Have you detected any signs of disease or pests? Wilt, borers, miners? Any possibility of animal damage to bark? How far out are your trees mulched - any chance of mower damage. Could there have been ice damage previously? Nearby construction stress?

The trees are very important it sounds like - perhaps an arborist could take a look?

Rachel

We live in Indianapolis. The area surrounding the trees is well mulch, and has not been disturbed for many years. One of the trees could have received a lightning hit, but that is not the tree that is in worst condition.

There is no sign of insect infestation.

Get an arborist to look at them. There is a disease/condition called Maple Dieback that especially hits Sugar Maples. Sometimes an extreme cutback can "shock" them into reeastablishing growth. Sometimes they just slowly die.