Trees, and their derivative products, have been used by societies around the world for thousands of years. Contemporary construction of tall buildings from timber, in whole or in part, suggests a growing interest in the potential for building with wood at a scale not previously attainable. As wood is the only significant building material that is grown, we have a natural inclination that building in wood is good for the environment. But under what conditions is this really the case? The environmental benefits of using timber are not straightforward; although it is a natural product, a large amount of energy is used to dry and process it. Much of this can come from the biomass of the tree itself, but that requires investment in plant, which is not always possible in an industry that is widely distributed among many small producers. And what should we build with wood? Are skyscrapers in timber a good use of this natural resource, or are there other aspects of civil and structural engineering, or large-scale infrastructure, that would be a better use of wood? Here, we consider a holistic picture ranging in scale from the science of the cell wall to the engineering and global policies that could maximise forestry and timber construction as a boon to both people and the planet.

The wood from the trees: The use of timber in construction

As they fulfil many ecological and social functions and services better than even-aged monocultures, heterogeneous pure and mixed-species stands are on the advance in Central Europe. Even so, knowledge of how different stands compare in terms of the quantity and quality of the produced wood remains limited, as forest research has been focused on pure stands in the past. Therefore, the still limited comparative studies on timber quality in mixed versus pure stands were reviewed. Further, approximately 100 studies on the morphology of mixed versus pure stands have been reviewed. As is known, the close connection between morphology and timber quality from many studies in pure stands as well as the morphological and structural properties of trees in mixed stands is used as proxies for their timber quality. The number of studies reporting a decrease or increase in timber strength and stiffness in complex stands compared with homogeneous stands was balanced. Knottiness is mostly higher in complex stands. Wood density behaves indifferently. Distortion, as indicated by eccentricity of crown, bending of stems, or irregularity of the tree-ring width, is generally higher in complex forests. This rather ambiguous pattern becomes clearer by typifying the findings depending on the species-specific morphological plasticity of the trees and the spatial conditions they are exposed to. When growing in strong lateral restriction in even-aged pure or mixed-species stands (type 1), trees follow a “keep abreast” strategy which results in high-quality timber especially in case of species with low plasticity. Trees in uneven-aged forests with vertically restricted growing space (type 2) often use a “sit-and-wait” strategy that may result in tapering stem shapes, wide and long crowns with low branch diameters, and high wood density. Distortion may be low in case of species with low morphological plasticity but increase with increasing shade tolerance and plasticity. Growth in widely spaced and heavily thinned pure and mixed stands (type 3) may let trees follow the “stabilisation” strategy. Because of their strong dominance, these trees develop tapering stem shapes, knots of big size and wide appearance along the stem axis, as well as lower wood density, especially in the case of conifers. In arrangements of types 1–3, the “transition” strategy may also emerge, which leads from the “sit-and-wait” stadium to the “keep abreast” strategy. It starts when trees strongly increase their height growth at the expense of the stem diameter growth. It results in slender stems, low knottiness, high wood density, and low distortion, with the result that the tree gets access to the upper canopy at the expense of lateral expansion of stem and crown. In fact, it is not primarily the species mixing that modifies the morphology, structure, and wood quality of the trees but the species-specific morphological plasticity and the structural heterogeneity of the stand. The latter is often higher in mixed than in pure stands and in uneven-aged than in even-aged stands. The more variable the stand structure, the wider the range of wood attributes. The discussion is focused on the relevance of the results for stand management and interdisciplinary research at the intersection of forest growth and yield science and wood science.

Wood quality in complex forests versus even-aged monocultures: review and perspectives

Because wood density is an important indicator of the end-use mechanical properties of sawn timber, detailed knowledge of its variation is important for optimizing value in processing. The aim of this study was to develop a model for predicting wood density variation in plantation-grown Scots pine (Pinus sylvestris L.) using position in the stem and radial growth increment as independent variables. Seventy trees, ranging in age at 1.3 m from 9 to 99 years, were sampled from 12 even-aged Scots pine stands in Scotland, UK. Pith-to-bark density profiles were obtained on 193 radial samples using scanning X-ray densitometry. We developed a mixed-effects model of average ring density (expressed as basic specific gravity) based on an exponential function of cambial age, height along the stem and annual ring width. Basic specific gravity ranged from 0.274 to 0.697, with a mean of 0.423, and increased rapidly from the pith until approximately rings 20‐30, before stabilizing. In addition, there was a weak negative relationship between basic specific gravity and annual ring width, and for a given cambial age, basic specific gravity decreased with sampling height. The fixed effects of the final model were able to explain �57 per cent of the within-stem variation in wood density (78 per cent if random effects were included). The final model is intended for integration into a growth, yield and wood quality simulation system.

