Pruning trees correctly is important to maintain their health and prevent safety issues. It requires understanding a tree's biology and using proper techniques. When pruning branches, one should either locate the tree's collar and cut just outside of it, or use the branch bark ridge method which cuts at a 45 degree angle outside the wrinkled tissue separating branch from trunk. It is best to hire an arborist for high branches and only prune using clean, sharp tools. Bad pruning can allow pathogens to enter and decay the tree over time.

1. Good Tree Pruning Techniques

2. Pruning trees is a skill and a very important one to learn
because if you do it incorrectly, you can severely limit the
life of the tree. Trees are magnificent and a little
understanding their biology will help you manage them
properly. Trees can not heal themselves as we do when
we cut our skin. When a tree is wounded, the tissue does
not repair itself, it does not heal, instead the trees
reaction is to seal the wound. Two things happen, the
damaged area is compartmentalised and a barrier zone is
created around the wound. Trees do not heal from the
inside, rather callus tissue forms on the outside at the
edges of the wound and the new wood eventually grows
over, sealing it off from pathogens.

3. Pruning trees is dangerous especially if you are pruning
high up. Realistically, tree pruning is for the qualified
arborists who are trained people who manage and
understand how to prune trees correctly. They know
how to handle their equipment properly and how to
climb trees safely. For any pruning jobs above your
head, I suggest you hire an arborist. If you do do it
yourself make sure your ladder is firmly attached to
the ground and you do not take risks. Chain saws
should only be used by qualified arborists. They are
very very dangerous in the hands of amateurs.

4. There are many cow boys out there, so beware of people
claiming to be tree surgeons. When you are engaging an
arborist ask to see their qualifications, if they have
personal and property insurance and whether they
understand the Branch Bark Ridge (BBR) technique. If
they look at you blankly on third question, DO NOT HIRE
THEM! It is the correct technique used today for pruning
branches off trees.

5. The Collar Method
If you are going to prune a branch off a tree, the first thing
you do is try and locate the collar. It is a swelling near
the base of the branch near where the branch joins the
trunk. Sometimes they are subtle and very hard to find.
It is the point where a trees will naturally shed a limb
from. If you find it, use it as a guide to prune off the
branch. YOU NEVER CUT INTO THE COLLAR,
instead you use it as guide. With your pruning saw you
remove the branch following the line of the collar about
5 - 10cm (1/2 an inch) away from it. Don't worry if you
can't find it, because you can use the BBR method to
remove the limb.

6. The Branch Bark Ridge Method (BBR)
The BBR is the tissue, which separates the branch from the main
stem and it forms what I call wrinkles between the branch and
the trunk. All trees have a collar but with some species like
Magnolia grandiflora the BBR is really easy to find, whereas
others like Eucalypts sp. are really hard to find. When you find
the BBR, work out a 45 degree away from the BBR and that is
the spot where you start cutting.
It takes a little practice to start recognising these points, but once
you get the hang of it, you will never do a bad pruning job again.
You will also notice that the final pruning cut looks right and the
job is finished. Another thing you will start noticing is all the bad
pruning jobs around. You will be amazed and surprised at how
forgiving trees are.

7. How to remove the branch safely
Remove most of the branch, leaving about 1/2 meter
stub
Note: Becareful - branches can be really really heavy
Located the collar or BBR
Undercut the branch, so that you don't tear the bark
when you are doing the final cut
Final cut, remove the rest of the branch

8. Tools of Trade
When you are pruning you need a good clean and
sharp pair of secateurs. If yours are old rusty and
blunt, treat your self to a new pair and the same
goes with pruning saws. Pruning saws can not be
re-sharpened so if yours is over 3 years old, it is
time to get a new one. It is also a good idea every
six months to sterilise your pruning tools with some
bleach or methylated spirits. This kills all the nasty
pathogens that can be spread through dirty tools.

9. Bad Pruning Cuts
Bad Pruning cuts can shorten a tree's life dramatically. Pathogens
such as fungi, bacteria and timber eating insects can enter
wounds and over time cause decay of the branch which can
spread eventually through the entire tree. This is okay in the
bush and is vital in the ecology of the environment. It provides
homes for birds, possums and lots of other creatures. But in the
cities, where people live closely together, trees weakened by
bad pathogens can be dangerous and may need to be removed.
Trees shows signs of distress very very slowly and we call this tree
time. Where you and I bleed immediately and know we are hurt,
trees may not show signs they are damaged for many many
years. A bad pruning cut today may not have any effect for
years, but it has turned a perfectly healthy specimen into a
potentially dangerous tree, that needs to be removed
prematurely.

10. Branch attachment
How the branches are attached is really really important
because if they are poorly attached they can easily fall off
and damage your house, roof or you. There are two types
of attachments - the U and V. The U shape is a good
branch attachment and means they are structurally sound
and well enmeshed into the structure of the trunk. A V
attachment is not a good attachment and is structurally
unsound. Bark and moisture gets trapped in the V and
this starts up a beautiful environment for fungi, bacterial
and insects to start decomposing the wood, thus making
the V prone to splitting. This is called included bark and if
you start looking up into the trees, you will begin to notice
this.

11. Sources:
http://www.72tree.com/services.html
http://ezinearticles.com/?Good-Tree-PruningTechniques&id=6016233