The research was conducted to determine the suitability of the application ofpartially mechanized and mechanized biomass harvesting systems in conditionsof non thinned spruce forest culture. As a prerequisite for the interpretation of theresearch results of biomass harvesting systems the potential and characteristics ofaboveground biomass of spruce trees, that define it in terms of feedstock in theproduction system were explored. Mathematical models were constructed tocalculate the amount and structure of aboveground biomass and the water contentin individual segments the tree. The studies carried out have resulted in adynamic model for comparative analysis of the efficiency of the the same meansof work in different methods of work, or comparable methods of work anddifferent means of work.The field part of this research was conducted at the experimental plot (4437' N,1519' E; altitude 573 m) in the area of management unit "Žitnik" managed by theForest office Perušić, Forest administration Gospić of "Croatian Forests" Ltd.Studied spruce forest culture was established by planting seedlings in rowsspaced 2 × 2 m 42 years ago. Tree branches were not pruned, and first thinningwas conducted during this study.In accordance with the goals of the research four harvesting systems wereformed. In partially mechanized harvesting systems (by assortment and full treemethod) felling and processing of trees was done motor-manually by a chainsawand skidder was used for extraction, In mechanized harvesting systems (byassortment and load-length method) felling and procesing was done by aharvester and extraction was performed by a forwarder. Systems werecomplemented with a chipper to produce wood chips at the landing.It was found that the unit cost per tree of felling, processing and extraction wasthe smallest when assortment method with mechanized system was applied,followed by the application of the load-length method of mechanized system andthe full tree method of partially mechanized system. The cost of application ofassortment method with partially mechanized system is twice as high. For thefinal production of forest products (wood chips), costs of full tree and load-lengthmethod have to be increased with the relative costs of chipping. But despite that,their application is still more cost efficient than the application of assortmentmethod with the partially mechanized system. Mechanized harvesting system,due to much greater productivity, in terms of cost has no alternative inconducting thinning in the studied forest culture. Using partially mechanizedharvesting systems can be justified only in the case of necessity of geometricthinning with felling of single rows, and it has the advantage of causing lessdamage to the remaining standing trees (when cutting duplicate rows).