A lightning strike seam on a White Pine -  London , Ontario ( photo credit T. Briggs)  

The explosion occurs in the transport system of the tree (the xylem and phloem).  The discharge leaps between the tree to the ground almost immediately and the roots are usually not the main conduit.   So the damage to the tree depends on how much damage is done to the main trunk  ( and sometimes roots) and therefore the tree's ability to move water and nutrients. 

 The size of the lightning strike and how and where the damage occurs in the transport system and roots dictates if the tree will survive.  More often than not,  the tree will survive and sometimes the visual damage will be minimal.  

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There are two issues with a lightning strike.  1) How much physical damage  has occurred to the tree to make it a physical hazard for potential breakage and falling down and 2);  The full impact of the damage may take up to a year to show in the tree canopy - The strike may send the tree into decline.

After a lightning strike,  watering the tree through the drier parts of summer and fertilizing in the spring will reduce stresses and encourage repair (especially to the roots that you cannot see) You may want to have it inspected by a tree service  if you are unsure of its structural integrity. 

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If a tree is very important to you, and you wish to avoid a lightning strike to it, you can install a  discharge conduit (basically a lightning rod) into the tree but note the conduit will need regular adjustment as the tree grows . 

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The main message is if your tree is hit, don't assume your tree is going to die. As noted above there is a very good chance your tree will survive  (most  references suggest 50% survival or slightly better).  When in doubt, talk to a tree service. 

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INTERESTING FACT.   The reason you don't stand under a tree in a thunderstorm is that trees invariably attract strikes because of their height.  Lightning electricity discharges in the soil and spreads out  and the the discharge from lightning is so large that it causes a distinct, massive, and rapid voltage gradient in the ground travelling approximately 60 feet in all directions from the strike .  Electricity always looks from a voltage gradient to move along, especially the path of least resistance.   So much so , that the distance between your two legs , though your body, may act as a path for the electricity to travel as it searches for a discharge current path from high voltage to low voltage in the soil .   Horses and cows are especially vulnerable because they have a greater distance between their front and rear legs than bipedal humans making a much greater voltage gradient.  Also, if you are close to the trunk, the lightning may leap to your body from the tree to get to  the ground, again as it looks for a path of least resistance. 

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