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You use "on" when something is on a surface. When climbing a tree you are surrounded by leaves and branches so it is like you are inside the tree itself.
You use "on" when something is on a surface. When climbing a tree you are surrounded by leaves and branches so it is like you are inside the tree itself.
Technically, if the tree had been felled (Chopped down) and one climbed onto it, one would then be on the tree. But what you say regarding a standing tree is right...he could also be up the tree though!
in the tree 
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