Models for predicting wood density variation in Scots pine

To maximize utilization of our forest resources, detailed knowledge of wood property variation and the impacts this has on end-product performance is required at multiple scales (within and among trees, regionally). As many wood properties are difficult and time-consuming to measure our knowledge regarding their variation is often inadequate as is our understanding of their responses to genetic and silvicultural manipulation. The emergence of many non-destructive evaluation (NDE) methodologies offers the potential to greatly enhance our understanding of the forest resource; however, it is critical to recognize that any technique has its limitations and it is important to select the appropriate technique for a given application. In this review, we will discuss the following technologies for assessing wood properties both in the field: acoustics, Pilodyn, Resistograph and Rigidimeter and the lab: computer tomography (CT) scanning, DiscBot, near infrared (NIR) spectroscopy, radial sample acoustics and SilviScan. We will discuss these techniques, explore their utilization, and list applications that best suit each methodology. As an end goal, NDE technologies will help researchers worldwide characterize wood properties, develop accurate models for prediction, and utilize field equipment that can validate the predictions. The continued advancement of NDE technologies will also allow researchers to better understand the impact on wood properties on product performance.

https://www.fpl.fs.fed.us/documnts/pdf2019/fpl_2019_schimleck001.pdf

Non-Destructive Evaluation Techniques and What They Tell Us about Wood Property Variation

Summary This paper describes the development of models to predict wood density in Sitka spruce (Picea sitchensis (Bong.) Carr.) growing in Britain. The models were designed to be practical and widely applicable and useful for large-scale resource mapping and investigating the impact of silvicultural treatments. Two models (and a variant of the second) were developed using data obtained from a 52-year-old spacing trial in Clocaenog Forest, North Wales, and a 32year-old re-spacing experiment in Kershope Forest, Northern England. The model equations were based on models used to describe the within-tree variation in the wood density of Norway spruce (Picea abies L. Karst.) in France and are functions of ring width and ring number from the pith. The models were able to explain up to 48 per cent of the variability in density from Clocaenog but only 27 per cent of the variability in the density from Kershope, although they were able to accurately represent the radial trends in wood density. Some of the difficulties in explaining the variation in density may be due to the differences in the site conditions, silviculture and age of the trees at the two sites, but it is clear there is significant inter-tree variation in the data.

/pdf/models-for-predicting-wood-density-of-british-grown-sitka-41k6xn21xc.pdf

Models for predicting wood density of British-grown Sitka spruce

Information on wood properties variation is needed by forest growers and timber processors to best utilise the available forest resource and to guide future management. This study aims to quantify the variation in selected properties of Sitka spruce (Picea sitchensis (Bong.) Carr.) structural timber. Twelve harvest-age stands were selected, ten trees per site were felled and processed into 301 logs. Dynamic modulus of elasticity (MOEdyn) was measured on each tree and log using portable acoustic instruments. Logs were processed into structural timber and its MOE and bending strength was determined. Overall, the timber satisfied the MOE, bending strength and density requirements for the C16 strength class. Approximately 25 % of the total variation in timber mechanical properties was attributed to between-stand differences, with the remaining 75 % attributed to within-stand differences. A series of equations were developed to predict site, tree and log-level variation in timber properties. Knowledge of the site and stand factors that are associated with differences in timber properties can assist with segregation of the current resource. Portable acoustic tools can also be used to increase the stiffness of sawn timber by segregating out individual trees and logs that will yield low stiffness timber.

