Answer: It is common for newly planted trees and shrubs to go through an adjustment period; losing leaves and just generally looking puny are some of the symptoms of stress. Healthy trees will develop leaves to replace those they lost, which is what your tree has done. Foliage is important to the continued health of the tree. The heat is adding extra stress, but I still think your tree will recover - once the root system is firmly established. The best way to help it cope is simply to leave it alone (no pruning, no feeding). Water it deeply once (or twice) a week, depending upon weather. It should recover and begin to perk up in a few weeks. By next spring it should be well established and ready to grow.
Best wishes with your flowering cherry tree!
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