The paper provides research results concerning the relationship between age, absolute height and relative crown length of black alder. Analysis of the ratio of relative crown length to tree height of black alder trees allowed to develop a crown length model. Alder is capable of producing a secondary crown, which entails the shortening of the non−branched section of the trunk. A relationship has been found between the length of the primary crown and the length of the secondary crown depending on elongation of the secondary crown at the cost of shortening of the primary crown