What causes my tree splits into two trunks and it looks like it's coming apart where they meet?
In a normal branch attachment the branch is smaller than the trunk, so each year the trunk lays down wood that wraps around and locks it in. When two stems grow at the same rate neither can wrap the other. They press together, and the bark pinched between them gets buried instead of pushed out. Every season of diameter growth drives the two stems apart like a wedge while the trapped bark decays into a weak seam. Trees that were topped, lost their leader to storm or deer, or were grown fast and unpruned in a nursery row start life with the defect already built in.
What you'd see
Two stems of roughly equal size leaving the trunk at the same height, meeting in a narrow V instead of a rounded U. Down in the seam you see a dark, tight line of ingrown bark rather than a raised, wrinkled ridge of wood. Sometimes there's a vertical crack running down from the notch, a bulge of wood swelling out on either side, or a dark wet stain weeping from the notch after rain. Bradford and Callery pear, silver maple, and river birch often show three or four stems all leaving one point.
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