When pruning trees taller than 15 feet, I am just as mindful of their natural grace and complexity as I am with smaller ornamental trees, and apply generally the principles of aesthetic pruning. However, it is especially important with such trees to pay close attention to safety, tree health, and tree integrity. In this way one must clarify one's pruning objectives from the outset.
For larger medium-aged and mature trees, the types of pruning and their purposes are:
-- Structural pruning, to treat defects.
-- Crown-cleaning, to remove dead, dying, diseased, or broken branches.
-- Crown-thinning, to allow more air and light to pass through the canopy.
-- Crown-reduction (as minor as possible), to reduce end-weight and thus risk of breakage; to direct growth away from a structure; to bring an errant branch or two into balance with rest of tree.
-- Crown raising, to provide space under tree, as for a patio or walkway.
In addition, it is best to remove no more than 15-20% of a large tree's live foliage.
For young trees that are meant to grow large, the most important objectives are:
-- to correct structural defects while main stem and branches are relatively small in diameter.
-- to encourage trunk taper and lateral root growth, for a strong foundation.
I adhere to ISA standards when pruning larger trees (over 15 feet), and when performing structural pruning on young trees that are meant to grow large. Please see the ISA link in the index to learn more about the International Society of Arboriculture.