Fig. 5

a The principal component analysis (PCA) scores plotted on the first and second principal components on the basis of the second derivative NIR spectra in the 7,300–4,000 cm⁻¹ region. b The spectrum obtained from PC1 loading in PCA. The bands at 5,970, 5,800, and 5,220 cm⁻¹ are assigned to lignin, hemicellulose, and the absorbed water, respectively. c Second derivative spectra obtained from unaging heartwood and aging wood of P. thunbergii designated as KYOw19176 and KYO_ID_5197