Diameterklassen

Diameter classes, or diameterklassen, are predefined intervals used to group trees by trunk diameter at breast height (DBH). DBH is measured at 1.3 meters above ground on standing trees. Each class is defined by a lower and upper bound in centimeters, and the width of classes can vary (commonly 5 or 10 cm). Examples: 0–9 cm, 10–19 cm, 20–29 cm, etc. Some inventories use finer 2 cm or wider 20 cm classes depending on objectives.

Diameter-class distributions describe the size structure of a forest stand. They are used in stand inventory, growth and yield modeling, and habitat assessments. Analysts assign each measured tree to its diameter class and compute class frequencies, densities, and, using species-specific allometric equations, estimated volumes and basal areas per class. The distribution reveals patterns such as a typical reverse-J shape in self-regenerating stands or gaps indicating disturbance or aging.

In interpretation, changes in the distribution over time indicate recruitment, thinning, mortality, or shifts in age structure. For example, a strong concentration of trees in larger diameter classes with few small trees may indicate past thinning or an aging stand, whereas many individuals in small classes reflect ongoing regeneration and potential future growth.

Measurement practices can vary; diameters are measured on standing trees, with multi-stemmed trees requiring a representative stem, or using alternative metrics when trunks are irregular. Diameterklassen are a standard tool in forestry and ecology for characterizing size structure and informing management decisions.