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The Wood From The Trees - Issue Five - NIR provides new insights into wood quality

The percentage of cellulose and other compounds in wood affects pulp yield and also solid-wood properties. Cellulose content, pulp yield, and cellulose microfibril angle—another important property of wood—provide critical information for resource assessment and tree breeding.

Near infra-red (NIR) spectroscopy offers a low-cost, rapid way of predicting cellulose content, pulp yield and microfibril angle. Geoff Downes and his team in Project 2.4 have developed a NIR calibration for cellulose and have used CSIRO’s NIR calibration for pulp yield (see CRC Technical Report no. 171) to predict cellulose and pulp yield in several thousand wood samples of blue gum and shining gum for breeding and wood quality evaluation studies.  When used to predict cellulose content of a new set of samples, we have found that the CRC calibration usually explains more than 75% of the variance in cellulose content in the sample set (as determined by chemical analysis of these samples).

Hand-held NIR instruments cost in the region of $20 000, and the CRC’s cellulose calibration is available to CRC partners. Currently we are analysing several large data sets of over 500 wood samples, using both the laboratory and hand-held Phazir NIR instruments, to assist CRC partners with their breeding programs and help move towards industry adoption of NIR as a routine tool.

Our existing cellulose and pulp yield calibrations require the NIR spectra to be captured from powdered woodmeal samples.  Grinding contributes about half the cost of making an NIR prediction.  We are therefore developing calibrations for pulp yield and cellulose content, and also for microfibril angle, based on NIR scanning of the surface of radial (pith to bark) wood samples. 

Radial profiles of kraft pulp yield and cellulose in a shining gum tree

Figure 1: Radial profiles of kraft pulp yield (KPY) and cellulose in a shining gum tree.  Notice how pulp yield and cellulose tend to increase toward the bark, and vary seasonally over the course of each year by as much as 3–4%.

Radial NIR scanning, as well as potentially reducing the cost of assessment, enables us to study fine-scale variation in wood chemical properties within the tree, as you can see in Figure 1.

These radial calibrations have some obvious applications:

  • Understanding radial variation in pulp yield and wood density, together with the growth, will help pulpwood growers to select an economically optimum rotation age.
  • Detecting tension wood: Bands of tension wood, identified by their high cellulose content and low microfibril angle, indicate that excessive shrinkage and board distortion is likely during solid-wood processing.  Tension wood is a particular problem in plantation-grown blue gum.


Geoff Downes
03 6237 5662