Cellulose absorption index (CAI) to quantify mixed soil-plant litter scenes

Quantification of plant litter cover on the soil surface is necessary in both agricultural and natural systems because the presence of litter influences the flow of nutrients, carbon, water, and energy in terrestrial ecosystems. Although remote sensing methods for measuring plant litter cover provide both a wider area of coverage and a more objective estimate of the spatial variability of litter than manual methods of quantifying the nongreen vegetation landscape components (e.g., litter or soil percent cover), it has been difficult to assess the efficiency of detecting partial litter cover over different soil types. The objectives of this study were (i) to acquire spectral reflectance data for four crop residues and two forest litter types in mixed scenes of soil and plant litter, (ii) to derive relationships that show the spectral variable, cellulose absorption index (CAI), as a function of the amount of litter on the soil surface, and (iii) to test whether the variability of soil background reflectance inhibits the detection of residues and/or the ability to quantify residue cover. Scenes of known amounts of plant litter covering three contrasting soils were prepared and their reflectance spectra (0.4-2.5 μm) were measured with a hyper-resolution spectroradiometer. Litter from four crop (corn, soybean, rice, and wheat) and two tree species (coniferous and deciduous) were included. The CAI (0.5(R2.0+R2.2)-R2.1) describes the average depth of the cellulose absorption feature at 2.1 μm in reflectance spectra. Positive values of CAI indicate the presence of cellulose. The mean CAI of the soils was -2.0 while the mean CAI of the plant litter was 5.2. CAI increased linearly for each plant litter as the amount of plant litter in the scene increased from 0% (bare soil) to 100% cover. The CAI values of mixed scenes with more than 10% litter cover were significantly larger than the CAI values of the soils. The results of this study indicate that CAI is useful for quantifying plant litter cover, even at low percent cover.