It was found that mixed stands of the Niepłomice Forest cannot be included among highly productive stands, since their mean biomass amounts to 158.5 t·ha^-1, ranging from 40.1 t·ha^-1 in age class I to 347.2 t·ha^-1 in age class III. The percentage of individual biomass components (underbrush, wood of trees, assimilative apparatus) changes with age, and depends on the main species forming the stand. Biomass of the underbrush is 2.4 t·ha^-1 on the average, and its percentage is significant only in stands of age classes II (3.41%) and III (1.79%). The percentage of wood in the total biomass is 84.4% on the average (from 81.9% in stands of age class II to 86.4% in stands of age class IV and older), while that of the assimilative apparatus is 2.2%. The annual production of the tree layer reaches 8.3 t·ha^-1 on the average. The greatest increment (mean 9.2 t·ha^-1·year^-1) is in stands of age class III, while the smallest one (mean 4.7 t·ha^-1·year^-1) in stands of age class I. Biomass of the assimilative apparatus is of a considerable importance (from 19.2% in stands of age class I to 44.9% in stands of age class V) in the total annual production of biomass.

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