All tree and their timbers have their strengths and weakness, be it due to the wood itself or the shape the tree has grown after the history it has had to that point. What is the ideal ? Most true trees started with a central leading shoot aiming to grow tall and straight with side branches that have thinned themselves out to achieve an nicely spaced and evenly balanced scaffold structure. But there are myriad ways that this could have been spoilt, whether caused by an organism or environmental factors.
What we are aiming to do with pruning is remedy or avoid a situation where the tree or part of the tree can fail undesireably or present a hazard. In the case of a crown reduction, the more the crown size is reduced, the safe the tree becomes – and that was the old ‘lopping’ method. The trouble, we now know, is the more you take off the more the future health of the tree is affected. The root system dies back, the rampant regrowth is weakly attached, columns of wood become functionless and decay sets in the open wounds or dying roots.
So the modern arborist goes right out on the tips of a tree and prunes the outer branches, reducing the crown size by a specific percentage whilst leaving leaves on each branch to maintain sap flow and tree health, 10-30% is the norm……so that is the other reason we groan when customers say “Can you take a bit more off !!!!!! “
Pollards are another matter……thats enough for today!!!!