Evaluation of the Photochemical Reflectance Index in AVIRIS Imagery

In this paper, we evaluate the potential for extracting the “photochemical reflectance index” (PRI; previously called the “physiological reflectance index”) from AVIRIS data. This index, which is derived from narrow-band reflectanceat531 and 570 nm, has proven to be a useful indicator of photosynthetic function at the leaf and canopy scales (Gamon and Field 1992, Gamon et al. 1992, Pcfiuelas et al. in review). At the leaf level, PRI varies with photosynthetic capacity, radiation-use cfficicncy, and vegetation type (unpublished data). This finding is consistent with the hypothesis that vegetation types exhibiting chronically reduced photosynthesis during periods of stress (e.g. drought-tolerant evergreens) invest proportionally more in photoproketivc proecsses than vegetation with high photosynthetic capacity (e.g. crops or deciduous perennials). Vcrlical transects in tropical and boreal forest canopies have indicated declines in PRI associated with downrcgulation of photosynthesis at the canopy tops under sunny, dry midday conditions (unpublished data). Tlris reduced PRI in upper canopy lCVCIS provides a further basis for examining this signal with the “view from above” afforded by aircraft overflights. Although many factors could eonfound interpretation of a subtle physiological signal at the landscape scale, wc conducted a preliminary examination of PRI cxtractcd from existing AVIRIS imagery of Stanford Universit y’s Jasper Ridge Biological Preserve obtained on the June 2nd, 1992, overflight. The goal was to usc the hyperspectral capabilities of AVIRIS to evaluate the potential of this index for obtaining useful physiological data at the landscape sealc. The expectation based on leafand canopy-level s[udics was that regions containing vegetation of reduced photosynthetic capacity (e.g. chaparral or evergreen woodland) would exhibh lower PRI values than regions of high capacity (e.g. deciduous woodland).