Spectral Bio-indicator Simulations for Tracking Photosynthetic Activities in a Corn Field
2011
Accurate assessment of vegetation canopy optical properties plays a critical role in monitoring natural and managed
ecosystems under environmental changes. In this context, radiative transfer (RT) models simulating vegetation canopy
reflectance have been demonstrated to be a powerful tool for understanding and estimating spectral bio-indicators. In this
study, two narrow band spectroradiometers were utilized to acquire observations over corn canopies for two summers.
These in situ spectral data were then used to validate a two-layer Markov chain-based canopy reflectance model for
simulating the Photochemical Reflectance Index (PRI), which has been widely used in recent vegetation photosynthetic
light use efficiency (LUE) studies. The in situ PRI derived from narrow band hyperspectral reflectance exhibited clear
responses to: 1) viewing geometry which affects the light environment; and 2) seasonal variation corresponding to the
growth stage. The RT model (ACRM) successfully simulated the responses to the viewing geometry. The best
simulations were obtained when the model was set to run in the two layer mode using the sunlit leaves as the upper layer
and shaded leaves as the lower layer. Simulated PRI values yielded much better correlations to in situ observations when
the cornfield was dominated by green foliage during the early growth, vegetative and reproductive stages (r = 0.78 to
0.86) than in the later senescent stage (r = 0.65). Further sensitivity analyses were conducted to show the important
influences of leaf area index (LAI) and the sunlit/shaded ratio on PRI observations.
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