Use of near Infrared Spectroscopy to Predict Lignin Content in Tropical and Sub-Tropical Pines

Near infrared (NIR) spectroscopy was used to predict lignin content for trees from five species of tropical and sub-tropical pines (Pinus caribaea, P. maximinoi, P. oocarpa, P. patula and P. tecunumanii) grown in Brazil and Colombia. Breast height disks were taken from 174 trees and wedges from the disks were sectioned to sample juvenile and mature wood. The sections were ground into woodmeal and NIR reflectance spectra were measured on both unextracted woodmeal and woodmeal with extractives removed. Klason lignin content was measured on the woodmeal samples and partial least squares were used to fit calibration equations to predict lignin content from the reflectance spectra. Good prediction models were obtained regardless of which data set (i.e. combinations of species and regions) was used for the model calibration. A model using reflectance spectra for woodmeal with extractives removed and combining data for all species across both regions had an R² of 0.90 and standard error of cross-validation of 0.43% lignin for the calibration data set and an R² = 0.91 and standard error of 0.40% for the validation data set. Calibration equations developed using only Brazilian or Colombian data were tested on the other data set. Predictions were very good, with prediction R² ranging from 0.83 to 0.90 and standard errors of prediction from 0.43 to 0.54% lignin.