One layer is formed by all trees that share a canopy and exhibit at least a 10 % degree of coverage.
Hence, a two-layer forest is one that has two crown canopies above and not touching one another. For example, this can be young growth below the shelter of an old stand.
Plenter forests are particularly demanding for forestry and structurally diverse. They are always multi-layered, but they have special site requirements and tree species compositions and occur at only negligible area percentages in Germany.