Hazardous trees -Web sites with good information:
How to recognize hazardous defects in trees
Tree defects and evaluations -This was written for Alaska but it is good and can be useful at any location.
Tree decay and compartmentalization (This booklet covers information on compartmentalization of trees. It is a bit long but an excellent resource. )
Learn to recognize hazardous trees - you can see right through the crack in this tree!
Remove hazard trees before they fail!