what do the trees symbolize for the boys?

Do you mean the tree which the "accident" happened? I think that is alluded more to than trees in general. This particular tree is central to this novel. To Gene the tree seemed "tremendous". It was a "steely black steeple" on the riverbank. The tree symbolises risk and adventure. It is larger than life. It embodied his time at Devon and, in particular, his relationship with Finny. The tree changes in significance as the novel progresses. By the end of the novel when Gene re-visits Devon, the tree seems small and shrunken.