I'm asked about pruning for many different reasons. Lots of clients want more light in their garden or property. Some have safety concerns or worries about a dodgy looking feature or mechanical fault in a tree. Some worry about the roots and what’s going on underground. Some don’t like the branch that the pigeon perches on to poo on their car!
Formative pruning is carried out thinking about the future of a tree and its growth. Removing branches that might become a problem in the future whilst their small and the cut will be minor. Rather than letting it grow into the problem and having to remove a bigger branch or limb, leaving a bigger cut to try and heal up. For example, one might remove or reduce a co-dominant stem, crossing or crowded branches of a young tree. Or a branch which is growing towards somewhere unsuitable. Like a building or where it might become an obstruction, like across a path or road. Formative pruning might also be carried out to encourage growth in a certain part of a tree. Maybe for good growing direction or to provide screenage.
Correct pruning can be very affective to prevent limb or entire tree failure, normally by reducing an over extended or a faulty branch/limb or thinning the crown entirely. A little weight off the end of a branch significantly reduces leverage on that branch. Therefore, reducing the risk of the branch/limb breaking off. And thinning or reducing the crown will reduce the sail area (i.e. catch less wind) so the tree is less likely to be blown over.
Pruning can greatly improve aesthetics or it can easily ruin a tree forever. I think a tree is most beautiful when it looks natural and unpruned! That is one reason why generally it’s best to take off a little and take branches back to good take-over points (to retain decent tracery). The other reason is that so many species will respond with profuse regrowth from pruning cuts which results in a bigger and denser tree as it grows back. The classic story is a tree owner wants more light, they call someone in who cuts the top off, thus ruining the natural shape of the tree, then a few years later have less sun-light than before and an ugly topped tree to look at. Big cuts also may not heal up, resulting in decay and the potential decline of the tree. And some trees, like Beech, will get sun burned/scorched after a topping! Good pruning can provide more sun-light over and through the tree (giving that lovely dappled shade), and a feeling of space, whilst leaving the tree healthy, happy, looking good and natural.
Tree work isn’t all black and white though and there are certain situations where a heavy reduction is required or necessary, and also some species that coppicing or pollarding is a good management technique for. Another thing to think about is heavy pruning/topping, often results in large decay pockets (holes), excessive ivy or dense foliage, all of which can be great habitat for birds, bats, tree dwelling mammals and invertebrates. So when I see a butchered tree, rather than get upset, I just think of the benefits in habitat creation and that at least the tree was not cut down completely.