He cuts the bears foot off, then later chooses not to shoot the bear in the animal's elderly age. Do you believe the bear that finally kills him is a descendant of the previous bear? or that concept is silly/pointless to be considered?
I always thought it stood for the 'wildness' in Tristan. The thing pulling him back to nature (and all it's beauty and savagery) Tristan's true love was the wilderness and he spent a lot of time in the movie fighting this desire.
Hence his trying to 'kill' (nature) the bear, as it always came between him and everybody else in his life. Isabelle 2 was the only one who was able to put 'the bear asleep' for a time, but one gets the feeling it would have eventually awakened, as it was just part of who Tristan was.
In the end, he realizes that he can't fight it anymore, and gives into who he is by giving Alfred custody of his children, and running back into the wilderness.
It ends for Tristan the only way it can end...with him and the bear dying together as they were always one (Tristan/Nature).
Tristan is the type of guy who technically should never have a normal/conventional life...he tried for a time but it inevitably ended tragically for him.
He's just too tied to nature, and that's where he really belongs. Roaming the woods like the bear does.