Submitted to: Geoscience and Remote Sensing Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 22, 2010
Publication Date: October 13, 2010
Citation: Wang, L., Hunt, E.R., Qu, J.J., Hao, X., Daughtry, C.S. 2010. Estimating dry matter content of fresh leaves from residuals between leaf and water reflectance. Remote Sensing Letters. 2(2):137-145. Interpretive Summary: Remote sensing can be used to estimate vegetation cover, leaf area index, and canopy water content using indices that compare one wavelength of a reflectance spectrum with another wavelength. Determining foliar dry biomass using remote sensing is important to: determine growth, determine nutrient contents, and predict vegetation susceptibility to wildfire. Recently, the Normalized Dry Matter Index (NDMI) was developed for estimating the foliar dry biomass for fresh green leaves, but the relationship between NDMI and dry biomass showed large variation due to the presence of liquid water in leaves. A linear regression with respect to wavelength between the natural logarithm of leaf reflectance and the specific absorption coefficient of liquid water was used to reduce the effects liquid water, enhancing the signal from dry matter. The relationship between NDMI and foliar dry biomass was much more accurate for a variety of data sets. This method can be used with hyperspectral sensors on aircraft and future satellite missions.
Technical Abstract: At 1722 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 due to the absorption spectrum of liquid water. We applied a method originally proposed by B. -C. Gao and A. F. H. Goetz (1994, Remote Sensing of Environment 47:369-374) 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 (R2 = 0.636) and for the Leaf Optical Properties Experiment, LOPEX (R2 = 0.684). Lignin and cellulose contents were also measured during LOPEX, but NDMI only 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 be able to calculate the fuel moisture content for prediction of wildfire.