If you desire a tree with multiple trunks, such as a birch clump, make certain that the trunks are well-separated at the ground line. Remember, trunks do expand in diameter as they grow. Two trunks may be slightly separated when small, but as they grow in girth, the trunks will squeeze together. Look for early signs of vertical trunk cracks. Examine branch unions carefully for small cracks below the unions.
Cracks are major starting points for fractures of branches and trunks.
The small cracks could be present for many years before a fracture happens. Always keep a close watch for vertical cracks below squeezed branches and squeezed trunks. Corrective pruning helps. If your tree has only a few minor problems, corrective pruning may help. Start corrective pruning one year after planting.