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Negative environmental impacts related to climate change and social pressure to raise the rotation age of forests contribute to the quality deterioration of harvested wood, including its decomposition by fungi. This paper presents the results of laboratory experiments investigating the decomposition of Norway spruce (Abies alba) and silver fir (Picea abies) wood by selected species of decay fungi: Hericium flagellum and Bondarzewia mesenterica (causing white rot) as well as Rhodofomes roseus (causing brown rot). The decay process affects wood density, elemental composition, net calorific value, as well as the amount and price of the energy contained in the wood. Wood density was measured conventionally using an electronic caliper and laboratory balance. Energy parameters were determined using elemental analyzer and combustion calorimeter. Wood decay was found to adversely affect energy content per unit of wood at a given volume, thus leading to higher prices per unit of energy. Over the 120 days of the experiment, wood parameters were not affected considerably by Hericium flagellum or Bondarzewia mesenterica. Of the three examined fungal species, Rhodofomes roseus caused the highest weight loss of wood and the greatest decrease in its oven-dry density (spruce - a decrease from 409 to 214 kg m−3; fir - a decrease from 420 to 306 kg m−3), unfavorably reducing energy content per 1 m3 of wood (spruce – decrease by 3.8 GJ m−3; fir – decrease by 2.3 GJ m−3) and raising the unit price of energy. [Display omitted] [ABSTRACT FROM AUTHOR]