I have read this book a long time ago and remember it being beautifully written and full of amazing imagery. I don't recall, however, how the symbolism of the cedar tree effects the story and/or the characters. The website below gives the symbolic meanings of many trees, as do other websites for the symbolic meanings of flowers, etc. Hope this helps!
Cedar trees mean healing, cleansing, and protection. It is widely grown as an ornamental tree, planted in parks and large gardens for its drooping foliage. It is the national tree of Pakistan. The name "deodar" is derived from the modern Indian language derivatives of the Sanskrit name "devdar," meaning "timber of the gods." http://www.livingartsoriginals.com/infoforests.htm
The significance of the cedar tree is to create bonds and relationships within the story. However, I only recall Hatsue and Ishmael having association with the cedars, no other character. Anyway, the cedar tree is where Ishmael and Hatsue used to go when they were young children and the flame of love was beginning to be ignited. When Ishmael first made a move, the fire blew out, and they stopped going to the cedar tree. But it quickly rose up again, as Ishmael and Hatsue reunited, and Ishmael came to realize that Hatsue really did like him.