NIR-hyperspectral camera analyses for differencing agroforestry and forestry poplar woods

Abstract

Wood characteristics of trees grown in agroforestry systems are still little studied, while their growth conditions are different from conventional stands. This work focused on the impact of the agroforestry system on the lignin/cellulose ratio of hybrid poplar trees. One disk sample was harvested on 6 agroforestry (AF) and 6 forest control (FC) poplar trees, at breast height ground level (1.30 m). Every disk was analyzed by Near Infrared Hyperspectral imaging using a Specim FX17 (Specim, Spectral Imaging Ltd.). Images from hyperspectral camera analyses corresponding to absorbance spectra were collected at the wavelength of 1450 nm, attributed to first overtone O–H stretching vibration of lignin/extractives compounds, in order to clearly observe the chemical difference between AF and FC poplar woods. The results indicated significant difference between the chemical composition, based on estimated lignin content, of AF and FC poplar woods. According to the results from NIR-hyperspectral images analyses, the lignin content appeared to be lower in AF poplar wood (9.8 ± 1.1 pixels/mm2) than in FC poplar wood (16.1 ± 3.8 pixels/mm2). These results could be explained by the different tree growing conditions between the both systems. AF poplar tended to produce more tension wood and more juvenile wood than FC poplar, which resulted in a lower concentration in lignin.