Severe (hard) pruning can kill or seriously weaken trees by removing too much foliage.  The typical response of trees pruned in this manner is profuse sprouting (epicormic) along the remaining or denuded branches.  This is an attempt to restore lost foliage.  For this to occur, the tree must shift resources from other essential functions, e.g., root growth and defense, etc., to repair "damage" and restore "balance", e.g., root-shoot ratio.  This may result in energy depletition, dieback, increased susceptibility to secondary pests, or decline.  Depending on severity, excessive pruning can also lead to sunburn injury to the bark.  As a general rule, remove no more than 25% of a tree's foliage or branches.

Young trees, however, can tolerate more severe pruning.  Aging trees, on the other hand, are less tolerant of pruning.  Such trees should be pruned only as needed.  Restrict pruning to remove dead, suppressed, structurally weak, diseased and insect damaged branches or to lighten heavy, horizontal branches.