Long-term drought can cause dieback in the upper branches of a tree.
One common long-term effect of drought is stem dieback, which is a result of the loss of fine feeder roots. As soils become dry during the hottest summer months, the fine roots in the upper soil surface may be stimulated to increase in number to get what little water is available. However, they will begin to die if soils remain dry, thus putting the root system out of balance with the amount of foliage found aboveground. When rain does return, the plant may not be able to take full advantage of this much-needed water because of its reduced root mass. The result is a resizing of the canopy through branch die-back. If drought persists into the next growing season or recurs before the tree can fully recover, it may die.
Pest problems are another result of long-term drought. Many pests, like wood borers and bark beetles, cannot survive in a healthy tree. As a tree or shrub becomes weakened from drought, these pests invade rapidly. Other pests that take advantage of drought-stressed plants include the bronze birch borer, black turpentine beetle, and many conifer bark beetle species.
Some pests, like spider mites, lacebugs, and aphids, can also be more detrimental to their hosts during extended hot and dry periods. The increased injury is a result of the plant's inability to grow faster than the rate of damage, due to the lack of water. Also, many beneficial insects, such as predatory mites, slow or cease foraging activity under these conditions.
Drought-stressed trees also exhibit a reduced ability to compartmentalize or isolate small wounds, which allows pathogens to invade and colonize successfully. It is common to see more incidences of stem canker diseases in the years following a drought. If you see sunken, grayish cankers on branches, it is likely your tree was too weak to ward off the disease.