Shakespeare wrote As You Like It to represent a very popular pastoral novel of his time, Rosalynde. Nature is a dominant theme in any pastoral literary work because one of the ideas the genre presents is that pastoral life, or country life, is a form of utopia, while city life, or courtly life, is corrupt. Shakespeare uses the Forest of Arden to represent the nature setting needed in a pastoral work. More importantly, the Forest of Arden is portrayed as having healing attributes, which helps to represent the forest as a form of utopia, or at least as being less corrupt than life at court.
We see one example of the forest being portrayed as having healing attributes with respect to Oliver's transformation as a character. In the beginning of the play, Oliver is shown as being exceedingly jealous of his younger brother Orlando's qualities. Specifically, Oliver jealously describes Orlando as being gentle, intelligent despite lack of education, noble, and so loved by all that Oliver himself is hated, as we see in Oliver's lines:
Yet he's gentle, never schooled and yet learned, full of noble device, of all sorts enchantingly beloved, and indeed so much in the heart of the world ... that I am altogether mispris'd [despised]. (I.i.166-71; Shakespeare Navigators)
Oliver is so jealous of Orlando at the beginning of the play that he denies Orlando the inheritance left for him in their late father's will. He even tries to kill Orlando twice. It is when Oliver pursues Orlando into the woods to try and kill him once and for all that Oliver experiences a very sudden and complete change of heart. Orlando sees Oliver sleeping and about to be attacked by a lioness; Orlando nearly walks away, but decides to rescue his brother instead and wrestles the lioness to death. Oliver is so moved by this act of self-sacrifice that he is filled with love for his brother, which gives him a total transformation in character. Hence, since Oliver is so transformed once in the forest, we see that the forest is being represented as having healing qualities.