Tom Creswell, P&PDL Director, Purdue University
Tall trees are natural targets for lighting strikes and the results can be a dramatic explosion of the bark as shown in these photos. When most of the moisture in the tree is concentrated in the water-conducting vessels (phloem) just below the bark the electrical charge may cause such rapid expansion of the water that the bark is literally blown off the tree. The damage may be minimal if the tree bark is water soaked from rain so that the lightning follows the water on the outside of the tree in it's path to the ground.
For more detailed information see these articles:
When Lightning Strikes, Is The Tree Out? - Rosie Lerner, Extension Consumer Horticulturist
Trees and Lightning, FNR-FAQ-9-W (pdf file) - Forestry and Natural Resources
Click image to enlarge
Images courtesy of Doug Hyland