Maple decline describes the loss of vigor and dieback in forests or urban plantings of maple trees. Sugar maple is the species most commonly affected, although the condition is also often seen in old urban plantings of Norway maple. Early signs of decline include small or scorched foliage, and premature fall colors on some of a tree's branches. Later, dead twigs may become visible as signs of distress become evident throughout the crown. As the tree's condition deteriorates, whole branches die. The tree may attempt to compensate for its diminishing crown by producing leafy shoots on its trunk. Fruiting bodies of bracket fungi may appear on the trunk or in the crown. Eventually, the whole tree dies. The process from first signs of trouble to total death may span anywhere from a few years to several decades.

Urban trees suffer decline due to soil compaction, soil pollution (caused by discarded cigarettes, pet droppings, careless or malicious use of herbicides, and road salt), air pollution and unfavorable moisture conditions. Sugar maples in such situations decline quickly, so they are rarely planted along city streets. Norway maples may look healthy for half a century before declining.

In rural areas, maple decline is often attributed to soil acidification caused by acid rain. Soils which have developed from nutrient-poor parent materials such as sandstone, quartzite and granite are most sensitive to acidification. Fertile areas often show maple decline where large numbers of livestock such as cattle are allowed to roam in woodlots, as herds or flocks of livestock can compact a soil to a degree which is unfavorable to many of the trees.