If the dead patch of bark is on the south or southwest side of the tree, it's likely that your Japanese maple was damaged by sunscald. Sunscald may develop in winter or summer. It is due to sudden changes in temperature caused by exposure of the bark to intense sun. In summer it's most likely to happen during dry periods of extreme heat. In winter it happens when sunny winter days alternate with cold nights. Thin-barked trees such as maple, linden, apple, crabapple, cherry, and plum are most likely to be affected. Once the bark is injured, there is little you can do. Remove any loose bark, which may harbor insects or trap moisture underneath it. To prevent the problem from developing, wrap the trunk of newly planted trees over the winter with tree wrap, or paint the trunk with white latex paint to reflect the sun's rays. Remove the wrap during the spring and summer months. Water recent transplants throughout the summer, but especially during hot spells.