Moisture in Sitka spruce trees and logs (PhD thesis)

Congratulations to Mike Yerbury who graduated with a PhD.  The thesis contains commercially valuable information so is embargoed for a period – but you can read the entry on Glasgow University’s repository.

Mike was working with SIRT research partner Mike Jarvis – while also working his day job as general manager for EGGER forestry harvesting.


“Within the UK, levels of timber processing are set to increase as the plantation grown forest resource reaches financial maturity. Made up predominately of exotic coniferous species, this resource has been the focus of many recent studies as wood users look to improve their fundamental knowledge of the primary characteristics that will enable them to make more appropriate decisions and capitalise on future opportunities. As a further contribution to this knowledge base, it was the aim of this thesis to determine moisture content variation within Sitka spruce (Picea sitchensis (Bong.) Carr) standing trees and roundwood logs. Within standing trees, moisture content was found to vary significantly in both the longitudinal and radial directions (P<0.001). Longitudinally, the base and top of the trees contained higher quantities of moisture than did the lower and mid sections. Radially, the moisture content at the centre of each disk was very low and this extended outwards before rapidly increasing mid-radius and then reaching a plateau near to the cambium. A sigmoid regression model successfully explained two-thirds of the radial moisture content variation as well as explaining the longitudinal moisture content variation. The radial variation with tree height was described by a shortening of the low moisture content extending outwards from the pith and a softening of the steepness of the increase with the duration extending as tree height increased. Seasonal variation within standing spruce trees was assessed within a plantation forest in northern England. Removing samples from 81 trees throughout a period covering 42 calendar weeks of 2012 revealed no significant effect of season on the moisture content although a link was identified with short-term variation in temperature. However, further analysis on the radial sections confirmed that when the moisture content data were converted to a percentage of total lumen saturation, a significant effect of seasonal variation was observed within the outer sapwood region (P<0.001). Whilst the standing trees as a whole did not demonstrate seasonal variation in moisture content, variation due to seasonal weather did have an effect on the drying of roundwood within the forest. Analysis of c. 25,000 samples, collected over a 6-year period, confirmed that season (dormant or growing) had a significant effect on sample moisture content (P<0.001). Temperature was recorded as having the greatest effect on moisture variation within roundwood whilst the effect of rainfall was not significant, due principally to the unpredictable nature of this variable throughout the year. To identify the radial and longitudinal moisture content change of roundwood during drying, an experiment was carried out to accelerate the drying process on short logs using an industrial oven at a constant temperature (40°C). Total drying time varied between 7 and 28 days prior to post drying analysis of the logs to determine the longitudinal and radial moisture profiles. All logs lost a constant quantity of moisture each day with the logs dried for 28 days losing more than 50% of their starting weight. Longitudinally, moisture was lost from all points along the length of each log with the log ends remaining driest, although these dry areas only accounted for 40% of the total log length. Radially, moisture declined across the log with the greatest amount removed from the outer radius. In order to provide some relevance of the impact moisture content variation has on the forest industry value chain, data collected from a working timber harvesting operation confirmed that variation existed between volume to weight conversions of different products. Within this even-aged Sitka spruce clearfell, volume to weight conversion factors ranged from 1.02 to 1.19. Differences of this magnitude can not only lead to a lack of understanding of between tree variation within a single site, but can also result in an impact on the optimum value recovery expected by each of the parties engaged within the value chain. The results presented within this thesis will help guide timber growers, agents, managers and purchasers in their future decision making when considering the most suitable markets for standing trees or roundwood logs. Improving knowledge of spatial and temporal moisture content variation will potentially result in maximising the forestry value chain and enable the forestry sector in the UK to make more from wood.”

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