In most of Europe, bread made from bark was a famine food. It was more regularly eaten Europe’s far North. The “bark” in bark bread is actually the cambium layer that grows under the bark. Pine was a common tree to use for bark breads. The cambium layer is pealed from the tree, dried, and ground into a flour. If used alone then it makes a cake — an unleavened bread — and if mixed with flour — rye would be a good choice in keeping with the breads of the Northern Europe — it bakes into a loaf bread.
I have never done this. The author of this video suggests tasting the cambium before stripping the tree to be sure it has a good taste. Taste is apparently variable.
If you have made bark bread, please leave your report in the comment section, below.