9 minute read
Alternative UK timber species
Steven Adams and Dan Ridley-Ellis discuss the findings of a recent research project into the properties and potential uses of lesser-used timber species in the UK.
European silver fir logs being marked up prior to cutting into test specimens. Photo: Edinburgh Napier University
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The UK’s commercial forestry is based largely on one species, Sitka spruce, which provides wood fibre for use in a wide range of uses, including construction, pallets and packing, fencing, panel products and paper. There are other important economic species, such as Norway spruce, Scots pine, larches, and Douglas fir, but a little more than half of the total volume is Sitka spruce because this species grows and processes so well.
With more than 15 years of research at Edinburgh Napier University, the properties of these timbers are now well known, but this does not necessarily provide certainty for the future. Just these few species, together with two more pines (Corsican and lodgepole), account for as much as 95% of the total home-grown softwood available for harvest, which raises concerns about the resilience of the forest, and the industries and livelihoods that depend on them, against the effects of climate change, pests and diseases.
Disease and diversity
There is now a different, and more personal, perspective on the need to keep a disease outbreak under control, but there is also no shortage of high-profile examples for trees too, such as ash dieback and Phytophthora ramorum, which are causing enormous damage in the UK, and Xylella fastidiosa, pinewood nematode and spruce bark beetles in Europe. One way to protect forests, which also brings many other benefits for modern multipurpose forestry, is to plant a much wider range of tree species – but how can we prepare our wood products industries for a more diverse raw material?
UK forests already have a number of other tree species that are currently under-managed for the production of wood, but which could potentially be brought into the supply chain in the near future, though the real challenge is preparing for what is being planted now. There is information in books and databases about these other species when grown in other countries, but data is sparse about what the properties would be like when grown in the UK (except that there is a good chance that they will be different), or how suitable they would be for processing in mills that have optimised their operation for the relatively small list of current species. >>
Research project
A study by the Strategic Integrated Research into Timber (SIRT) project took a preliminary look at certain potential alternative species, to gauge what properties could be expected from growth in UK forests, and how they might complement the existing commercial species for the wood processing industries. Initially, seven species with promising forestry potential were selected through consultation with the Forestry Commission (as it was then known), and members of the SIRT project management board and sponsors representing the forestry industries.
The study covered Pacific silver fir (Abies amabilis), European silver fir (Abies alba), grand fir (Abies grandis), Japanese red cedar (Cryptomeria japonica), Serbian spruce (Picea omorika), sycamore (Acer pseudoplatanus) and silver birch (Betula pendula). A previous study, with additional funding from the Scottish Forestry Trust, covered Norway spruce (Picea abies), noble fir (Abies procera), western red cedar (Thuja plicata) and western hemlock (Tsuga heterophylla). A Forest Research project also enabled some testing of Nordmann fir (Abies nordmanniana).
Some of these species had, so far, only been grown in small amounts, but thanks to the work of the Forestry Commission there were some reasonably representative stands in forest gardens and other experimental plots. Nevertheless, it requires a large amount of testing across several sites to properly characterise a timber species for commercial production from a particular geographical origin, so the data that follows can only be an indication of potential, pending more comprehensive study. This was especially true, even for the very common species birch, because the available trees, not being managed for timber, were too small to produce many timbers of the 100mm x 50mm size we used for evaluating strength.
The European grading system for structural timber is based on density, strength and stiffness (usually in bending), and it is usually one of these that will limit the achievable grades. For example, Sitka grown in the UK tends to be limited by stiffness when grading to standard BS EN 3381 strength classes, but other species can be limited by strength (such as UK-grown Douglas fir) or by density. This preliminary testing is enough to indicate the most likely limiting property, but the strength class estimates are more tentative. >>
The timber is broken in a bending test to obtain strength and stiffness data. Photo: Edinburgh Napier University
Outcomes
Softwoods
Results of this study indicate that UK-grown Japanese red cedar, which is one of the most economically important species in Japan (where it is known as sugi), would struggle even to make C14, due to low stiffness and a remarkably low mean density of just 310 kg/m3 . 2 This does not mean that it does not have its uses, where its low weight could come in handy for use as studs in partition walls, or even super-lightweight crosslaminated timber. It is an attractive wood, but also rather soft.
