Estimating dry matter content of fresh leaves from the residuals between leaf and water reflectance

At about 1720 nm wavelength, there is an absorption feature of leaf dry matter based on a C-H stretch overtone, which is difficult to detect in fresh green leaves because of the absorption spectrum of liquid water. We applied a method originally proposed in Remote Sensing of Environment by B.-C. Gao and A.F.H. Goetz (Estimating dry matter content of fresh leaves from the residuals between leaf and water reflectance, pp. 137-145, 1994) that used linear regression between the natural logarithm of leaf spectral reflectance and the specific absorption coefficient of liquid water over wavelengths from 1500 to 1800 nm to calculate an expected reflectance spectrum. The residual difference between the measured and expected leaf reflectance spectra enhanced the absorption feature of dry matter. The absorption feature was quantified using the normalized dry matter index (NDMI) based on the contrast between the residual leaf reflectance at 1649 and 1722 nm. NDMI was linearly related to leaf dry matter content (g cm(-2)) for data obtained in the field (coefficient of determination r(2) = 0.636) and for the leaf optical properties experiment (LOPEX; r(2) = 0.684). Lignin and cellulose contents were also measured during LOPEX, but NDMI was weakly correlated to these variables, indicating NDMI was sensitive to all leaf biochemical constituents. Estimation of dry matter content combined with estimates of water content will facilitate the calculation of the fuel moisture content for prediction of wildfire.