Trees are susceptible to a variety of problems, including pests and diseases, that cause premature defoliation. Leaf loss that begins at the top of the tree indicates a disease problem or an issue in the tree's growing environment. Proper care of trees and disease treatment are crucial to keeping leaves healthy and growing.
Trees in drought conditions are likely to start shedding leaves over time. A lack of moisture in the soil prevents trees from getting nutrients, and the leaves on the upper branches lose moisture first, because they are furthest from the roots of the tree. The longer a tree goes without water, the more leaves it will lose. Leaves turn yellow and begin to brown at the edges before dropping from the tree. Water the tree regularly during periods of drought.
Verticillium wilt is a fungal disease caused by Verticillium alboatrum, and it causes noticeable problems beginning in the early summer. Leaves will be light and dull in color and start dropping from the tree without first wilting; leaf drop can begin at the top of the tree or along one side. Interior leaves begin to fall first, and the leaf loss moves outward along the branches. Pruning and good soil culture help control verticillium wilt, but there is no chemical treatment.
Root girdling is a problem for some maple trees, including the Norway maple, according to the University of Rhode Island Extension web site. Girdling of roots occurs when roots wrap around one another, and one root becomes choked off and unable to transport nutrients and moisture to the top of the tree. Symptoms of girdling roots include top dieback, leaf loss, stunted leaves and dieback of twigs. Root girdling is treated by removing the root that is causing the girdling.
Some insect pests begin feeding at the upper portions of the tree. The emerald ash borer, which affects ash trees and is a cause of serious problems for native ash populations, is one such pest. As emerald ash borers feed, they cause leaf loss at the crown of the tree, which spreads to the lower branches of the tree. Bronze birch borers are another type of pest that causes leaf loss at the tops of trees. Pesticides help control the presence of pests, although there is little possibility for control of emerald ash borers.