Growth strains assessment of various Eucalyptus species by near infrared spectroscopy

The efficient use of plantation Eucalyptus timber is particularly limited by the development of checks, splits and deformations during primary processing operations. These very penalising defects are usually frequent in hardwood plantation species with an initial fast growth rate and logged at an early stage. Longitudinal growth strain (LGS), which is an intrinsic property of wood, can explain the considerable internal effort generally known as "growth stresses"- sustained by wood of standing trees. These stresses are released during processing operations (from felling to log conversion) and can damage the wood by causing end splits, warping and broken boards. Growth stresses originate from surface growth strains induced in the cambial layer during the differentiation and maturation of new cells and impeded by the mass of the whole trunk. These stresses help to reorient the tree in a more favourable position. Longitudinal growth strain at the stem surface is appraised on the basis of stress released on the stem periphery by drilling into wood under the cambium. It involves measurement of the distance between two reference points before and after drilling a hole equidistant from these two points. The LGS measurement for assessing the suitability for solid wood production or for breeding purposes is currently hampered by the destructive and time-consuming aspects of the method. Near infrared diffuse reflectance spectroscopy (NIRS) is one of the most promising methods that could be adapted to obtain rapid and slightly destructive measurements. The results presented in this study indicate that NIRS could be used to predict LGS because it depends closely on the chemical composition of wood. The main problem remains the reference measurement inaccuracy or repeatability that could make the prediction not operable. Methodological and metrological improvements are nevertheless possible. Consequently, the quality of the calibration equation depends strongly on the choice of experimental design (training samples) and also on the accuracy of the reference methods. The prospects for better and more useful NIRS calibrations on wood depend on: improvement of the sampling method by selecting a wider variability span within the calibration set, improvement of the accuracy of the reference method, determination of the best measurement volume for samples in order to match the reference measurement volume and avoid local effects. (Texte intégral