Split Bent Scots Pine
(courtesy Mats Warensjö, SLU)
- Strength properties of individual fibres and solid wood
- What is reaction wood?
- Structure of wood fibres
- Importance of fibre properties
- Modifying wood properties
- Studying fibre structure
- Lack of information on the chemistry and structure of wood fibres
- Natural variability as an obstacle for the industrial use of wood and fibres
- Why focus on reaction wood?
- Related COST actions
Strength properties of individual fibres and solid wood
The major components of plant cell walls are cellulose, hemicelluloses and lignin. A cellulose chain consists of about 10,000 glucose units. In general, 30-40 parallel cellulose chains associate to form microfibrils containing crystalline and amorphous parts. The orientation of microfibrils in the various cell wall layers determines the strength properties of individual fibres and solid wood. Moreover, it is a key parameter for the biomechanical function of wood in the living stem.
What is reaction wood?
Compression wood cells
(courtesy Alan Crossley, CEH)
Trees control the shape of their stems through the generation of growth stresses during the process of wood formation. In situations demanding high orientation control, trees produce the so-called reaction wood, defined by the International Association of Wood Anatomists (IAWA) as:
“wood with distinctive anatomical and physical characteristics, formed typically in parts of leaning or crooked stems and in branches, that tends to restore the original position of the branch or stem when it has been disturbed; also known as tension wood (in deciduous trees) and compression wood (in conifers)”.
This definition combines a functional and a structural aspect of reaction wood, but also refers indirectly to the problems associated with reaction wood occurrence: namely heterogeneity of physical and mechanical properties within wood products, often leading to instability in use and rejection by the end-user.
Structure of wood fibres
Wood fibres have a complex ultrastructure and are composed of several cell wall layers. These cell wall layers vary in their respective amounts of cellulose, hemicelluloses and lignin. The chemistry and ultrastructure of this complicated network of wood components determines the properties of the resulting lignocellulosic fibre of both annual and perennial plants, as well as the amount of longitudinal growth stress generated during cell-wall maturation. This is the result of the biosynthetic and biochemical processes during cell wall formation.
The various types of reaction wood represent extreme cases of macromolecular and ultrastructural organisation: compression wood is characterised by high lignin content and high microfibrillar angle, while tension wood has little lignin and a low microfibrillar angle. Approximately half of the angiosperms species produce tension wood where the secondary wall is partially replaced by a so-called “gelatinous layer” from which lignin is absent and made up of axially oriented cellulosic microfibrils.
Importance of fibre properties
The final fibre properties are of great importance for the quality of pulp and paper as well as timber and sawn products. More information on the variability of cell walls in different wood species and new ideas on how modifications can be made at the ultrastructural level (scale from 1 nm to about 500 nm resolution of the fibre) are required in order to develop new cellulose and wood-based products and composite materials, to improve production processes and thus, to optimise the use of the industrial potential of wood fibres.
Modifying wood properties
In trees and other woody plants, the biosynthesis of lignin has been modified by genetic engineering to change the enzymatic machinery. Different treatments such as high temperature drying and microwave treatments, modify cell wall chemistry and wood properties.
The structure of lignin in certain trees has been altered producing fibres that are easier to pulp, requiring less chemical and energy inputs. In contrast, the content of lignin should be increased and its structure should be modified in order to grow trees being more resistant to wind, rain, decay and pathogens as well as to produce stronger timber and resistant solid wood products. All these modifications are likely to influence growth-stresses generation and reaction wood production, due to the possible role played by lignification.
Studying fibre structure
Selective removal of cellulose, hemicelluloses or lignin from lignocellulosic fibres with specific enzymes may be used for studying fibre structure at all levels of resolution and to modify their technological properties. Polysaccharide hydrolysing enzymes (cellulases, hemicellulases) and lignin-oxidising enzymes (peroxidases, laccases) are important in this respect and may reveal new details of fibre morphology and ultrastructure.
Lack of information on the chemistry and structure of wood fibres
Despite the knowledge resulting from earlier and ongoing research, there still exists a lack of information on the chemistry and structure of wood fibres. Large variations can be found within a single tree, from the pith to the bark and from the base to the top of a tree. Often the chemistry and structure of a wood cell are extremely heterogeneous and difficult to investigate with conventional techniques.
Rapid developments in molecular biology, microscopy and spectroscopy have now provided techniques, which will allow the detailed study of the basic building elements of plant fibres as well as the influences of chemical, enzymatic and mechanical treatments. For example, recent studies with antibody labelling have shown that lignin structures vary in different cell wall layers. Moreover, these patterns vary between normal wood and compression or tension wood.
Natural variability as an obstacle for the industrial use of wood and fibres
For example, the major problems encountered with compression wood relate to the heterogeneity of wood and fibre products and concern both processing and utilization:
- Drying-induced warp
- Surface quality and dimensional stability of wood and paper products.
Similar problems have been reported in relation to tension wood for a variety of hardwood species. Tension wood is usually associated with a higher log-end cracking risk following cross-cutting and steaming, and other growth-stress related problems.
Several EU sponsored and national projects have highlighted the limited understanding of reaction wood as well as the necessity to combine knowledge of tree physiology, biomechanics, wood science and wood technology to address these problems.
Why focus on reaction wood?
A consideration of secondary metabolites (extractives), although important for some species and practical applications, has been excluded from the scope of this Action devoted to aspects related to the major components, polysaccharides and lignin, regarding wood formation and cell wall properties.
Reaction wood will be given special attention as it provides a key for the understanding of the structure-properties relationships; it is by nature a quantitative trait and one goal of this Action will be to study the environmental and genetic factors that account for the observed phenotypic variation.
Related COST Actions
This COST Action built on the outcomes of completed as well as running Actions of the COST Domain on Forests and Forestry Products (FFP), for instance:
- COST Action E8: “Mechanical performance of wood and wood products” (completed)
- COST Action E10: “Wood properties for industrial use” (completed)
- COST Action E11: “Characterization methods for fibres and paper” (completed)
- COST Action E20: “Wood fibre cell wall structure” (completed)
- COST Action E28: “Genosilva: European Forest Genomics Network” (running)
- COST Action E35: “Fracture mechanics and micromechanics of wood and wood composites with regard to wood machining” (running).
COST Action E20 worked out new aspects on the distribution of cell wall components and on the structure of wood cell walls. Building on these anatomical and chemical fundamentals, this Action focused on the better understanding of the structure and biosynthesis of macromolecules and methods that influence those structures for a more efficient utilisation of wood as well as for the development of new innovative products.