Tree researchers are wierd. They don't just cut down a dead or dying tree--they DIG it up and look at its roots.
Rather simple, what they found out though. According to Gary Johnson from the University of Minnesota at St. Paul, dysfunctional roots are the problem. What makes a root dysfunctional? Planting the tree too deeply.
Mr. Johnson addressed this question and told us the answer at the Shade Tree Short Course, sponsored by Iowa State University Extension,last week at Ames.