Sylvia finds that the arrival of the young, handsome hunter kindles a desire within her to please him and to tell him the information that he is looking for. Fortunately, her knowledge of the woods and of its inhabitants and her close relationship with nature allows her to find out the information that he is after: the location of the nest of the white heron. Note the way we are told Sylvia thinks of the climb up the tree:
Now she thought of the tree with a new excitement, for why, if one climbed it at break of day, could not one see all the world, and easily discover whence the white heron flew, and mark the place, and find the hidden nest?
Sylvia contemplates her "triumph" as she is able to tell the hunter the information he wants. Her subsequent climb up the perilous tree that is so high and dangerous therefore symbolises her desire to please the hunter and enter the world of men, in opposition to the world of nature that has been her home up until now. It therefore helps us to understand the massive internal conflict that she is experiencing as it looks as if she will choose men at this stage over nature. Notice how as she descends the great tree, having learnt the "secret" of the white heron, all she can think of is the hunter and what he will say:
Wondering over and over again what the stranger would say to her, and what he would think when she told him how to find his way straight to the heron's nest.
The climb up the tree is therefore used to advance the conflict that Sylvia undergoes internally as the story forces her to choose between nature and the world of men.