SPECTRAL CHANGES WITH LEAF AGING IN AMAZON CAATINGA

Significant gaps exist in thp knowledge of tropical leaf spectra and t he manner in which spectra change as leaves age in their natural envir onment. Leaf aging effects may be particularly important in tropical v egetation growing on nutrient poor soils, such as Amazon caatinga, a w hite sand community common in the Amazon Basin. Spectral changes obser ved in six caatinga dominants include decreased reflectance and transm ittance and increased absorptance for epiphyll-coated older leaves. Ne ar-infrared (NIR) changes were most significant. More detailed spectra l and physical changes were studied in one dominant, Aldina heterophyl la. Over 16 months, Aldina study plants produced one or two leaf flush es. During leaf expansion, leaf water content and Specific Leaf Area d ecreased rapidly. Over the first 6 months spectral changes occurred ac ross the spectrum, resulting in decreased transmittance and increased absorptance in the visible and NIR and decreased visible and increased NIR reflectance. In contrast, significant spectral changes were restr icted to the NIR over the last 9 months, which showed a 10% absorptanc e increase associated primarily with increasing epiphylls and necrosis . At the canopy scale, increased NIR absorptance provides a mechanism for producing seasonally varying forest albedo and changing NIR to red ratios, independent of changes in other canopy attributes. In the Ama zon caatinga studied, all canopy dominants were subject to epiphyllic growth providing a mechanism for distinguishing these forest types spe ctrally from more diverse terrafirme forest or forest types with more rapid leaf turnover, such as second growth. These changes are observab le using remote sensing and could be used to map caatinga and monitor interannual or seasonal variability in phenology. If these results can be extended to other communities with long-lived foliage, they may of fer a means for mapping vegetation on the basis of leaf longevity.