/pdf/within-and-between-stand-variation-in-selected-properties-of-13njrhemeo.pdf

Within- and between-stand variation in selected properties of Sitka spruce sawn timber in the UK: implications for segregation and grade recovery

The extent and sources of variation in the wood quality of Sitka spruce (Picea sitchensis (Bong.) Carr.) were quantified using data collected from 64 stands in northern Britain. These stands were selected on the basis of elevation, latitude, longitude, yield class, initial spacing and the presence or absence of thinning. Dynamic modulus of elasticity (MOE) was calculated from measurements of stress wave velocity made on standing trees and qualitative descriptions were made of stem form. Dynamic MOE of individual trees ranged from 3.81 kN/mm 2 up to 12.29 kN/mm 2 , with a mean of 7.71 kN/mm 2 . Approximately 55 percent of the variation in dynamic MOE was due to differences between individual trees within a site, while 35 percent was due to differences between sites. The remaining 10 percent was due to differences between the measurements made on opposite sides of each tree. Variation in dynamic MOE at the site level was significantly influenced by yield class, elevation as well as by a number of the interactions between these factors and latitude, longitude and initial spacing. A multiple regression model incorporating these variables was able to explain 45 percent of the variation in dynamic MOE. Ramicorn branches were the most commonly recorded defect (37.2% of all live trees), followed by stem scarring and basal sweep (6.9% and 6.3%, respectively). Dynamic MOE was not influenced by stem straightness (p = 0.10) which indicates the utility of stress wave velocity measurements for segregating Sitka spruce stands based on potential grade recovery.

The effects of site and stand factors on the tree and wood quality of Sitka spruce growing in the United Kingdom.

Sitka spruce (Picea sitchensis) is the United Kingdom’s main commercial tree species accounting for nearly one-third of the UK's total woodland area and half of its conifer estate. Sawn timber from this species readily grades to the C16 strength class, but there are factors beyond the structural grade that can influence its acceptance as a construction material. This paper summarises the results from resource characterisation studies that have investigated the properties of Sitka spruce at the standing-tree scale down to the scale of a few microns. These studies have substantially improved the understanding of the impact on mechanical performance of structural timber of factors at the micro-structural level (e.g. cellulose structure and abundance) and at the forest-level (e.g. genetics, the environment and forest management). End-user requirements for timber are discussed in terms of what is, and is not, provided by current grading practice and some of the main misconceptions about UK-timber are challenged.

Strategic Integrated Research in Timber: Getting the most out of the UK's timber resource.

This paper is concerned with the extent to which portable stress-wave-based instruments can be used to segregate Sitka spruce trees and logs; in particular the relationship between acoustic velocity and the properties of sawn timber. Measurements were made at 64 sites which were selected to span a range of conditions. At each site a portable stress wave timer (IML Hammer) was used to estimate the dynamic modulus of elasticity (Ed) on a sample of ten standing trees. Differences between sites accounted for 36 percent of the total variation in Ed, while differences between trees within a site and sides of the tree account for 55 and 9 percent of the variation respectively. Trees were felled from a representative sub-sample of 12 sites and the Ed of logs estimated using the HM-200 resonance instrument. At an individual tree level, a strong relationship was observed between measurements made on standing trees and those made on the butt log. The mean bending stiffness (Es) of timber cut from an individual site was strongly related to the mean stress wave velocity of both trees and logs from that site showing that it is possible to directly manipulate the characteristic stiffness of a population of sawn timber by segregating logs based on stress-wave velocity values. Taken together, these results demonstrate how non-destructive measurements made on standing trees and logs can assist the forest products industry to improve timber properties and that such approaches work for Sitka spruce grown under UK conditions. The average Ed of logs at a site was also found to be negatively associated with the average live crown ratio of trees at the site offering the possibility of partial sorting, even without acoustic tools.

Gregory J Searles

Papers

Within- and between-stand variation in selected properties of Sitka spruce sawn timber in the UK: implications for segregation and grade recovery

The effects of site and stand factors on the tree and wood quality of Sitka spruce growing in the United Kingdom.

Strategic Integrated Research in Timber: Getting the most out of the UK's timber resource.

The use of acoustic-based NDT to predict the wood properties of UK-grown Sitka spruce at different stages in the wood supply chain.