Tree repair, the installation of bolts and cables to mechanically assist in strengthening cracked trees, is complicated, sometimes difficult to perform, and I recommend that you use an experienced arborist to complete this work. (But go and see his work.)
Most tree breakage occurs at included bark crotches, caused by wind, weight or snow loading. Whatever the cause, the attachment's ability to hold up under strain has failed. At this point the cracked branch or trunk is either hung up in another tree, caught in the tree itself or lying on the lawn or in the garden.
STOP here and be careful, slow down; most people's natural reaction is to grab a chain saw. Sometimes this will be the answer, but there are a number of cases where the tree can be put back together and live a long time afterwards. The number one critical factor is this, is the bark still viable, intact? It may well be bent, but not broken. If the bark has not been snapped or cut from the action of the fall, then once the broken section is lifted back into place and firmly held with bolts through the trunk, and/or cables higher up, the tree will continue to function as normal.
Trees that can be repaired usually have the split closer to the ground rather than higher up. The reason for this is that the branch is "caught " by the ground and not allowed to bend down too far, which would form a sharp fold in the bark, thus cutting through all the connecting pipes of the phloem. The phloem is essential tissue within the bark and is responsible for all downward moving liquid transport.
If the broken branch is high up and on the outside of the tree, once the weak crotch lets go and the branch starts to fall, it will fall through a full arch, perhaps 90 degrees, leaving it hanging almost straight down, and surely with cut, snapped, sharply folded bark that is not repairable. This situation is dealt with by pruning the remaining attachment from the tree and disposing of the branch.
If the branch is on a small tree or shrub and working on it is not dangerous or requiring too much heavy lifting you can attempt this at home on your own. Many times for small split branches, if they are put back into place, and held in place with a rope, then a small carpenter's screw can be inserted to hold the branch permanently in place.
(Continued in Tree Repair - 2)