The Forest of Arden helps to create both a magical and jovial atmosphere in As You Like It. Shakespeare wrote As You Like It to represent the very popular book of his time Rosalynde, a book that is also part of the pastoral literary genre, a genre stemming all the way from Ancient Greece and which was also very popular in Shakespeare's day. Shakespeare uses the Forest of Arden to represent the country, or pastoral setting that is necessary in pastoral literature. It's also important to note that the pastoral literary genre depicts pastoral life as a type of utopia; hence, in order to represent the forest as a type of utopia, Shakespeare also gave the forest healing properties, which help portray the magical atmosphere of the forest.
There are many different ways in which the forest acts as a healer for the characters. One of the biggest ways is that Oliver has significant character change while in the forest. When we first meet Oliver, his jealousy of his brother's attributes drives Oliver to mistreat Orlando, denying him his inheritance and even threatening to kill him. When Oliver is sent by Duke Frederick into the woods to pursue Orlando to try and kill him again, Oliver experiences a sudden transformation. Orlando sees his brother Oliver asleep in the woods and about to be attacked by a lioness. Though Orlando nearly decides to walk away, he turns back and wrestles the lioness to death, saving his brother. Oliver is so touched by this act of self-sacrifice that he transforms into a new man who now loves his brother, as we see him explain in the lines addressing Celia's statement that she has heard Orlando describe Oliver as an evil man, followed by her question asking Oliver if he himself is Oliver:
'Twas I; but 'tis not I.--I do not shame
To tell you what I was, since my conversion
So sweetly tastes, being the thing I am. (IV.iii.135-37)
Beyond Oliver being healed and transformed, even Duke Frederick becomes healed by a religious man he meets in the forest, making him repent and relinquish the dukedom back to Duke Senior.
The healing properties of the forest also help to create the jovial atmosphere of the forest. One example of the jovial atmosphere is that Duke Senior and his courtiers are seen being gay and making the most of their bad situation. Touchstone especially helps create the jovial atmosphere in the forest through his foolery. Plus Rosalind also especially helps create the jovial atmosphere through her love pranks on Orlando as she pretends to be Ganymede. Essentially, all of the happy characters in the forest help to portray the forest as possessing a magical and jovial atmosphere.