Comparative Study of the Height and Volume Structure of Douglas-Fir Forest Plantations in North-West Rhodopes, Bulgaria

Comparative study on the variation curves of height and volume distribution in mid- aged Douglas-fir plantations, created after mid-20th century in State Forestry Alabak – Velingrad- North-West Rhodopes was carried out. The plantations are located at different altitudes and various forestry activities were conducted in them. Their mean height curves and the unified height curve of Tyurin are compared. It is concluded that the height curves are roughly the same, and their course does not depend on tree species and environmental conditions. The curves in the tended sample plots are close to each other and, although varying in wide interval, they are closer to the common variation curve than to the variation in the untended plot. Curves and straight lines of the masses are built and they are recommended for use during cubic measurement of middle- aged plantations dominated by Douglas-fir and other similar tree stands, growing at comparable to the studied environmental conditions. The results obtained have theoretical and practical importance. The revealed features of the height and volume structure complement the research carried out so far and can be used to select the correct and appropriate management of the forest plantations with participation of Douglas-fir in third age class, located at relatively high altitudes.