You're going for 6-7-8 aesthetically, right? Between the number being out of order on the card and not immediately seeing the number 6 in the mana cost (I started this comment before even noticing the CMC), it was hard to tell. I think 7-7-7 would be clearer and probably fine power-wise, but if you like this I'd change the wording to "At the beginning if your upkeep, put a 7/7 green Elemental creature token onto the battlefield if you control eight or more Forests." It's almost functionally the same, except I guess you could cheat in the eighth Forest in response to the trigger. You couldn't with your wording.

I had not aimed at the 6-7-8 aesthetic but just going with what felt right. I'll go with your wording since I want to keep the CMC at 6. The goal was to give a little warning that big creatures are coming (mush like you'd hear them coming well before they were visible in the woods.)