A long vertical split opened up in my trunk over the winter — what should I do?

A straight vertical crack in the bark, usually on the south or southwest face, often several feet long. It typically shows up in the coldest stretch of winter, sometimes with an audible bang. In summer it closes nearly shut; the next winter it reopens in the same line. Over years a raised, lipped ridge of callus wood — a frost rib — builds along both edges. On maples it may weep dark sap in spring. This is a split in the wood itself, not just the bark, running from the outside toward the center. For the tree it means the outer wood is now open to air and water, and it will respond by walling the injury off and growing over it. Most trees live with a frost crack indefinitely and a healthy one closes it and carries on. The lasting significance is that the crack is a permanent line of weakness and a doorway for decay, so a trunk with one is a trunk you monitor rather than a trunk you fix.