Miller says that
the virgin forest was the Devil's last preserve, his home base and the citadel of his final stand.
This is perhaps one of the most significant narrative details that puts this story in motion. The Puritans believed that the forest contained the Indians who had every intention of harming them. In fact, Abigail later claims that her own parents were scalped right in front of her. These Indians did indeed hurt many Puritans, but the Puritans were a rare breed whose religious snobbery probably caused many of their own problems.
This impacts the first act because the event that sets the story in motion occurs before the first act, but is regularly referred to AND it occurs in this terrible forest. The girls' act of naked dancing and trying to conjure spirits occurs and is reported about during this first act and it is what apparently paralyzes Betty Parris. If it weren't for the Devil at work in the forest, these pure Puritan young ladies wouldn't have sinned. (That's at least how Puritans would view it.)