Main types of tree surgery

Crown lifting / crown raising: the removal of the lowest branches, side branches, or parts of branches. The process is usually defined by the height of the lowest branches after surgery, for example "raise crown to 3 m above ground level". This lets more light and wind under the tree, and gives more room for people to walk underneath

Crown thinning: The even removal throughout the crown of selected branches, side branches, or parts of branches back to growing points. It is usually defined as a fraction of the crown to be removed, for example "thin crown by up to a third". Thinning lets more light and wind through the tree.

Crown reduction: The removal of the outermost parts of branches, or side branches, back to growing points, to reduce the tree’s height and spread. Reduction is usually defined as a fraction of the crown to be removed, for example "reduce crown by up to a fifth", and/or by the maximum length of branch to be removed, and/or the size of the tree afterwards, for example "reduce crown, removing up to 4 m, to leave crown 15m high by 10 m wide". The process lets more light and wind above and around the tree.

Often a combination of crown lifting, thinning, and reduction may be used.

Other work may include dead wooding, removal of dead and/or dying branches, and cleaning out (removal of broken branches, crossing branches, stubs from previous bad pruning, and climbers like honeysuckle, ivy, wild clematis).

Coppice and pollards
Trees can be cut back regularly to the same points and stay healthy only if this is done from an early age by somebody with experience. Coppicing (cutting to ground level in woods) and pollarding (cutting to a 6'-15' stump in wood pasture) were traditionally used to produce poles for building and fuel. In gardens, coppicing can be useful to stabilise steep slopes. In streets or gardens a form of pollarding can be used which leaves a framework of branches. Like pollarding, it should be started when the tree is young.

What not to do - topping
"Taking the top out" is not the same as pollarding. It will not stop the tree growing. If a mature tree is suddenly cut for the first time, the trunk and branches are cut off midway, rather than to a growing point. This weakens the tree, and decay will increase. The new growth appears vigorous, but it will be weakly attached to the trunk, and will be more likely to break in future, so the tree then has to be cut regularly for the rest of its life. The tree can become dangerous and unhealthy as well as ugly. If you want a tree to be made smaller, don’t get it topped - ask for a proper crown reduction with cuts made to growing points.

Where to cut, how much, and when:
 Cuts should be made at a side fork to leave a flowing branch line, and at the natural division between the branch and its parent stem, just outside the branch collar and branch bark ridge. This leaves a ring of tissue which will grow as wound wood over the cut.
Don’t remove more than a third of the tree’s leaf area. Don’t remove branches which are more a third of the thickness of the parent branch or trunk.
The best time is different for different species, so ask the contractor for advice. Avoid the time around budburst in spring and around leaf-fall in autumn.

Precision Cutting 
As seen above, this is a very important practice to ensure that the wound heals properly and that the end product is neat and tidy.
In the case of Plate 1 a broken and damaged branch has been corrected to the best possible point. It is very important for branches to heal and form a callus that would protect the wound from invasion by viruses and diseases