When facing the tree, go to your Pokemon and select the one that has the move HEadbutt. Then use it.
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you go to Ilex forest and find a man in there and he will give you headbutt and teach it to you Pokemon and now you could headbutt a tree
headbutt tree
Note: You need Headbutt.New Bark Town Headbutt Trees.CherryGrove City Headbutt Trees.search him on your pokedex and it will be red instead of green that means you need to headbutt a tree using headbutt.
There Is No Exact Tree(s) You Should Headbutt As All Trees Have A Possibility Of A Pokemon Appearing. The Pokemon Appearing Is Decided By The Game Itself So The Only Advice I Can Give You Is, Headbutt Until Something Pops Out.
It means, get a pokémon that knows headbutt, like psyduck, or sudowoodow or cranidos and headbutt a stupid tree! yay!