I would like to point out the distinct spiral pattern of the lightning strike. From all the trees I have seen that have been struck by lightning, they all have the same distinctive spiral pattern. I don't know why follows the spiral pattern, but I do know why lighting causes trees to explode like this. If the tree is dry (i.e. no rain has fallen) then the lightning will travel down through the living part of the tree, where the sap and water is. The water super-heats and expands rapidly (it literally explodes) causing the outer layer of the tree to blow out. In some cases, such as this, the damage is extensive enough that it will kill the tree outright or do enough damage to severely reduce its ability to survive.
|You can see two separate spiral patters here where the lightning traveled down the tree.|
|If you look carefully you can see the splintering going all the way up the tree.|
|From the exact opposite side of the tree from the previous picture.|