Arborists distinguish two main styles of growth habit in shade trees: “excurrent” and “decurrent.” An excurrent tree has a strong central leader or trunk, which usually grows faster than the lateral branches. This allows the tree to maintain an upward directional growth habit which needs minimal pruning. Examples of excurrent trees include most conebearing evergreens, some oaks such as pin oak and willow oak, tulip poplar, sweet gum, and southern magnolia.
A decurrent tree has a weak central leader, which allows lateral branches to grow as quickly as the terminal shoot on the leader. These trees have more rounded outlines because there are many more branched stems with numerous V-shaped crotches that are structurally weak. Therefore these trees generally need more pruning than excurrent trees, especially during the first 5 to 15 years. Examples of decurrent trees include most broadleaved trees such as ash, elm, hackberry, hickory, silver maple, and many oaks.
Other trees that need more attention are fast growing trees, those that may be only marginally hardy in your area, those that have weak wood, and those that were improperly sited and need controlling, lest they outgrow their space.