Since we know that at least some trees have been planted by humans it means now we have a valid scientific theory that all forests have been planted by humans?
No, it means that we have proof that forests can be planted by humans, hence, a human-planting theory couldn't be ruled out based upon that evidence alone.
As applied to the two theories in question (Intelligent Design and Evolution), evidence to date rules neither theory as "unscientific" (a wild-eyed claim that some have made on this very thread)...