Nature’s hierarchical materials

1. Introduction.- 2. Tree Form and Stem Taper.- 3. Tree-stem Volume Equations.- 4. Tree Weight and Biomass Estimation.- 5. Quantifying Tree Crowns.- 6. Growth Functions.- 7. Evaluating Site Quality.- 8. Quantifying Stand Density.- 9. Indices of Individual-tree Competition.- 10. Modeling Forest Stand Development.- 11. Whole-stand Models for Even-aged Stands.- 12. Diameter-distribution Models for Even-aged Stands.- 13. Size-class Models for Even-aged Stands.- 14. Individual-tree Models for Even-aged Stands.- 15. Growth and Yield Models for Uneven-aged Stands.- 16. Modeling Response to Silvicultural Treatments.- 17. Modeling Wood Characteristics.- 18. Model Implementation and Evaluation.-

Modeling Forest Trees and Stands

The term microfibril angle (MFA) in wood science refers to the angle between the direction of the helical windings of cellulose microfibrils in the secondary cell wall of fibres and tracheids and the long axis of cell. Technologically, it is usually applied to the orientation of cellulose microfibrils in the S2 layer that makes up the greatest proportion of the wall thickness, since it is this which most affects the physical properties of wood. This review describes the organisation of the cellulose component of the secondary wall of fibres and tracheids and the various methods that have been used for the measurement of MFA. It considers the variation of MFA within the tree and the biological reason for the large differences found between juvenile (or core) wood and mature (or outer) wood. The ability of the tree to vary MFA in response to environmental stress, particularly in reaction wood, is also described. Differences in MFA have a profound effect on the properties of wood, in particular its stiffness. The large MFA in juvenile wood confers low stiffness and gives the sapling the flexibility it needs to survive high winds without breaking. It also means, however, that timber containing a high proportion of juvenile wood is unsuitable for use as high-grade structural timber. This fact has taken on increasing importance in view of the trend in forestry towards short rotation cropping of fast grown species. These trees at harvest may contain 50% or more of timber with low stiffness and therefore, low economic value. Although they are presently grown mainly for pulp, pressure for increased timber production means that ways will be sought to improve the quality of their timber by reducing juvenile wood MFA. The mechanism by which the orientation of microfibril deposition is controlled is still a matter of debate. However, the application of molecular techniques is likely to enable modification of this process. The extent to which these techniques should be used to improve timber quality by reducing MFA in juvenile wood is, however, uncertain, since care must be taken to avoid compromising the safety of the tree.

Cellulose microfibril angle in the cell wall of wood fibres

Microfibril angle (MFA) is perhaps the easiest ultrastructural variable to measure for wood cell walls, and certainly the only such variable that has been measured on a large scale. Because cellulose is crystalline, the MFA of the S2 layer can be measured by X-ray diffraction. Automated X-ray scanning devices such as SilviScan have produced large datasets for a range of timber species using increment core samples. In conifers, microfibril angles are large in the juvenile wood and small in the mature wood. MFA is larger at the base of the tree for a given ring number from the pith, and decreases with height, increasing slightly at the top tree. In hardwoods, similar patterns occur, but with much less variation and much smaller microfibril angles in juvenile wood. MFA has significant heritability, but is also influenced by environmental factors as shown by its increased values in compression wood, decreased values in tension wood and, often, increased values following nutrient or water supplementation. Adjacent individual tracheids can show moderate differences in MFA that may be related to tracheid length, but not to lumen diameter or cell wall thickness. While there has been strong interest in the MFA of the S2 layer, which dominates the axial stiffness properties of tracheids and fibres, there has been little attention given to the microfibril angles of S1 and S3 layers, which may influence collapse resistance and other lateral properties. Such investigations have been limited by the much greater difficulty of measuring angles for these wall layers. MFA, in combination with basic density, shows a strong relationship to longitudinal modulus of elasticity, and to longitudinal shrinkage, which are the main reasons for interest in this cell wall property in conifers. In hardwoods, MFA is of more interest in relation to growth stress and shrinkage behaviour.

