Firewood for this experiment was harvested from March 2007 to October 2008 with intervals ranging from one to five months depending on the time of year. Harvested species included Norway spruce (Picea abies (L.) H. Karst.), beech (Fagus sylvatica L.), pendunculate oak (Quercus robur L.) and sycamore maple (Acer pseudoplatanus L.) (Fig. 1).