European silver fir. Photo: Edinburgh Napier University
If grown in the UK, four of the North American species – grand fir, western red cedar, noble fir and Nordmann fir – also look like they might not be able to achieve the usual UK market strength class, C16, with high grading yields, the first two due to low stiffness and the second two due to low strength. This, however, might well be due to the limited testing, particularly for grand fir and Nordmann fir. The other properties measured for grand fir showed more promise, with strength suitable for C16 and density suitable for C18, and similarly with Nordman fir, where stiffness was equivalent to C18 and density C30. The other two North American species look like they would be able to achieve viable C16 yield for grading, if grown in the UK. Pacific silver fir is limited by its density and western hemlock by its stiffness. However, western hemlock, being rather similar to Sitka but with higher density, is a good candidate for blending in with existing British spruce supplies. Pacific silver fir, with strength and stiffness both looking to be around the C18 mark, might have potential for lightweight construction, even compared to Sitka spruce – which was historically favoured for building aircraft.
Three UK-grown European species of conifer were included – of these, European silver fir looks limited to C16 by stiffness, but strength and density again far exceeded this grade (C20 and C27 respectively). Serbian spruce looks to be limited to C16 by strength, but had good stiffness (C18) and density (C40). Norway spruce is already known to be similar to Sitka, which grades very well to C16, perhaps with slightly better stiffness and strength.
Many of these conifer species look like they could have potential to be blended in to the existing C16 ‘British spruce’ market, ideally by mixed processing, subject to adequate grading approaches. This is encouraging, as relatively small amounts of these species could start entering the mainstream markets with little disruption.
Hardwoods
Two hardwood species, sycamore and birch, were looked at to see if they could be the equivalent of and be graded along with the conifers. Both species were found to have good properties; with sycamore, stiffness is the limiting factor and would fit the ‘softwood’ grade of C18. For birch, both the stiffness and strength are limiting factors, but fit the >>
high grade C40. However, it proved difficult to get enough birch of the dimensions required, which not only meant that only limited testing could be carried out but also that it may be difficult to find enough suitable material for processing. Density for both these species is, unsurprisingly, much higher than the softwood grades require, but it is also not enough to grade well into the hardwood D-classes where it would become the limiting property, reducing the declarable design strength and stiffness.
Industry survey
At the same time as this research, a survey was carried out asking for the thoughts, opinions and experience of sawmillers with these alternative species. The results showed that, while there is little or no experience of using some of these species, such as Japanese cedar, Pacific silver fir and Serbian spruce, this did not preclude everyone from saying that they would not be open to using them if they became available in the future.
A stool made from birch and sycamore, reclaimed from the broken test specimens. Photo: Edinburgh Napier University There was some experience of the other conifer species, European silver fir and grand fir, with the latter being the least popular due to the presence of large knots and cracks, for example, which weaken the wood and create processing problems. Birch and sycamore were also known, with both species receiving a favourable opinion.
Thoughts for the future
The information gained from this research into the mechanical properties of alternative timber species, together with information from the concurrent survey gathering the thoughts and experiences of professionals using these species and working within the industry, will help to inform future research. This research will feedback to industry to help make informed decisions for new planting and preparing the UK’s timber industry for the future. n
About the authors
Dan Ridley-Ellis Head of the Centre for Wood Science and Technology Edinburgh Napier University
Further information
Research into properties of several species mentioned in this article can be found at: www.forestresearch.gov.uk/research/ timber-properties-of-noble-fir-norway-spruce-western-redcedar-and-western-hemlock-grown-in-great-britain
Further reading
WIS 2/3-67 Specifying British-grown timbers, BM TRADA, 2017
References
2.Adams, S. H. and Ridley-Ellis, D. J., ‘A brief look at alternative tree species in the UK’, Proceedings of the 16th
Annual Meeting of the Northern European Network for Wood
Science and Engineering – WSE2020, 1–2 December 2020