/pdf/microfibril-angle-measurement-variation-and-relationships-a-524gkdmy8w.pdf

Microfibril Angle: Measurement, Variation and Relationships – A Review

https://www.repository.cam.ac.uk/bitstream/1810/279810/1/Authors%20copy.pdf

Towards the design of high-performance plant fibre composites

Juvenile Versus Mature Wood: A New Concept, Orthogonal to Corewood Versus Outerwood, with Special Reference to Pinus radiata and P. taeda

SilviScan-2 was used to estimate and map microfibril angle (MFA) and density in twenty-nine 15-year-old Eucalyptus nitens trees. Over 4000 MFA measurements were made at about 100/hour. Each measurement represented a weighted average for about 50000 fibres. After an initial decrease near ground level, density increased with height in the stem. MFA decreased with height in the stem, reached a minimum around 30 to 50% of tree height, then increased towards the top. In the radial direction, density first decreased for a few years, then increased towards the bark. MFA was in the range 20 to 30 degrees near the pith at all heights and generally decreased towards the bark over most of the height of the stem. The lowest MFA values (approx. 10 degrees) were found close to the bark at 30 to 50% of stem height. Both density and MFA varied more rapidly near ground level, increasing the uncertainty of correlations between breast-height properties and whole tree properties. On some samples, high-resolution (0.2 mm) MFA scans were performed to confirm an earlier finding that MFA is strongly inversely correlated with density over a few growth rings but not over larger distances. X-ray diffraction was also used for estimating the orientation of the fibres within the samples. The radial variation in fibre orientation generally decreased with distance from the ground. Breast-height sample properties were moderate to good predictors of whole tree properties. Better correlations were obtained using samples from 5.5 m. Whole tree average density and MFA were found to be uncorrelated, indicating that tree improvement strategies could be designed to simultaneously optimise these properties.

Variation of microfibril angle, density and fibre orientation in twenty-nine Eucalyptus nitens trees

Abstract Changes in lignin and carbohydrate content with radial direction, growth-layer number, compression wood (CW) severity, and earlywood (EW) and latewood (LW) origin are described for one near-ground position in each of a severely bent and a nominally straight ramet (tree) of a clone (genotype) of Pinus radiata. Bark-to-bark strips were taken through the pith and the longest radial dimension of the CW side of the discs. Separate EW and LW samples were obtained for most growth layers, yielding a total of 95 samples. Differences in lignin and carbohydrate content between EW and LW were large where CW formation was moderate and small where it was severe. Mannose content was consistently different in the EW and LW of opposite wood (OW) and CW. Results suggested that the inner juvenile wood of OW rings might contain either a galactose-rich galactoglucomannan or a β-1,4-galactan. Consideration of all 95 samples showed that although the contents of galactose, lignin, glucose, and mannose were linearly and strongly correlated with one another, their relationship with xylose and arabinose content was non-linear.

Lignin and carbohydrate variation with earlywood, latewood, and compression wood content of bent and straight ramets of a radiata pine clone

The fraction of the available fibre surface in a sheet that is bonded to other fibres is the relative bonded area (RBA), which is an important determinant of sheet mechanical properties. The main method for estimating RBAhas been to extrapolate data for the light scattering coefficient as a function of tensile strength to zero tensile strength. This method can produce significant errors. In this paper, the light scattering coefficient, corrected for the total surface area of the fibres available for scattering, was plotted against sheet density, corrected for fibre shape. This was carried out for sheets made from several series of pulps, including single furnishes of long- and short-fibre bleached and unbleached pulps, as well as sheets made of blends of bleached long- and short-fibre pulps. The corrected scattering coefficient was inversely linearly correlated to the corrected density for each data set. A theory was developed to allow RBA calculation from the intercept and slope of the fitted line.

https://users.monash.edu.au/~batchelo/Downloads/hfsg.60.3.253.pdf

Calculation of the relative bonded area and scattering coefficient from sheet density and fibre shape

Two morphologically different market kraft pulps, a New Zealand radiata pine medium grade and a benchmark northern hemisphere spruce pulp from interior British Columbia, were evaluated and compared for their reinforcement potential following supplementation at different levels to a Eucalyptus market kraft pulp. A full factorial experimental design including softwood pulps types, three levels of both wet-pressing and refining, and six different softwood proportions in softwood/eucalypt furnish mixtures (0, 5, 10, 15, 20, and 100%), was employed to generate two independent regression models using SAS General Linear Model procedure, from data obtained from handsheet evaluation. It was conclusively shown, with statistical significance, that the reinforcement potential of softwood market kraft pulps in softwood/hardwood mixtures can readily be characterized and predicted through the measurement of fracture energy index of handsheets made from 100% softwood fiber.

/pdf/characterization-of-the-reinforcement-potential-of-different-dlimvup46y.pdf

R. Paul Kibblewhite

Papers

Juvenile Versus Mature Wood: A New Concept, Orthogonal to Corewood Versus Outerwood, with Special Reference to Pinus radiata and P. taeda

Variation of microfibril angle, density and fibre orientation in twenty-nine Eucalyptus nitens trees

Lignin and carbohydrate variation with earlywood, latewood, and compression wood content of bent and straight ramets of a radiata pine clone

Calculation of the relative bonded area and scattering coefficient from sheet density and fibre shape

Characterization of the Reinforcement Potential of Different Softwood Kraft Fibers in Softwood/Hardwood Pulp Mixtures