Sometimes there is no other option but to remove a tree. Trees that are structurally weak can pose serious threats to people and property. It is crucial to have a trained professional evaluate the risk of any tree that appears to be in decline if it poses a danger to anything in the area. It is important to remember that anything within a distance of 1.5x the height of the tree is within it's potential target zone. Here are a few things to look for in a declining/hazardous tree:
Decay, especially on main trunk
Leaning towards a target, posing an obvious threat
Tree planted too close to house, potential to cause structural damage
Severe storm damage
Other signs to look for:
The appearance of abnormally small leaves, pale green coloration of leaves, unusually slow growth, premature development of fall leaf coloration, early leaf drop, dieback of twigs and branches, wilting of leaves and tender new growth, peeling bark, and presence of fungi protruding from stems and branches.