how 2 stick 2 wood ??????

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I had a poster hanging on my (plasterboard, paimted) wall, held by scotch tape at the corners, holding up fine for about 10 years. Now I moved, and I have wooden walls (pic related).
They have been treated but not painted or varnished as far as I'm aware. They're pretty smooth though (definitely miles smoother than the paint from my previous walls). However, my newly erected poster fell down within 48 hours.

What's the best way to hold it up? I don't really want to screw/nail things into the wall if I can avoid it.