I wonder if my hypothesis about what happened is correct. Those polypore #fungi grow in different planes, one over the other. I know those birch trees were cut down only a few months ago and I assume one fungus was already growing on the tree before it was cut down, but once on the ground another (or a part of the same?) started growing in the right orientation, which is perpendicular to the initial inhabitant of the tree. I hope some #fungi specialist confirms or falsifies this.