Sometimes the symptoms of tree decay are obvious. Leaves fail to appear in the spring. Large swaths of bark disappear from the trunk. Branches become dry and riddled with holes from wood-boring pests.

But other times, it’s less clear when trees are in poor health. Signs of internal rot include mushrooms growing on brittle bark, branches falling off, and discolored leaves.

Decaying trees can be dangerous, as recent events have shown. Since February, five people have been killed by falling trees in Canton, Lyndeborough, N.H., and most recently, Abington, where a tree crashed onto a passing car, killing the couple inside.

The rotted tree was felled by high winds and snow, authorities said. Residents said the tree had been dead for years and should have been removed. Town officials have said it is unclear whether it was on private or public land.

Trees near the end of their lives are more likely to be knocked over during storms or by significant gusts and should be removed if they pose any danger to property or people, arborists say.

“The responsibility falls on the homeowner to maintain or remove dead or hazardous trees,’’ said Marc Hansen, president of the Massachusetts Arborists Association.

He recommends that homeowners and business owners survey their property for any obvious signs of decaying trees and call a certified arborist for trees that look as if they may be in distress. Other signs that a tree may be decaying include:

¦ Dead leaves clinging to branches of deciduous trees through the winter. On healthy trees, they should fall to the ground.

¦ A tree that is beginning to lean, or has bare branches on one side, may have root damage.

¦ Vertical cracks, or seams, on the trunk.

¦  Areas of smooth wood where bark has fallen off. In healthy trees, new bark would grow in its place.

¦ Small branches sprouting from the base of a tree.

Many residents often overlook such warning signs.

“My frustration is that people want to spend more money on their grass than on their trees; they want to look down instead of up,’’ said Bob Kelley, an arborist at Arborway Tree Care in Hyde Park. “Trees are a lot more expensive to take care of. But it can be a real tragedy when they don’t take care of them.’’

Still, enough wind or stress can cause healthy trees to fall as well, he said. “Sometimes, there’s no way to predict when it will happen,’’ he said.

David Abel can be reached at dabel@globe.com.