Question

we should never stand under a tree during lightning

Answer 1

 If you're under the tree when it's hit by lightning, you have chance of
(a) being hit by the lightning along with the tree,
(b) getting electrocuted by the electric field set up on the ground around the tree, (c) getting hit by a falling branch (or the tree itself!) after the strike. 

Answer 2

if near a short tree no problems . If a tall tree don't stand near by because it transfer the charge to the earth