A lightning strike on a tree can have a variety of effects, depending on the path the electricity takes to the ground. If the bolt passes through the center of the tree, it may kill the tree and possibly even blow it apart from the inside. An arcing path down the outer skin of the tree, on the other hand, may just blow off bark and leave a gaping wound.
When lightning strikes a tree, the electricity tends to follow the path of least resistance. Moist tissues conduct electricity better, so the bolt tends to travel through water and sap vessels. Since most trees are drier in the center than near the surface, this usually results in a pathway near the bark, superheating the water and sap, and blowing off the skin of the tree in a long scar. The tree may recover, but in many cases, the scar is permanent.
Another danger of a lightning strike on wood is that it can ignite leaves and dry branches, creating a fire that can engulf the tree and possibly spread. A powerful strike can cause branches to explode, severing them from the trunk and potentially dropping heavy fragments on cars and nearby houses.