Changes in volumetric energy densities during storage of whole-tree feed stocks from silvicultural thinnings

• Energy densities of whole-tree feed stocks were determined.
• Changes in basic density play a bigger role than moisture content.
• Determination of basic density of fractionated material contains errors.
• Covering feed stocks helps lower moisture content.
• Pine benefitted from the storage time but birch feed stocks lost energy.

The quality issues of woody feed stocks usually report changes in the moisture content, heating value or dry matter, but these factors are rarely summed and reported as changes in energy densities for a given feed stock. This paper reports the results of ten whole-tree and two stem wood storage field trials on energy densities. The material consisted of eight Scots pine (P. sylvestris), and four downy birch (B. pubescence) trials, all after the first thinnings. Other factors studied were the length of storage, pile cover and seasoning in the stand. The two variables under scrutiny were the basic density and moisture content. It was shown that even a good degree of drying could not counterbalance the reduction in the basic density. The Scots pine feed stocks dried well, and the basic density losses were less than those of downy birch. As a result, the pine feed stocks increased in energy density from 0.7 to 17.6% over the trial period, whereas all of the birch feed stocks suffered losses from −3.4 to −9.6%. Although covering the feed stocks had a beneficial effect on moisture content, it proved to have a positive effect only on pine when the storage was extended beyond one year. There was some gain on energy density if the feed stocks were seasoned in the stand, but this advantage was lost at the roadside landing. Furthermore, transpiration drying in the stand requires a two-phase harvesting technique, and this expense may be difficult to justify.