Sitka truths

Summary
• SIRT is being carried out in collaboration with Forest Research and Glasgow University Chemistry Department.
• Key timber properties are density, modulus of elasticity and modulus of rupture.
• Research is examining genetics, silviculture, environment and rotation length.
• The benchmarking study will be extended to cover all of the UK.

Since its inception three-and-a-half years ago, the primary focus of the Strategic Integrated Research in Timber (SIRT) project has been to identify the causes and extent of variation in the wood properties of Sitka spruce that affect its use as a structural timber. In this context, the key properties are density, modulus of elasticity (MOE) and modulus of rupture (MOR), and four studies are under way to examine the different factors that are known to affect these – genetics, silviculture, environment, and rotation length.

Two of these studies give some idea of the scope of work being undertaken. The first concerns the effects of rotation length in UK-grown Sitka spruce since this can affect the mechanical characteristics of timber, particularly the MOE and the MOR, due to their effect on two important and intrinsic wood properties: density of the timber and microfibril angle in the wood cells.

In many conifer species, including Sitka spruce, there is a general increase in wood density from the pith to the bark and a reduction in the angle of cellulose microfibrils in the secondary cell wall (otherwise known as microfibril angle or MfA). Wood found within the first 10-15 rings from the pith is generally referred to as corewood (or juvenile wood) which has higher MfA and lower density, making it less stiff and more prone to distortion, while wood in the remainder of the cross-section is referred to as outer wood (or mature wood) which has lower MfA and higher density making it more suitable for use in construction.

As trees get older, not only does their diameter increase, but the proportion of the total cross-section area containing outer wood also increases. The combination of these two factors means that it is possible to cut battens that are entirely composed of higher stiffness outerwood, but the age and size of trees grown in UK forests, coupled with current sawing patterns used in UK sawmills, tends to result in a large proportion of battens being cut from the juvenile core of the tree, with the resulting timber considered to have an inherent strength class of C16.

Timber cut from the outerwood of older and larger trees might be expected to have superior physical and mechanical properties, which in turn could mean an acceptable percentage being graded to C24.

Rotation length study

Birkley Wood in the north-east of England is being used to examine the effect of rotation length in UK grown Sitka spruce. The wood was planted in 1923 and contains the oldest plantation timber in the UK. The project is looking at several aspects of this timber including radial variation in MOE (stiffness), MOR (strength) density and MfA.

Logs cut from each tree have been processed into 4x2in structural battens that in turn have been statically tested to give the MOE and MOR of the timber. From this, it is hoped to provide answers to many questions, such as the quality of 80-year rotation UK timber compared to imported 80-year rotation timber, and whether UK timber is inherently C16 or can C24 be achieved with increased rotation length?

Strange as it may seem, given the sheer extent of Sitka spruce being grown in the UK, this resource has never been categorised, nor have the effects on it of the environment or silviculture ever been properly examined. A benchmarking study is therefore being carried out to assess the wood properties of standing trees at 64 sites throughout Scotland and northern England (Kielder Forest).

Sample trees will be felled and processed into logs and subsequently sawn into battens. The aim is to investigate the effects of site (environment) and silviculture on the timber properties and also look at different sawing strategies (for example, taper sawing to maximise the recovery of high stiffness material on the outside of the log versus conventional sawing which maximises volume recovery). The battens will be graded and tested and the results compared between sites and cutting patterns.

In the next stage, the benchmarking study will be extended to cover all of the UK and review the silvicultural and genetics trials already undertaken; improving the understanding of acoustic tools (an important part of all the SIRT studies so far); carrying out batten performance modelling, developing a robust and reliable strength grading system for local industry; and synthesising the available knowledge on Sitka spruce wood properties in order to make this information available to the wider industry.

To achieve these goals means also extending the research funding base from government support to one that includes a representative section of the industry itself.