Plant Litter and Soil Reflectance

The presence of plant litter on the soil surface influINTRODUCTION ences the flow of nutrients, carbon, water, and energy in Ecological Importance of Litter terrestrial ecosystems. Quantifying plant litter cover is Litter is senescent (or dead) plant material that gradually important for interpreting vegetated landscapes and for decomposes into soil. It is difficult to classify litter beevaluating the effectiveness of conservation tillage praccause there is no particular point in time where it shifts tices. Current methods of measuring litter cover are subfrom one state of organic matter to another. In this jective, requiring considerable visual judgment. Reliable study, litter is considered to be both senesced tree leaves and objective methods are needed. The spectral reflecand the portion of annual crops left in the field after tance (0.4–2.5 lm) of wet and dry soils (six types) and harvest. plant litters (2 crops, 14 forest, and 2 grasses) of different The decay of litter adds nutrients to the soil, images were measured. Discrimination of plant litters from proves soil structure and reduces soil erosion (Aase and the soils was ambiguous in the visible and near-infrared Tanaka, 1991). The annual loss of 1.25 billion tons of soil (0.4–1.1lm) wavelength region. An absorption feature asin the United States could be reduced by leaving litter sociated with cellulose and lignin was observed at 2.1 lm on bare soil (USDA, 1991). Litter also affects water infilin the spectra of dry plant litter, which was not present tration, evaporation, porosity, and soil temperatures (Reiin the spectra of soils. A new spectral variable, cellulose cosky, 1994). Thus, the presence of plant litter on the absorption index (CAI), was defined using the relative soil surface influences the flow of nutrients, carbon, wadepth of the reflectance spectra at 2.1 lm. CAI of dry ter, and energy in terrestrial ecosystems. Quantifying litlitter was significantly greater than CAI of soils. CAI ter is important not only to improve surface energy balgenerally decreased with age of the litter. Water absorpance, but also to improve estimates of net primary tion dominated the spectral properties of both soils and productivity and nutrient turnover rates. In agriculture plant litter and significantly reduced the CAI of the plant systems, quantifying crop residue cover is necessary to litters. Nevertheless, the CAI of wet litter was signifievaluate the effectiveness of conservation tillage cantly greater then CAI of wet soil. This study provides practices. a new methodology to discriminate plant litter from soils Crop residue can be identified and quantified using by differences in spectral reflectance produced by their manual residue cover measurement techniques. Morphysical and chemical attributes. This remote sensing rison et al. (1993) noted that the most widely used method should improve quantification of plant litter procedure to measure crop residue cover in the field cover and thus improve estimates of phytomass produc(line-transect method) is tedious and prone to human tion, surface energy balance, and the effectiveness of soil judgment errors. These methods need to be replaced by conservation practices. Plant litter reflectance is a verifimore objective, faster, and more accurate spectral meaable component in vegetative landscapes and should be labeled and modeled separately from soils in landscape surement techniques (Daughtry et al., 1996; McMurtrey studies. Elsevier Science Inc., 2000 et al., 1993).