Tower Based Measurements of Bio-indicators for Light Use Efficiency Estimation
Yen-Ben
Cheng, NASA GSFC, yenben.cheng@ars.usda.gov
(Presenting)
Elizabeth
M
Middltone, NASA GSFC, elizabeth.m.middleton@nasa.gov
Thomas
Hilker, University of British Columbia, thilker@interchange.ubc.ca
Nicholas
C
Coops, University of British Columbia, nicholas.coops@ubc.ca
T.
Andrew
Black, University of British Columbia, ablack@interchange.ubc.ca
Praveena
Krishnan, University of British Columbia, praveena.krishnan@ubc.ca
The use of remotely sensed measurements collected by satellite, aircraft, and ground instruments has been shown to improve our understanding of ecological and hydrological. We analyzed hyperspectral reflectance measurements collected at a coastal forest in British Columbia, Canada through the 2006 growing season. Diurnal and seasonal dynamics of the Photochemical Reflectance Index (PRI), a normalized difference spectral band-ratio index based on the xanthophyll signal at 531 nm which expresses protective responses to high light stress, were studied. This index has been shown to correlate with photosynthetic light use efficiency (LUE), an essential variable to model carbon uptake efficiency by plants. The measurements were collected from an automated system mounted on a flux tower under different sun and view geometries and atmospheric conditions through the 2006 growing season. Canopy structure was modeled using Light Detection and Ranging (LiDAR) technology, from which the sunlit and shaded canopy fractions were calculated as a function of incoming photosynthetically active radiation (PAR). These directional observations allowed us to: 1) compare the PRI with tower-based micro-meteorological measurements; and 2) separately investigate the PRI dynamics for sunlit and shaded partitions of the canopy which differ in response to their light environments. PRI showed an obvious correlation with the LiDAR-based shadow fraction estimates. In general, PRI for the sunlit foliage subset showed lower (more negative) values than shaded leaves. This was expected since leaves exposed to direct sunlight in their natural environment are likely under higher light stress. The difference was greater in April, the commencement of the growing season, than in August when water availability was at its lowest of the year. For other tower based measurements, PAR and GEP both showed apparent seasonal patterns. Better estimates of the actual PAR intensity illuminating the sunlit and shaded canopy fractions were retrieved using the shadow fraction to reduce the above-canopy PAR. A clear seasonal pattern emerged for this revised PAR that distinguished among the groups and was also used to estimate LUE for the leaf groups. The correlation between PRI and LUE was confirmed. Furthermore, the differences of PRI values between sunlit and shaded foliage were calculated and were named dPRI. This index showed significant correlation with LUE for the bulk canopy through the growing season. Air temperature showed slightly weaker correlation with LUE. From these results, better understandings of the dynamics of carbon exchange bio-indicators that can be derived from directional hyperspectral reflectance measurements were demonstrated.
Keywords: PRI, photosynthesis, PAR, GEP, LUE
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