Drought: Trees may lose as many as 10% of their leaves during a drought without being irreversibly affected. Typically, once drought conditions are in place, leaves begin to drop and continue to drop for the extent of the drought period. While a tree uses leaves to make food, this amount of leaf loss does little or no harm, and may actually be helpful to the tree. By shedding these leaves the tree loses less water through transpiration
Adjustment to summer conditions: Sometimes trees just make too many leaves! When cool moist spring weather turns to hotter, dryer summer conditions a number of leaves may drop suddenly. This is called "physiological leaf drop" and does not harm the plant's health. This leaf drop is simply a defense mechanism from further injury. When weather conditions are hot and dry, trees can take up an immense amount of water. If that water is not in the soil to support the root system the tree shuts down to preserve the moisture needed for continued life. This means eliminating the loss of water…which happens to be through the leaves.
Inner leaf drop: The observer should look carefully at the tree. If the falling leaves are from the inside of the tree they may have been "shaded out". Inner leaf drop occurs when the leaves on the outside and top of the tree are so thick that the leaves inside the tree do not receive enough sunlight. After such leaf drop, the larger branches inside the tree and close to the trunk look bare. Inner leaf drop is normal and not harmful.
Shading: If leaves can be lost due to shading from within a single tree, then it follows that leaves can be lost when an entire tree is shaded. Trees are living things and a tree may grow so large that it begins to cast shade on another tree which was once in sunlight. The smaller tree no longer receives enough light to support its leaves which begin to drop. Another form of shading occurs when closely planted trees grow so large that they begin to shade each other on the sides that face each other. Often, this leads to leaf loss on the sides of the trees where they are in close contact or are intermingling branches.
Adapted from: Ohio State Buckeye Yard and Garden Online and Brian Pugh, Oklahoma State University
