Possessive nouns show ownership, and "forest" is not something that can possess.
Instead, you need to add a possessive noun or pronoun to a phrase related to the forest:
* The forest's edge: This refers to the edge of the forest.
* The forest's trees: This refers to the trees that are within the forest.
* Our forest: This refers to a forest that belongs to us.
Here's how to use possessive nouns with "forest":
* The forest's canopy: This refers to the covering of trees over the forest.
* The forest's animals: This refers to the animals that live in the forest.
* The forest's secrets: This refers to things hidden within the forest.