A crown reduction involves only the outer part of the crown, and therefore provides only a modest reduction in overall height and spread.

Understanding tree physiology is key to establishing the appropriate amount of foliage to be removed, 30% is considered to be the maximum allowable amount of foliage to be removed, with a view to retaining enough photosynthetic area for healthy growth and the maintenance of defences against dysfunction and decay in the wood.

The 30% limit is however, a very arbitrary value, due to the great variability amongst tree species in their capacity to tolerate various cutting treatments.