- Cool autumn weather and shorter days with less sunlight signal deciduous trees to shed their leaves. The leaf veins gradually close off, trapping sugar in the leaf and encouraging color changes. Eventually, all leaf tissues are sealed off from the rest of the tree. Then the tree severs the connection between the leaf and its branch, causing it to fall. A leafless tree may be less attractive, but it's also more capable of surviving cold winter weather. The fallen leaves decompose and produce important nutrients for the tree and other nearby plants.
- According to Michigan State University, trees can shed up to 10 percent of their leaves during dry periods. This can be alarming, but it actually helps the tree survive with less water. Leaves have broad, flat surfaces that increase their exposure to the sun and ability to photosynthesize. Trees lose moisture through these surfaces, so dropping some of its leaves can help a tree conserve water. Some trees also lose leaves in mid-summer, even when conditions aren't extremely dry. These trees made too many leaves in spring, and are reducing their foliage for better summer water conservation.
- A tree's outer leaves can "shade out" its inner leaves, causing them to drop off. This is due to lack of sunlight exposure, and won't harm the tree. Most healthy trees have large, bare branches on the inside of the canopy and many smaller, leaf-bearing branches on the outside. Some trees shade out other, smaller trees, causing them to lose leaves on the shady side. In mild cases, this problem is merely cosmetic, but it can eventually kill the smaller tree if it doesn't get enough light. Prune large trees carefully to reduce the risk of this problem.
- Sooty mold, fungal leaf spot diseases, leaf blights and root fungi can kill leaves. Each one has its own set of symptoms, and most fungi target specific species of trees. Not all fungal conditions are serious, but some will weaken and even kill trees. Birch leaf blight produces brown spots on leaves, which then turn yellow and fall off the tree. In severe cases, the tree can lose up to 40 percent of their leaves. However, if the tree still has enough leaves to photosynthesize, it will remain relatively healthy. Other conditions, such as phytophthora root rot in citrus, can kill trees if left untreated. Tulip poplars and similar shade trees may develop fungus-like black spotting in dry conditions, but they soon recover.
- Insect infestations can encourage leaf loss in some tree species. Leaf borers and other leaf-damaging insects tunnel into the leaf stalks or consume large portions of the leaf, causing it die and fall off. In most cases, these problems affect relatively little foliage and require no treatment.
- Chemicals, such as herbicides and pesticides, can damage trees and cause them to lose their leaves. Non-selective residual herbicides are the most dangerous, and can actually kill the entire tree. Avoid using the same sprayer to apply herbicides and chemicals beneficial to the plant, since herbicide residue can hurt the tree. Some herbicides remove only the leaves, and the tree should recover once new leaves grow. Others cause more serious damage.
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