Evaluation of near infrared spectroscopy to non-destructively measure growth strain in trees

In order to quickly predict growth strain non-destructively in green stems of young eucalypt trees, the effects of mechanical strain on rewetted Eucalyptus regnans as well as green E. quadrangulata and E. nitens samples were examined by NIR spectroscopy. Non-destructive NIR measurements of green wood were conducted with a fibre optic probe in reflection mode. Results showed that, abundant free water in rewetted and green samples overlapped with the band of interest and weakened the structural information of crystalline cellulose in the NIR spectra. Despite the weak signal for rewetted and green samples, the 6286 ± 5 cm−1 band shifted linearly to higher frequencies with the increase of tensile strain, in both, transmission and reflection NIR spectra, because of the elongation of hydrogen bonds in cellulose. Further, NIR spectra contained information supporting lateral expansion of cellulose microfibrils when dried below 20% MC. Band shift rates were lower for never-dried green samples than for those previously dried, suggesting differences in macroscopic and molecular strain. Growth strain of 163 green wood samples was measured using strain gauges and correlated to NIR spectra. However, no correlation was found between growth strain of green stems as measured by strain gauges and the NIR spectra in reflection mode. Contributing factors included excessive signal overlap at high moisture content, lower signal to noise ratio of diffuse reflection compared to transmission NIR spectroscopy and local variation in surface properties and growth strain.