Crown thinning is achieved by selectively removing secondary branches within the canopy, this can reduce the sail area, while not necessarily having much effect on the overall shape and size of the crown. Crown thinning is a pruning technique used primarily on broadleaf trees. The most common reason for why this practice is carried out, is for increased light penetration and air.
Its value in mitigating a hazard is, however, more difficult to determine, in this instance the preferred option would be to crown thin in conjunction with crown reduction.
Crown thinning, can have undesirable effects if carried out badly or to excess. Excessive opening up of the crown structure may expose individual branches to increased wind penetration and turbulence, increasing the chances of branch failure. This is a particular problem when the inner and lower parts of the crown are heavily thinned, while the outer and higher parts are left densely foliated (lions tailing).