The day after Rochester proposes to Jane under "the great horse-chestnut at the bottom of the orchard," that same tree gets "struck by lightning in the night, and half of it split away" (2.8.119). It can’t be a good omen to have something that’s whole get violently split in half right after two people sitting beside it decide to unite themselves. The real question is whether the tree represents Jane and Rochester or just Rochester. If the tree represents the two of them and their union, then the half that gets split away is Jane, who is driven away from Rochester by her own desire to avoid temptation. But much later in the novel, Rochester compares himself to the splintered tree and Jane to a new plant: "I am no better than the old lightning-struck chestnut-tree in Thornfield orchard…And what right would that ruin have to bid a budding woodbine cover its decay with freshness?" (3.11.109) The point is not to make you choose between these two interpretations of the symbol, but to suggest that they coexist: the tree represents Rochester, but also Rochester and Jane together, because Rochester can’t be himself without her, and because he’ll never recover completely from the trauma of their separation (just like the tree will never be the same after getting struck by lightning).