stubs larger than three inches in diameter within the tree’s crown, to 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 obstructions where other pruning practices are impractical may be exempted from this chapter at the determination of the city’s Tree Board.