Foliage moisture content and spectral characteristics using near infrared reflectance spectroscopy (NIRS)

Foliage moisture content (FMC in % dry-weight) was measured in seven Mediterranean evergreen species during summer 2001. All samples were dried, ground and scanned using a NIR spectrophotometer. Two methods of calibration for FMC were compared, one using stepwise regression on selected wavelengths and the other using partial least squares (PLS) regression that takes into account all the spectral information. The PLS regression method provided a better accuracy and was thus chosen. For every species except one, PLS calibrations between spectral data and FMC values were accurate: r of the linear regression between measured values and values predicted by the model varied from 0.93 to 0.99, standard error of cross validation (SECV) values were 3 to 6 times lower than the standard deviation (SD) of the reference values. PLS Calibration of FMC in all seven species combined was also accurate: r = 0.94, SD/SECV > 3. It is thus possible to measure the initial moisture content of a fresh foliage sample from its spectral characteristics when dried, whatever the species.